Front Cover
 Title Page
 Table of Contents
 Galvanism, or voltaism
 Index and glossary
 Back Cover

Title: Scientific dialogues : intended for the instruction and entertainment of young people
Full Citation
Permanent Link: http://ufdc.ufl.edu/UF00002782/00001
 Material Information
Title: Scientific dialogues : intended for the instruction and entertainment of young people
Series Title: Scientific dialogues : intended for the instruction and entertainment of young people
Physical Description: Book
Language: English
Creator: Joyce, Jeremiah
Publisher: Thomas Allman
Place of Publication: London
Edition: New, single volume
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Bibliographic ID: UF00002782
Volume ID: VID00001
Source Institution: University of Florida
Holding Location: University of Florida
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Resource Identifier: ltqf - AAA3073
ltuf - ALH2744
oclc - 45891806
alephbibnum - 002232352

Table of Contents
    Front Cover
        Front Cover 1
        Front Cover 2
    Title Page
        Page i
        Page ii
        Page iii
        Page iv
        Page v
        Page vi
    Table of Contents
        Page vii
        Page viii
        Page ix
        Page x
        Page xi
        Page xii
        Page 1
        Page 2
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    Galvanism, or voltaism
        Page 496
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    Index and glossary
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    Back Cover
        Page 534
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Full Text


... ..

.1~ kn~d*

A 4&d


: i ' I rii,7.r,nl.iL`n 'i




~c~ ~4

~ ,;fI

42 1UflOM D.L





BY THU BY. .3. JOyc

Ssmw om m



Tnz Aaudwi drfi hmnslf estmndey Ip h
e.portlt wM le thi pgmicbom albri hbi of
a1-nowdging the oblpo em he bi uadr to &e
nathor d' Pral Eaetion,' fr the pl rmsr ml
iaMtnction which he 'M d"ivl fma that valuable
wort. To toi he a d oely idebted 6 r othe U a
writing oa the abjeot of Natul Philoopy hr &h
ure of child How du is plan eorspboad wi
that agsto d by Mr. Edgwoth, m his chapter
IaFechalu, m ta be ht wih a cead pAbl to

The Author comedv, at I hbe d Uall ie JUid
in malng, tHt no itnadalian to mateu t od .


perimental philosophy h been attempted in a
method so familiar and eay a that which he now
ofers to the public :-nme which appear to him
I property adapted to the capacitie of young people
of te or eleven year of ae, a period of life which.
fom the Author a own experience, he confident is
by no mean. too early to induce in children habit do
scientific reuonig. In this opinion he b sanctioned
by the authority of Mr. Edgeworth. "Parents,*
says he, "ae anxnou that children bsould be cover-
ut with mechanic, and with what are called the
mechanical owe. Certainly no species of know.
ledge i better suited to the tate and capacity of
youth, and yet it seldom form a part of early in-
strction. Every body talks of the lever, the
wedge, and the pulley, but most people perceive
that the notion which they have of their respective
e is unsatisfactory and indistinct, and many en-
deavour, at a late period of life, to acquire a sientiic
ad exact knowledge of the effects that an produced
by implemats which ar in every body's hands, o
that are alboltely necessary in the daily occupations
of makimld.

The Author trus that the whole work ill be
bead a complete compendium of atuul ad ex.

psrim al pulaosphy, 0e oly adapted the
mrdmuhrima of yougfleple, bdt well calculeod
alo to cmvey dtha kat,4 .miiar iruetism which
i absoluatly oamdos y IhM a pron cMa tread
public lectures in thee beaches of eme with
advantage. "UI says Mr. Edgerowth, m pl eng
o this sbject, "the lectmr doe not eemmaicat
much eo tht knowledge whih he aelmwn to
explain, it i not to be attribued either to hi want do
skill, or to the insmcicy dh bi appratus, but to
the novelty of t lerm which he is obliged to e.
Ignoance of the language in which any seiece is
taught, is an insuprable bar to its being auddely
acquired; besides a precise knowledge ofth ing
ofterm, we must have an instantaneou idea exzitd
in our minds whenever they ar npeated; and, -
thi can be acquired only by practice it is imponibl
that philophical lectures can b of much service to
those who are not familiarly acquainted with the
technical language in which they aM delivrd."

Mr. IBdreworats dhper ea Mechuanls sbd be
l,--r mald to the altemnl es o reader, but the
author uee willing to n sr to patf a wrk, tho
whole of which deserves the eareal peiM l at a p
mm nsaged In th edus eam ot ye th.


It is pwmad that an attentive perusal of the
DiloguM, In which the pridpal and most common
mea a sAle1 are ruefully explained and illus
treated, by a variety of familiar examples, will be the
meats ot obvating this objection, with respect to
pers who may be desirous of attending those pub-
Ie hilosophicl lectures, to which the mihabiuts d
lbh mopli have almost com ustt afr



I. Introducetimo . I
IL Of Matter. Of the D idity .t
Matter 4
III. Of the Attraction of Cobeion 7
IV. Of the Attction of Co i 11
V. Of the Attrction of rvitaio . 4
VL OftheAttmtio of Gmritatio 17
VII. Of the Attraction of Orritio 0
VIII. Of the Attraetinm f Gnvittioa .
IX. Ofthe Centreof GMity 9 7
X. Ofthe Cetrofraviy 30
XI. Ofthe Lawsf Motion 33
XII. Of the Law of Mtion 8
XIIL Of tLaws of Motio 41
XIV. Of th Mechanical Pow 44
XV. Of to 48
XVI. Ofthe Le~ .
XVII. Of th Wbeel ad A 6
XVIII.Of t Palley 6
xIX. O(thb laclied Pla B

Ni cowrfi~s.

X1. OftheWdge 66
XXI. OftheSrew .. 67
XXII. Of thPendulum 71

I. Of the fid Sh 74
IL Of th fxed Stan . .. 77
III. Of to xed Sta and Ecliptie 81
IV. Ofth Epbhemr 86
V. Ofthe Solar Sym . 90
VL Of th Figure of the Earth 94
V Ofte diunl Motion ofdte rEuh 96
VIIIOf Dayd Night .103
I. Of the annual Motion of the Ert .106-
X. Ofthe Sfeaf .. .10
XL Of theSe III
I. Of the qua ofTimr .117
XIII. Of Lap Yar 11
XIV. Of the Moon .. 124
XV. OfEclips 128
XVI. OftlTids 13
XVII. Of the Harve Mon .137
XVIIL Of Mrcury 140
XIX. Of VMnu 142
XX. Of Mr 146
XXI. Of Jpit 149
XXII. Of Satur 160
XXII. Of th Hebhel Pla 164
XXIV. OfComel 166
XXV. OftheSn 1
XXVI. Of theol 8ar .



I. Introduction . .. 10
II. Of th Weight a d Prame of F1l 16
III. Of th Weight nld Pmu of Fhuids 171
IV. Of the Laral Prre nof Fl d 176
V. Of the Hydretati Perado .178
VI Of the Hydvoeted Bdlow .13
VII. Of the Preue of luidb ag t the
Sides df Vme .186
VIIL Of the Motio ofMd .190
IX. Of theMotio fFlids .194
X. Ofthe Specile Grity of Bodi .198
XI. Of the Specie Grity of Bodie 01
XII. Of the Methods of lding the pecic
Oreity of Bodis. 06
III. Of th Methods of lading th Specii
Grnity o Bodis. .0
XIV. Of the Methodsof lading the Speci
Grity of Bodie 13
XV. Of the Methods of lading the Spele
Grity ofBodies .116
XVI. Of t Hydrometer 20
LVII. Of the Hydrometer nid Swimming 14
XVIII. Of the Syphon and Ttals's Cup 17
XIX Of the Di'" Bel l1
XX. Ofthd D AW 9B1 .14
XXL OfPmpm 3
xnII. Of thn Fordkg p-rrHmgi
Ropepmmp Chain-pump and
Wter-pre .240



common Paw
I. Of the Natre of Air . 46
II. Of the Air-pump .247
IIL Of the Tkiclli Experint 262
IV Ofthe P rodtLhe Air. .255
V. Of th P re oth Airk. 28
VL Ofth W t of Air .61
VII. Of th Eltici of Air .. 5
Ill. Of th Compneio of Air 70
IX. Miseellaeoup Experiante the Air-
pup 74
X. Ofthe Airg Sod .277
L OfSoud. .81
III. Of tl Speaig Trampet . 386
II. Ofth Eco .288
XIV. Of the Ech .
XV. Of the Winds .
XVI. Ofthe tUsma gine . 3
XVi. Ofth Sten-eagine. 06
XVIII. Of the Stem-agine sad Papin's Di-
gster 80
XIX. Ofthe uomter .318
XX. Of the ib metsr, su l iApplickatioto
toe ~Meuif of Aldita .317
XXI. Of te Thermometer. .320
XXI. Ofthe Thermometer 324
XXU.OftheTbsnDOar ss. .834
r Of the Pyroueter and Hypeter 27
IXIV. Of the Rain-Fa, ad RIl far
udgmgof the Wethbr. .332


I Light: the 8mlami ad Vcity of
o Puatil 87
II. Rays to Light:-R emiodm d B.

V. Diarmet Khie Im 83
VI. Puan "ibveigraatreraoi, R 36s
VIL Imns of Objrt. -kFLteI l,
&c. .
VIII. Nature Mad AdvaMlagf Lt 384
IX. Clom 7
X. *lectsd Light d Plla Mine 70
XI. Coca Mies .373
XII. CoacaveMinLo-Expemes. 870
XIII. CoGcamo and Convex Miras . 76
XIV. Optid Deceptions, Aninrpha, &. 3
XV. Dilst Pat of the Eoye .3
XVI. Manner of Viio . .. M8
XVII. Spectel, ud thir Um 3
XVIIL Riabow . .
XIX. BRIlebn Telescope .400
XX. BRldin Tdscope .404
XX. Microp .. .407
XXII. Cim s Obsear, Magic Leathmr,
and Maplyipi GOh . .418
L The M st .. .41
IL Magei Attracion and a i ,4
IIL Methods of ins Mpt .438
IV. MaSim' Comnpa 437


I. Early History of Electricity 431
11. Electrical Attraction and Repulsion 433
11L Electrical Machine 438
IV. Electrical Machine .442
V. Electrical Attraction and Repulim 446
VI. Electrical Attraction and Repulsion 452
VII. The Leyden Phial 464
VIIL Lane's Electrometer, and the Electrical
Battery 45
IX. Experiments with the Battery 463
X. Miscellaneous Experiments 468
XI. Electrophorus.-Electrometer.-Thun-
der.House, &c. 472
XII. Atmosperical Electricity .476
XIII. OfAtmospheric Electricity-of Falling
Stars Aurora Borealis- Water-
Spouts and Whirlwinds Earth-
quakes 479
XIV. Medical Electricity 485
XV. Animal Electricity-of the Torpedo-
of the Gymn"ou Elecicae-of the
Siur Electrias. .487
XVI. General Summary of Electricity, with
Experimets 491

I Of Oalvanim; a ri 0 .; E l ents
-of tbheDecepai o f Water 496
11. Galvanie Light and Shocks. 409
III. alvaic Cooductos.-Circle-Tables
-Experimuets 60s
IV. Miscellaneous Experimenta B
GOLsamar AND IssK *. 616


CHARLmS. Father, you told sister Emma aad me. tht
after we had finished reading the Evenigs at HArw,
you would explain to as some of the principles o
natural pophy: will yP begin this morning
Fathr. Y, I am quite t leisure; and I shall,
indeed, at all times take a delight in communicating to
you the elements of useful knowledge; and the more
so in proportion to the desire which you have of col-
tecting and storing these fcts that may enable you to
understand the operations of nature, a well as the
works of ingenious aruts. These, I trust, will lead
you isenibly to admire the wisdom and goodness by
means of which the whole system of the universe a
constructed ad supported.
Emms. But can philosophy be comprehended by
children so young as we are1 I thought that it hat
been the business of men, and of old men too.
F. Philosophy is a word which in its original sense
sigifi only a love or deire of wisdom; and you will
nt allow that you and your brother are too young to
wish for knowledge.
E. So far from it, thai the more knowledge I get
the better I eem to ike it; and the number of new
ideas which, with a little of your assistance, I have
obtained from the Eviasg ct Home," and the great
pleasure which I have received from the perusal of
that work, will, I am sure, excite me to read it again
and again.

F. You will And very little, in the introductory parts
of natural and experimental philosophy, that wi re
quir more of your attention than many parts of that
work with which you were so delighted.
C. But in some books of natural philosophy, which
I have occasionally looked into, a number of new and
uncommon words have perplexed me; I have also
seen references to figure, by means of large letters
and small, the use of which Idid not comprehend.
F. It i frequently a dangerous practice for young
minds to dip into subjets before they are prepared, by
some previous knowledge, to enter upon them; since
it may create a distaste for the most interesting topics.
Thus, those books which you now read with so much
pleasure would not have afforded you the smallest en-
tertainment a few year ago, when you must have
spelt out almost every word in each page. The same
ort of disgust will naturally be felt by persons who
should attempt to read works of science before the
leading terms are explained and understood. The
word agle is continually recurring in subjects of this
sort; do you know what an angle ai
E. I do not think I do; will you explain what it
means I
F. An angle is made by the opening of two right*
lines. In thi figure there are two straight
lines o and c meeting at the point b, and
the opening made by them i called an Fig. I
angle. Fig. 1.
C. Whether that opening be mail or great, is it
still called an angle
F. It is; your drawing cippae may miliarise
to your mindthe idea ofn eagle; the lies in thu
gure will aptly present the legs of the compasses,
and the pint b the joint upon which they move or
tarn. Now you may open the leg to any distance
S p please, even io ath tht t hall form one traht
Ine; in that position only they do mt form an angle.
Straight lHs, in woriu of sadee, me usay do.
Rebseto riH Uan.

nl every other situation a angle is mde by the open.
t of te ad thee engle is sid to be graer
orlei, u that opPg is greater or less. An angle
is another word for a cmnr.
E. As not aungles called right anglet
F. Angl are eaner right, ente, or oake. Whm
Sthe line a meet another line cd in such d
a manner a to make the angles abd and I
Abo equal to one other, then those angle i
are called right agles. And the lie
Said to be perpediculu to d. e mee
to be perpendiclar to, or to make rht angles with,
a line, means one and the sme thing.
C. Does it signify ow you call the letters of a
It is usual to call every angle by three letters,
and that at the angular point must be always
the middle letter of the three. There are
cue, however, where an angle may be deno-
minated By *a ingle letter; thuthe angle abi
may be called imply the angle b, for there ,
is no danger of mistake, because there is but
a inge angle at the int b. Fig. 3.
SC. I undenrstdth; farif, in tbe eced fig ,I
were to dcribe the angle by the letter b only, you
would et know whether I meant the angle *e or
F. That i the preMcie reo why it is necessary
n mot description to make use of three letters. An
aeut angle (Fig. 1.) abo is less than a right angle;
and an rk6teu angle (Fig. 8.) e is greater than a
right angle.
E. You s the reason now, Charles, why letter
are laced again or by the fguse, which pulled
C. I do; they ae intended to ditingish the sepa.
rate parts of each in order to rader the description of
them easer beth to the author and the reader.
L What i the different, papa. between an angle
and a triangle

F. As angle being made by the opema of two
line, ad as you know that two straight lie canst
encloea spa.o e trieagl ckis iaspece
bounded by thr straight line. It take
it name from the property of acoamg
three angle. Them are arioue rt of
trial, but it i not ae a to er upon thee .
prticulars, as I do not wish to bb u enyour memories
with more technical terms than we have occasion for.
C. A triangle, then, isa space or ngure containing
three angles, and bounded by a many straight lines.
F. Ye, that description will anwer our parent

or MATTR.--Ow ia DInuIUBTYurr or ATTS.
F. Do you understand what philoeophen mea
when they make ua of the word matter 1
E. Are not all things which we e and fel com-
poed of matter
F. Every thing which is the object of our seme is
composed of matter dieretly modified or arranged.
But a a philosophical se mater i delned to be an
stanIed, Jlid, insctiw, and msoesn e ubstance.
C. If by extension i meant length, breadth, aad
thickness, matter, undoubtedly, is an extended sub-
stance. Its solidity is manifet by the resistance it
makes to the touch.
E. And the other properties nobody will deny, for
all material objects are of themselves without motion;
and yet it may be readily coeived, that, by applica-
o of a poper ore, ithr i body which anoet
be moved. But I remember, pap, that you told ua
" .e n about the diiibility of matter, which
y said sihtb contiued without end.
I did, ome time back, mention this curious san
imtn iaghbject, and this a very fit tin for me
t expla it.

C. Can matter ideead be ii ly dWidr; Ar I
appse that ths is what i meant by a diviim wi.
P. Dilnlt as this may at Ant appear. yetr I ti
very capbl of proof. Can yo conceie of a par.
ticle of matter m small a not to hae an upper ad
under arfaeal
C. Certainly every ponrdo of matter, however i-
nate, mut have two arface at leat, and thea I Me
that it follows of cou that it i divisible; that s,
the upper and lower urfach may be sparated.
F. Your co ml ioa i just; and, though ter
may be particle of matter too mll for a actually
to diide, yet this ares from the imperfection our
instrument; they must neverthel in their nature,
be diviible.
E. But you were to give ams seemarkable m.
ancea of the minute division of matter.
F. A few yean agoa ady aile poadd od
wool into a thread 168,000 y= r lon. Ad Mr.
Boyle mention that two gn and a half of ilk was
pn ito a thread 300 yard in If a pound
d silver, which, you know, contams 6,760 ri
and a single grain of old, be melted together, the
gold will be equally diuaed through the whole il-
ver, inomuch, that if one grain of the mas be dis-
solved in a liquid called qua forti, the gold will fall
to the bottom. By thi experiment, it evident that
Sgain may be divided into 5,761 visible parts, r
nly the 6,761st prt of the gold i contained in a
ngle grain of the mass.
he goldbeater, whom you have en at work in
the shops in Long-cre, can spread a grain of gold
into a leaf containing 60 square ineb, and thi ef
may be readily divided into 600,000 pa each of
which i visible to the naked eye: and bthe help t
a mirocop which magnais the area or aurfhee of
a body 10 time, the 100th pat of each t the be
come viable; that is. the 10 millioth at of a
grain of gold will be viable. or a ingle gram of that

metal my be divided into 60 millions f visible parte
But the gold which coven the silver wire med is
making wt is called gold lace, spread over sameh
larger rface, yet it preserve, even if examined by
microeope, an uniform appearance. It has been
calculated that one grain of gold, under then eiraea.
stances, would cover a surface of nearly thirty square
The atrt divisions of matter m still more ar-
pinig. In odoriferous bodies, ucb a camphor,
muk, and fatid, a wonderful subtilty of pera i
perceived; for, though they are perpetlly a
considerable space with odoriferou perticle, ye these
bodies lose but a very mal part of their wght in a
great legth of time.
Again, it is said by tboe who have examined the
subject with the beat gluae, and whose accuracy m
be relid o, t t tthe an more animals in the milt
of a single cod-fs, than there ae men on the whole
earth, and that a single grain of and is larger than
four millions of thee annuals. Now if it be admitted
that these little animals ar poemmed of organized.
parts, such a a heart, stomach, muscles, vein, arts
nee, &c. and that they are posed of a complete
system of circulating fluid, umilar to what is foun
i larger animals, we eem to approach to an idea of
the ininie divibility of matter. It ha indeed been
calculated, that a particle of the blood of one of their
animalcule is a much miller than a globe one-tenth
of an inch in diameter, as that globe a smaller than
the whole earth. Nevertheless, if these puartl be
compared with the prtile of light, it i probable
at they would be ound to exceed them in bulk an
much u mountains do single grain* of sand.
I might enam rate many other instance of the
ame kind, but these, I doubt not, will be sndicient to
covne you into what very minute part matter i
capable of being divided.
Captain Scereby, hi Account of the Greenland
Ses, stat, that. in July 1818, his esl siled for

several leagues in water df a very unommon appea.
amce. The arface was varieated by larg p
of a yellowh-reen color. It was found to be .
duced by saimalcul and microsopes were app
to their examination. In a a*gle drop of the water,
examined by a power of 28,24 (mnguied msperi
cie), there were 60 in number, on a average, in
each squre of the micrometer glss of 1340th of aa
inch in diameter; and, u the drop occupied a circle
on a plate of glass containing 69 of these squam
there moat have been in this sngl drop of water.
taken at random out of the sea, and i place not t
most discolourd, about 26,460 animalcal. How
inconceivably minute must the vessels, orea, and
flid, of them animals beo A whale requme a sea
to sport in: knadred udisfty millions oftheu udd
Ames mUple sMeps for their satisu im s tabler of

F. Well, my dear children, have you reeeted
upon what we last conversed about Do you com-
prehend the several instances which I enumerated a
examples of the minute division of matter 1
E. Indeed, the examples which you gave as ver
much excited my wonder and admiration, and yet,
from the thianne of some leaf gold which I one
had, I can readily credit all yon have said on that
prt of the subject. But I know not how to conceive
ofsuch mall animals as you described; and I am tll
more at a los how to imagine that animals so minute
should pome all the properties of the larger oes,
se uas a heart, verin, blood, &e.
F. I can, the next bright morning, by the help of
my olar microscope, shew you very distintly, the
circulation of the blood in a fea, which you may get
from your little dog; and with better glames than
those of whah I am possesed, the sam soneanner

might be r n is imals still smaller than these ae
rbapevren in those which ar themeles invible
to the mnad eye. Bt we shall cnvees more .t
la o this mat ter, en we com to ceeside the
ubjct ofoptic, and the constretion and M of tb
ar miecrope. At rM wet w will tarm our
thought to that M.i in nature, which philoe
phers have do e grvit, or ttraction.
C. If the be no more difficulties in phio
y than we met with in our last kle I do t
r but t hat we shall, in general, be able to undr-
std it. Are there not several kinds of attraction
F. Ye there ar; two of whichit will be sfficit
for our present pp to describe; the oas i the
sttracion of ; the ether, that of grtatimn.
The satrMtims cf kiMn is that power which keeps
the puts of bodies together when they tech, ad
prevents them from eparating, or which inclines the
parts of bodies to unite, when they are placed suf-
eiently near to each other.
C. Is it then by the attraction of cohesion that the
parts of thii table, or o the penknife, are kept to-
gether t
F. The instance which you have selected ame c-
eurate, but you might have said the sme of every
other solid substance in the roos ; and it in propr-
tion to the different degrees of attraction wth which
different substances are affected, that ome bodies are
hard, others soft, tough, &c. 'A philosopher in Hl-
land, almost a centu ago, took great p s in ascer-
taining the dierest degree of cohebon which be-
longed to various kinds of wood, metals, ad many
other subtances. A short account of the epemrimean
made by M. Maseheabreek, you will herether fnd
in your own lnage, in Dr. Eaeld's Intitutes of
Natural Philoophy.
C. You once shewd me that two leader ballet
having a little scraped from the surfaces, would stick
together with great force; you called that, I believe,
the attration of cohesion?

F. Idid. snm philespher, who have made this
periomat with eat antelio ad seeway, srt,
that if thse at soAe, which an pesae to em
another, be but a quarter of a moh a diameer.
*aped vry aooth, ad forcibly Id r sptlh
with a twist, a weight of bhund pounds fe.
ladtiy reuimd to aento them.
Ai by this b dof tractio that the part of
solid bodies ar kept toher, s, whenm ay nbae
is separd broen, it s ely the attnrali of co-
hesio that is overcome in that particular put.
E. Then, when I iad the anfortua this meM
at brakfat to Iet my maer slip from my had, by
which it wu breoen into several pieces, wa it ly
the attactio of cohesion that was oeome by th
part of the acer being separated by its fl eM the
F. Jat s; for whether yea luckily break the
china, or cut a stick with yur knife, or melt ld over
the ire, as your brother sometimes does, in order to
make plummets; these and a thomand other instance
which are continually occurring, are bt examples in
which the cohesio is overcome by the fall, the knife,
or the ire.
E. The broke saucer being highly valued by
mamma, bhe has taken the pi* to jon it a*in wit
white lead; was this performed by meanof the at-
traction of cohesion I
F. It wa, my dear; and hence ye will eily
learn that many operation in cookery an i fet no-
thing more than diferef t methods of ca this
attraction to take place. Thus four, by il, has
little or nothing of is principle, but when ined with
milk, or other liquids, to a proper comency, the
part cohere to and thb eheri aany in-
tances becomes sall r er by mean of he heat
applied to it in boling or baking.
C. You put e ia mind of the fable the ma
blowing hot and cold; for, in the instance of the ld,
fin overcomes th attractio of cohesion; and the

to It0MANM10.
same power, heat, whe applied to puddins, bred
&e. causes their part to cohere more powerflly.
How an we to undersnd this
F. I will endeavour to remove your dificlly.
Heat expands all bodies without exception, a yu
ball see before we have finished our lectI ew
the fire applied to metals, in order to melt them, eases
such an expansion, that the particle are thrown out
of the sphere, or reach, of each others attract ;
whereas the heat communicated in the operation fd
cookery, is sufficient to expand the particle of flour,
but is not enough to overcome the attraction of co-
heion. Beside, your mamma will toll you, that the
heat of ilng would frequently disnit the prts of
which her pudding are comped, if he did not take
the precaution of enclosing them in a clo, leaving
them just room enough to expend without theliberty
of breaking to pieces; and the moment they ae taken
from the water, they lose their superabudant best,
and become solid.
E. When Aan the cook mke broth for little br-
ther, it is the eat then which overcome the attrac-
tion which the particles of meat have for each other,
for I have seen her pour off the broth, and the meat
is all in rags. But will not the heat overcome the
attraction which the parts of the bones have for each
other .
F. The heat of boiling water will never effect this,
bet a machine was invented several year ago by Mr.
Pai, for that purpose. It is called P i'a Dige
ana i umd in tavern, and in many g mili
for the purple of disolving bones as completely as
a leer degree of heat willliquefy jelly. On sme
future day Iwill show you an engraving of this ma-
chine, and explain its different parts, which are ex
timely simple *
See Pnoumatics. Conveation XVIII.


or T3 ATT ACTIO r conFaElm.
F. I will now mention some other instances of thi
great law of nature. If two polhed plate marble,
or bras, be put together, with a little oil between
them to fll up the pomre in their ~urhbos, they will
cohere so powerfully to require a very coderable
force to separate them.-Two globules of quicksivr,
placed very near to each other, will run toter and
orm one large drop.-Drop of water will do the
same.-Two circular p e of cork placed upon
water at about an inch distant will run together-
Balance a piece of smooth board on the end of a sale
beam; then let it lie fat on water, and ve or ix
time its own weight will be required to eparae it
from the water. If a small globule efqulcilvr be
laid on lea paper, and a piece of d broht
into contact with it, the merry wil adhere to it,
and be drawn away from the paper. But bring a
larger globule into contact with the smaller one, and
it will fonake the gla, and unite with the other
C. Is it not by means of the attracting of eobe-
aon, that the ltt tea which is generally lt at the
bottom of the cup instantly meds in the sgar when
thrown into it I
F. The sent of water or other liquids in ugar,
spone, and all porous bodies, i a species of tis at.
tractse, and i called empuo$uy strresim: it is thu
denominated from the property which tubes of a very
small bore, scareely lrger than to admit a hair, have
of causing water to stand above its level.
C. Is this property visible in no other tube ta
those the bohr of which are o eaeedingly Al e
F. Yea, it s very apparent in tube whose diame-
ter ae onetenth of a inch or more in length, but
the smaller ti ber, the higher the laid rime; iri
rom ctWu, the Latin word fur eAr.

enads in all instance, till the weight of the colu
of water in the tbe balances, or i equal to, the at-
traction of the tube. By immening tubeofdiffere
boree in a resel of coloured water, you will Me that
the water riea u much higher in the smaller tube,
than in the larger, as its bor is less than that of the
larger. The water will rie a quarter of an inch, and
there remain suspended in a tube, whose bor is about
oneeighth of an inch in diameter.
This kind of attraction is well illustrated, by taking
two pieces of glass, joined together at a
the side e, and kept a little open at
the opp ide adby a sallpiece
of cor In this poio immerse
them in a dish of coloured water f,
ad you will observe that the at-
traction of the glass at and near b, r. 6
will caum the lid to ascend to i whe about the
parts d, it sarcely rime above th eel of the water
i the veel.
C. I e that a crve informed by the war.
F. Them is, ad to t u th ere an many Cu-
rios property belonging, as you will hereafter bhe
abl to investigate for youelf.
E. I it not upon the principle of the atrac-
mo of cohsion, that carpenters glu their work to.
F. It is upon this principle that arpeate and
cabinet-makera make ue of gl; that braien, tin-
men, plumbers, &c. older their a; and that
miths unite different bae of iron by mean of heat.
These, and a thousand other operation of which we
are continually the witeses, depend on the sam
principle as that which induced your mamma to use
the white lead in meding her sacer. And u ht
to be told, that though white lead is fqMrontl du
a cement for broken cin glass, ud eawrteware,
et if the vlesel an to be beoht again into se, i i
ot a proper cement, being an active poion ; beid,
rne much stronger has been discov d, I believe, by

a vweyr a o d iePM ed pheM er, b t Itm Dr.
Ig.hou; t leaIhad it Am hmW ly

B. Wtl Im him? ,= lMJ`oopwi a Ihm
hw re my Dr, I"l wa, aeM *tl o s eh
killi hLnp a dI t
F. -Hew a m deeply killed ia may bMue
SrisM u; a IheL youd your bother wil
-M day Lmake bowm@ wMqm d iOth W mn T1
hi impoatu t diouene. Bat rel pleoopw
vill ceaidr it benMth hit maten toea tht cen.
C. ThiLs tsacti of cdeede ens to prd the
whola of batun.
F. It dom, bt ye wlle foetthtit c eiu
at vry mall dilr. slmse hbodi inded
to poeM a power th-ee ym e afttAreiem o
F. What is that, ppt
F. It iscal elrp Th watur npnl mt
boIr till toy ar wet. A small neede aefly
pluacd Mn wer will swi: lies walk upo it without
warming their et t:m drop of dd which ppwr io
a -nn m phM, putcularly a mbbapr plon%
Sa globularform, 6fim the mutuai attimr
tween tlh paleeof wamr; usd upc ezam- ti
it will be foud that the drp do t tech the l ,
for they wll u of empacmt bodies, which could
aet e if t hewe i tbe ihed amy diMr of at-
tracion betwem to walr d the iWt.
If a sml thi piee d iroe be lad peM qdiil-
wr, the nrpui between the dibret metal ill
cae the rfac of the quickilvmr ear the ien to
The pil force of t p dticka f a fnid i but
mall; dta if a ud di it aMily ul-m
agm. But if ga or ay hard ubtacn be
heeLen, the purt casmot bo =ma to cemn wilht

being Inb moisteed, became the repa o is too
gret to dadt ofa e-union.
The repelling frce between water and l Is ike..
wine grest, that it almost impoeible to mix them
in such a manner that the hall not prte agai.
Ifabll of light wood be dipped oil, and then
put into waer, the watr will neae so as to frm a
small chanel around the ball.
C. Why do cue, steel, and mr other things,
bear to bent without breaking, ad, when set t
liberty agin, recover their rigindl fnrm
F. That a p of thin teel, or caneeecoven its
sual fbm being bhet, is owing t a certain
po called lastiety, which may, porksa, ari
Sthe particles of those bodies, though disturbed,
not being drawn out of each others attraction; then;
fore, as sooa the frce upon them case to act,
they te thenelves to their former poion-Bet
our halfhour expired; I mustleveyM.

F. We ill now proceed to discs another very
important general principle in nature; the sttre-
tie of graitatim, or, as it i freqetly termed,
greity, which is that power by which diszsat bodies
tend towards each other. Of ti we have perpetual
instances in the falling of bodies to the eerth.
C. Am I, then, to understand that whether this
marble falls from my hand, or a looe brick from
Stop th e houn, or ua apple ftme the tee in the
rchaid, that all these happen by the attraction of
(rariy I
P.t is by the power which is commonly expressed
under the term gra iy, that all bodies whoever have
a teddeey to th earth; ud, ulem epparted, will
iall in lie nrly perpendiclear to its s4fce.
t. But am net mok, steam, and other ligh

bod., which we ase sd, neptios tohe gm
ni rule
F. It apear aat It eight, aud it ws fer rly
eceid a a eeral ofpini, that smoke, ta te.
pm d no weiht the diovy of the ur-pemp
n the f cy of this ma fr i an e.u
lasted eeier, that i, a glasm r frm which
the air is takes way by ms th a -pmp,
smoke ansd ena dby er own wght a co.
plaly api ece of s d. Whem we come to co-
vere em the objects of pamuais sad hydntatic
you will undertand that the mm why emoe and
other bodi ascend is imply becme they ar lighter
than t atmosphere wisk arimads them, and the
moment they mech tha t pat o it which hs the s
C. it, t-e, by th po that al test ial b
di ema m o the earth
F. By grvity, bodir em allU p f the earth
(which you kMow is of a global for) a kept el
it surface, because they all, whebmvw ituatd, ated
to the cotn; and, since all have a tedaency to the
cetre, the inhabitants of New Zealand, although
nerly poi to our fet, stuad s frm a we do
OnGreat Britain.
C. T7 is d&iie t to comprehed; erdthlms,
d bodies on aUll ar f the m of the earth have
a tademcy to the cetr, there eemo a why
bodies should et stead as rm on ne part as we
Another. Dos this power f grvit act aike eo
all bdie I
P. It does, without any read to thir fiA or
Iam fora ttrltim or gravity actlupeI bodal ia o.
ports to the quantity of netr which they contain,
that fhr times a greater force of gravity is ezrted
upeM a i t of four poond thae oe po e of a
fit e d. The Aquen of th priacple
tiall=bodi;s at equal distances from the auth fell
with mel velocity.
Wht do you nmWe, paper, by atelty

F. I wil explain it by an ample or two: if yet
and Chaies es t together, and pea walk a mile ia
half an hor, but h walk ad run two mie in the
me time, how much swifer will be go than you
E. Twice as swit.
F. He dos, beeaue, I the am tim, he pass
ver twice u much space; therefer, we ay hi velo.
city is twice a gatp a yos. Suppose atll, fed
from a cannot, pa t 0 rough 8 fet in a second of
time, and in the sae time you brothers rrow peas
through 100 feet ouy, bow much switer does the
cauao-bally thn the arrowt
E. Eight time swifter.
F. The it has eight times th velocity of the as
row; mld hence you understand that swiftness and
velocity are synonymous term; and that the velocity
of body measured by the pace it pae over in a
given time, u a second, a minute, an hour, &c.
E. If I let a piece of metal, as a peny-piece, and
a feather, fall from my hand at the omn time, the
penny will reach the ground much sooner than the
feather. Now how do you account for this if all bo.
dim are equally affected by gravitation, and deseead
with equal velocities, wen at the same distance from
the earth'
F. Though the penny and feather will not, in the
open air, fal with equal velocity, yet if the air be
taken away, which is easily done, by a little appear
tu connected with the air-pump, tey will descend in
the same time. Therefore the true reason why light
and heavy bodies do not fall with equal velocities, i,
that tbheormr, in proportion to its weight, meet with
a much ete resmtance from the air than the ltter.
C. It then, I maine, from the ame cause that,
Sif.I drop the penny ad a piece of light wood into a
vessel o water, the penny shall reach the bottom, but
the wood, after descending a nall way, rim to the
F. In this ase, the resisting medium is water in-
stead of air, and the copper, being about nine times

Mrm than its bulk of water, fl to the bottom
wit apparent resistance. But the wood, being
c light than water, cannot sink in therefore,
though y its moeniu it sinks a mallditanuc et,
a soo as that is overcome by the eiin medium,
it rie to the surface, being the lighter ubtce.

E. The term momentum, which you made ue of
yesterday, is mother word which I do not understand.
F. If you bae understood what I have aid re-
pti he velocity of moving bodies, you will easily
CO d wiht i. meant by the word momentum.
T muate, or moving force, of a body, i its
wight multiplied into it velocity. You may, or
instance, ploe this pouad weight upoo a chiaa-plate
without any danger of brcmng, but, if you let it fall
fram the height of only a few inches, it wil dah t
china to e. In tie fnt cae, the plate has only
the pound weight to dstain; in the other, the weight
mut be multiplied into the velocity, or, to speak in a
popular manner, into the distance of the height from
which it fel.
If a ball lean against the obsta-
cle b, it will not be able to overturn
it, but if it be taken up to e, and euf- .
(red to rdl down the inclined plaM F
I against 6, it will certainly overthrew it; in the
former eam, b would oly have to rmiat the weigt o
the ban th he latter it has to r- them witmul.
tilied late i moti, or veloity.
C. Then t momentum of a small body, whose
el is be retmY equal to that fa ery

S T esam l en o dd wh term wMi be nM i Uthe
Oda Omnmmml.

F. It may, and hence you the reen why im*
menu batteringrams, used by the ancient in the sa
f war, have given place to cannon balls of bt a few
wands weight.
C. I do, for what is wanting in weight, is made up
iy velocity.
F. Can you tell me what velocity a cannon bll of
18 pounds most have, to elect the ame prpoee, as
would be produced by a battering rm of 16,000
pounds weight, and which, by manual strength, could
be moved at the rate of only two feet in a second of
C. I think I can:-the imoumtum of the battering
ram must be estimated by it weight, multiplied into
the pace passed over in a second, which a 15,000
multiplied by two feet, equal to 30,000; now if this
momentum, which must also be that of the cannon
ball, be divided by the weight of the bell, it will give
the velocity required; and30,000 divided by 28, will
give for the quotient 107 eay, which is the num-
ber of feet which the cannon ball must pa over in a
second, in order that the moment of the battering
ram and the ball may be equal, or, in other words,
that the y have' the mae effect in beating down
an enemy's wall.
E. I now fully comprehend what the momentum
of a body is, for if I let a common trap-ball accident-
ally fall from my hand upon my foot, it occasions
more p thn the mere p aure of a weight several
time heavier than the bal.
F. If you let a pound, ora hundred pounds, fall on
the floor, only from the height of an inch and a quar.
Jr, it will strikethe oor with a momentum equal to
double its weight: and if you let it fall from our
time that heit, Ar ive inches, it will have double
that effect;-ad if it fall inetimes that height, or
eleven inches and a quarter it will have treble tr'
effet;-and by falling site time the height, or
twenty inches, it will have r times the effect, and
sa Hmce it i plain, that if o let the ball drop

from your hnd at the height f twenty inches eel,
will have eight tim man efet i causing pain mn
the me prem of the ball itself.
If th traction of gravitation be a power by
which bodies in eral teed toward each other, why
do all bodies tend to the earth a centre 1
P. I have already told yo that by the great law of
gravitation, the attraction of all bodies ie 'pr pport
to the quantity of matter which they eontam. Now
the e=uh, being o immeme lya in comparison of
all other betances in it vicinity, deroy the effect
of this attraction between mallbodi, by bng-
ing them all to itelf-If two bll are let fal bem
high tower at a mall distance apart, tho they
a ve attraction for oee another, yet it wil be as
nothing when compared with the attraction by which
they ar both implld to the earth, and co-eeqw y
the tendency which they mtually have of pprei
one another will not be percered in the fll. I.
however, any two bode were placed in fie peae, and
out of the sphre of the earth's attract they weM
in that cane auredly ll toward each other, nd
that with need veloty a they came nearer. If
the bodies wern equal, they would meet in the middle
point between t two; but if they were unqual, they
woold then meet a much neaer the larger one, a
that contained a geter quantity of matter than
C. According tohi, the eatth oght to move o.
ward lling bodies, as wrel a they move to it.
F. It ouht, and, injet theory, it doe; but when
you alclat how many million of ine largr the
earth i than anything belonging toit; ad if you
reckon the a diaancem rn which bodi ca hlal,
you will then nowa that the point where the falling
bodies and earth will meet, i removed only to an i
deaiely mall dietaen tem i him eea ; a dieta
mch too mall abe ceeaed b tbh huan imag
We willreme the bjeet of gravity e w.


E. Has the attraction of gravitation, paper, the
mame effect on all bodies, whatever be their distance
from the earth I
F. No; this, like every power which proved from
a centre, decreases as the qure of the distances from
that cetre increase.
E. I fear that I shall not understand this unle ye
illstrate it by example.
F. Suppoe you am reading at the distance of one
hot from a candle, and that you receive a certain
quatity of light on your book; now if you remove to
the distance of two feet from the candle, you will, by
tis law, enoy four times less light than you had be-
fore; here tha though you have increased your dis-
tece but twe-fold, yet the light is diminished tour-
fold, because four is the square of two, or two multi.
plied by itself. If, instead of removing two feet fro
the candle, you take your station at 34, 6, or 6 fee
distance, you will then receive, at the different di-
tances 9,16, 2, 36 times les light than when you
were within a ingle feet from the candle, for these, a
Snow, ar thesquaresof the number, 4, 6 and
The ame is applicable to the heat imparted by a
ire; at the dstce of oe yard from which, a per.
on will enjey fr times a much heat, as he who it
r stands two yards frm it; and nine time as much
a one that hall be removed to the ditace of three
C. Is tihe the a4tneties of gravity far times
Ie at a yrd distance frle t earthen it is at the
F. No; whatever be the cas of attraeione whob
o thi day nemais discovered, it act from the
mtre of the earth, and not from its surface, and hnce
the dilace of the power of gravity cannot be dis.

cerned at the small distances to whih we can have
ecces; for a mile or two, which is much higher then,
in general, we have oppltaitm at making aperi-
ments, is nohing in companion f 4000 miles, the
distance of the cent from the surface of the earth.
But could we ascend 4000 miles above the earth, and
ofcourse be double the distance thatwe aOw s from
the centa, we mshold there nd that the attractive
force would be but one-fourth what it i here; o
in other words tht a body, which at the surfaceof ta
earth weigh one pound, and, by the force of gravi
falls thrmgk sieen feet in a second of tinMe,
it 4000 mile above the earth weigh but a quarter of
.pound, and fall through only four feet in a scod.*
E. How is that kowa, papa, for nobody ever wa
there I
F. You are right, my dear, for Garneria, who sone
year ago etomihed all the people of the metropoli
and its neighborhood, by his flit in a balloo,
aeended but a little way i companon of te distance
that we are speaking of. However, I will try to se
plain in what manner piloopher have come b their
noledge on this subject.
The mo is a beay body connected with the earth
by this bond of attraction; and by the mat accurate
observation it is known to be obediet to the same
laws as other heavy bodies ae: its distane is als
clearly ascertaned, being about 140,000 miles, or
equal to about ity mi-diametes of the earth, and of
coune the earth's attraction apo the moon ought to
diminish in the proportion of the quare of thi dis
zL. lpmws It were required o iad the weiht of
a leas ball at the top of a motsin three miles high,
which, ea the urftce of the earh weighs Mib.
It the sem-dameter of the earth be taken at 4 ,
thea ad t t his the hesiht of the moenti, l m a, am
the qmuar or M is to the square m e m is b. to a
(tour thpeportloos: r as ImA : the5 ts: : s I
or something mole then ib s., w h ob the weight
if the laden ball at the Up e the meeslas.

anee; that is, i oght to be 60 times 00, or 300
times ler at the moon than it is at the surface of toh
earth. Thisiss found to be the e, by the mas
of the deviation of its orbit from a right in.
Again, the earth is not a pe t sphere, but a
pheroid, that is, of the oan orange, rather Bat
atthe two ends called the pol, and the distance from
the centre to the poles is about seventeen or eighteen
miles les than its distance from the centre to the equa-
tor; consequently, bodies ought to be something hea-
vier at, and near the poles, than they are at the equa-
tor, which i also found to be the cae. Hence it is
inferred, that the attraction of gravitation varies at all
distances from the centre of the earth, in proportion
as the squares of those distances increase.
C. It seems very surprising that philosophers, who
have discovered so many things, have not been able
to ind out the cause of gravity. Had Sir Iaac New-
ton been asked why a marble, dropped from the hand,
falls to the ground, could he not have ssiged the
reason ?
F. That great man, probably the greatest man that
ever adorned this world, was as modest as he was
great, and he would have told you he knew not the
The late learned Dr. Price, in a work which he
published fort-five year ago, ass, who does not
remember a tune when e would have wondered at
the question, why dom water run d .m hMilr What
ignorant man is there who is not persuaded that he
understand this perfectly 1 But every improved man
knows it to be a question he cannot answer." For
the descent of water, like that of other heavy bodies,
depends upon the attraction of gravitation, the cause
of which is still involved in darkness.
E. You jst now aid that heavy bodies by the
foe of gravity fall siten fat in a second of time;
s mat always the eae t
F. Yes, all bodies ear the surface of the earth fall
Is Cover. TI. As brmay.

at that rate in the t second of tie bout t attme
tion of gnravitedoa is continually acting, so t vlocy
of fraling bodies i an increeng, or, k it is ualy
called, an s velocity. It i floud, b very
accurate experiment, that body, deeceodi from
coiderble height by the force of gravity, fall
feet in the ut ed of time; 3 times 16 feet in .
next; 6 time 16 feet i the third; 7 time 16 feet in
the fourth secod of time; and m on, continually in-
creasg according t the odd numbers, 1, 3,5 9,
11, &c. In our latitude the true distance fallen is
16 feet oe-twelfth; but, by reason of the centrifuge
force, th distance varies a little in different latitude.
But tdir shll be explained to you hereafter.

E. Would a ball of twenty pounds' weight he.
weigh half an ounce lees on the top of a moatein
three miles high 1
F. Certainly; but you would not be able to s-
certain it by mean of a pair of scales and another
weight, because both weights beingin similar sit
ti would loe equal portion of their gravity.
E. How, then, would you make the experiment
F. By means of one of tboe teel spir pring in-
struments which you have seen occonally used, the
fact might be ascertained.
C. I think, from what you told us yesterday, that
with the distance of your top-watch, I could tell
the height of any place, by ob ing the number
of eonds that a marble or other heavy body would
take in falling from that height.
F. How would you perform the calculation
C. I should go through the mSltipliatwes ard.
n to the lber of ecads, d th add tese

F. Rxplain yourself more particularly:-ppoed g
ye, wer to let a marble or penny-pie f an down
that deep well which we saw last summer in the brick
field near Ramegate, and that it was exactly ve
seconds in the descent, what would be the depth of
the well I
C. In the first second it would fall 16 feet; in the
next 3 time 16 or 48 feet; in the third 6 times 16 or
80 feet; in the fourth 7 times 16 or 112 feet; and in
the fifth second 9 times 16 or 144 feet: now if I add
16, 48, 80, 112, and 144 together, the sum will be
400 feet, which according to your role is the depth of
the well. But was the well so deep
F. I do not think it was, but we did not make the
experiment; should we ever go to that place again
you may atisfy your curiosity. You recollect that
at Dover Castle we were told of a well there 360 feet
d ough your calculation was accurate, yet it was
not done as nature effects her operations; it was not
performed in the shortest way.
C. I should be pleased to know an easier method;
this however as very simple, it required nothing but
multiplication and addition.
F. True, but suppose I had given you an exam-
ple in which the number of seconds had been fifty in-
stead of five, the work would have taken you an hour
or more to have performed; whereas, by the rule
which I am going to give, it might have been done in
half a minute.
C. Pray let me have it; I hope it will be easily
F. It will; I think it cannot be forgotten afer it
a once understood. The rule is this, the
described by a body fi faru ly from a state f me
increa use a th seQnas= the timn inmr .* C."
quetl yu he ly to quare the number of
, thi, yo know, to multiply the number ins
itself, and them mltiply that agun by sixteen fst,

the pa which it deenbes in the m s moed, ad
yo a tmh required anwer. Now try teaumple
of the U.
C. The square of 6, for the the, x IS, which m*I.
tpied by 1 gives 400, just a I bought it out bei s.
Now if the coods had been 60, the answer weld
be 80 times 60, which is 000, sad this mulhipied by
1, give 40,000 for the pace required.
F. I ill now ask our siter a qui, to t how
she has understood thu nbjeet. ppose obeerv
by this watch that t thtime of the fight o your bro-
ter's arrow is exactly six seconds, to what height does
it rise t
E. This s a different question, because her the
. Oe as well u the fal of the arrow s to be en-
F. But you will remember that the time of the
ascent is always equal to that of the descent; for as
the velocity the descent is generated by the force
gravity, so is the velocity of the act destroyed by
the ame force.
E. Then the arrow was three seconds onl in fall-
ing; now the square of 3 is 9, which multiplied
16, for the numberof feet described in the Arst seco
is equal to 144 feet, the eight to which it rose.
F. Now, Charles, if I get you a bow which will
carry an arrow s high a to be fourteen conds
in it light, can you tell me the height to which it
ascends t
C. I can now answer you without beitatio ;-it
will be 7 seconds in falling, theequa of which is 49,
and this again multiplied by 1 will give 784 feet, or
rather more than 261 d, for the answer.
F. If yo will now consider the example which you
did the long way, yo will ee that the loe which I
hae given you m we very completely. In the rM
coud the body ell 16 feet, and in the eat 48, them
added together make 4, which the sqoar of the
aseods multiplied by 16. The am hols true of the
fnt seconds, for m the third secod it fell 80 feet,

which added to the 64, iv 144, equal to the square
of 3 multiplied b 16. Again the fourth ecod it
fell 112 feet, which addedto 144, give 26, equal to
the squae of 4 multiple by 16; and in the fift
second it fell 144 feet, which added to 286, give 400.
equal to the square of 5 multiplied by 10. Thus yo
will and the rule holds in all cases, that the sptee
dewribd by bodi f~ salfrey frm a ste of ,
increMs as the eqUAM of t timen icrow.
C. I think I hall not forget the rule. I will also
shew my cousin Henry how he may know the height
to which hb bow will cary.
F. The surest way of keeping what knowledge we
have obtained, is by commuicating it to our friends.
C. It is a very plant circumstance indeed, that
the giving away s the bet method of keeping, for I
am sure the being able to oblige one's friends s a
most delightful thing.
F. Your sentiments ae highly gratifying to me;
fain would I confirm them by ddin to sor tock of
knowledge. And, in reference to this ubje, it may
be necessary to guard you against the noc, that be-
came the ps described by falling bodies are a the
square of the times, the worcuie increase in the same
ratio. This is not the case. The velocity acquired
bya body falling freely, at the end of the frt second
of its motion, s such as, if it continued uniform,
would carry at over 32 feet in the next second.
And in all succeeding intervals the velocities are as
the timeW that is, at the end of 2, 3, 4, and 6
seconds, the velocities acquired will ba respectively,
twice, thrice, four times, and five times 32 feet; or,
64, 96, 128, and 10 feet.
E. Before we quit this pt of the subject, papa,
lot me try if I thoroughly comprehend you. A fall.
.ag body having been in motion 4 seconds, will have
descended 26 feet, and will then have a velocity of
128 feet; but the motion eill accelerates and causes
the body to pes over nine times 16, or 144 feet, in
the 6th second making i all 400 feet : it will the

have ase uirl velocity 6 tmm w 1O0 fees i
Second, which if it continued unite m r another
scoods, would carry the body over 0 fee, or just
itol the pace described by the body in the AMt 6
secods, during which sU motion wa equbly accele-
rated by raity.
F. Yo hae meet aceutaly caught the ditine.
tion I wished you to udertad.
With this we conclude our preat conveatin.

F. We an now going to tret npon the Cnami of
Graity, which is that point of a body in which its
whole weight is a it wer concentrated, and upon
which, if the body be freely suspended, it will rest
and in all other positions it will endeavour to deend
to the lowest place to which it can gt.
C. All bodies then, of whatever shape, have a cen
tre of graity
F. They ae and if you conceive a line draws
from the centre of gravity of a body toward the ce
tre of the earth, that line w called the liu f iirma,
along which every bod, not supported, edeavoumn
to l. If the i..dim ll within the be of
any body, it will stand; but if it does not fall within
the bess, the body will fall.
If I place the piece of wood on the edge
ofa table, and from a pin c at it centre of
gravity be hung a little weight d, the line
of direction ed falls within the bae,, and
therefore, though the wood loans, yet it '
stands secure. But if upon a another pa ce .
of wood b be placed, it s evident that the centre of
gnvit' of the whole will be now raised to r, at which
oint if a weight be bung, it will be bad that te
ine of direction falls out of the base, and threor
the body must fall.

E. I think I now e the reason a the advice
which you gave me, when we were going acr the
Thames i a boat.
F. I told you that ifever you wre overtaken by a
storm, or by a quall of wind, while you wre o the
water, never to let your feast so get the better of you
a to make you rise frm your seat, became, by s
doing, you would elevate the centre of gravity, and
thee, a i evident by the last experizmet, increase
the danger: whereas, if all the person in the vesel
were, at the moment of danger, instantly to slip from
their places on to the bottom, the risk would be ex-
ceedingly diminished, by bringing the centre of gra-
vity much lower within the vessel. The same princi-
is applicable to those who may be in danger of
eg overturned in any carriage whatever.
E. Surely then, papa, those stages which load their
top with a doea or more people, cannot be safe for
the paaegem .
They are very unsafe; but they would be more
so wre not the roads about the metropolis remarkably
even and good; and, in general, it is only within
twenty or thirty miles of London, or other great towns,
that the top of cariages are loded to excess.
C., I understand, then, that the nearer the centre
ofgravity is to the base of a body the irmer it will
F. Certainly; and hence you learn the reason why
conical bodies stand so sure on their bases, for, the
tops being small in comparison of the lower part., the
centre of gravity i thrown very low; and, i- the cone
be upright, or perpendicular, the line of direction falls
in the middle of the base, which is another fendames.
tel property of steadines in bodies. For the broader
the bese, and the near the line of direction is to the
middle of it. the more hrmly does a body stand but
if the lin of direction fall ear the edge te body is
eaily overthrown.
C. Is that the eman why a bal i' o easily rolled
along a brintal plane


P. It for u all spherical bodies the ba is bt
a pont, cospeqaently almost the smalest forest a f-
sact to remove the line of direction out of it. Hee
it is evident that heavy bodies situ-
ated on an inclined plane will, while
the line of direction falls within the
base, slide down upon the plane;
but they will roll when that line falls
without the base. The body a will
slide down the plane de, but the bo- Fig. 8.
dies b and a will roll down it.
E. I have een buildings lean very much oat of a
straight line; why do they not fabill
F. It doe not follow, because a building leans, that
the centre of rvity does nt fal within the base.
Theb i. high tower at Pi, a town in Italy, which
lan feen fee out of the perpendicular; stranger
trouble to pas by it; sll it is foand by epement
that the line of direction falls within the bse, Mid
therefore it will staid while i materials bold together.
A wall at Bridgenorth, in Shrophire, which have
een, stands in a similar situation, for so long as a
line ob, let fall from the centre of gravity c of tht
building* As, passn within the base CD, it will remain
irm, unless the materials with which it is built go to
C. It mut be of grat use, in many cases, to know
the method of finding the centre of gravity in different
kinds of bodies.
F. Thee are many easy rules for this with reect
to all manageable bodies: I will mention on, which
depend the oty which the ntrof gravity
kd ofalwa ndvouainto de nd to thelowes
If a bedy e fa ly pleaded o 6
a pin b, and plumb line be be hug by
the sau proill aw thre&MOO tille
Contr of grad i, r tat ent ,,
a the low itl, ti it Al, the ame 1F. 0.
ee O S VIete t tle wiae.

line u the plumb line. Mark the ine be; then hang
the body up by any other point, s d, with the plumb
hedf, which will also pas through the centre of
gravity, for the sae reason u before; and therefore,
a the centre of gravity somewhere in be, nd also
in some point of df, it must be in the point e where
those linr erem.

C. How do those people who have to load carts
and waggons with light good, as hay, wool, &e. know
where to find the centre of gravity 1
F. Perhaps the generality of them never heard of
such a principle; and it seems surprising that they
should nevertheless make up their loads with such ac-
curacy as to keep the line of direction in or near the
middle of the base.
E. I have sometime trembled to pas by the hop
waggonswhich we havemet on the Kent Road.
F. And without any impeachment of your courage,
for they are loaded to such an enormous height, that
they totter every inch of the road. It would indeed
be impossible for one of these to pass with tolerable
security along a road much inclined; the centre of
gravity being removed so high above the body of the
carriage, a small declination on one side or te other
would throw the line of direction out of the base.
E. When brother James flls about, is it because
he cannot keep the centre of gravity between his feet
F. That as the precise reason why any person,
whether old or young, fal,. And hence you learn
that a man stand much wiher with his feet a little
Than if they were quite close, for b p ting
be increases the bse. Hence also the d lty
ef sustaining a tall body, as a walking cane, upon a
narrow foundation.
E. How do rope and wire dancer, whom I have
seen at the Circs, mange to balance themselves

They generally bold a long pole, wit. weigbt
at each end, ears the rope on which they dance,
keeping their es Axed on some object parallel to the
rope, by which mean they know when their centre
of rvity declines to one e of the rope or the other,
and thu, by the help of the pole, they are enabled to
keep the centre of gravity over the base, narrow u it
is. It s not, however, ropedancer only who pay
attention to this principle, bt the moot common ac-
tions of the people in general are regulated by it.
C. Inwhat respects
F. We bend forward when we go p tairs, or rei
from our chair, for when we are sitting our centre of
gravity is on the seat, and the line of direction falls
beh our base; we therefore lean forward to bring
the line of direction towards our feet. For the same
reason a man carrying a burthen on his back leas
forward; and backward if he carries it on his breast.
If the load be placed on one shoulder he leans to the
other. If we ip or stmble with one oot, we natu-
rally extend the opposite arm, making the same us of
it as the rope-dancer does of his pole.
This property of the center of gravity always en-
deavouring to defend, will account for appearances,
which are sometimes exhibited to excite the surprise
of petators.
&. Whatarethose papal
F. One i, that of a double cone, appear to roll
up two inclined planes, forming an angle with each
other, for as it rolls it sinks between them, and by that
means the centre of gravity is actually deciding.
Let a body ef, consting of
two equal conee united at ir
":sbe placed upon the *
of two straight smooth ruler ..
sh and ad, which at one end
meet inan angle at ndrst
an a hoismntal plane, and at g. 11.
the other ae raised a little above the plane; the body
will roll towards the elevated end of the rulers, and

apato ascend; the pru of the eom that rest o
mu rulers growing mailer as they go oer a ,arger
opening, and thus letting it down, the centre of gra.
dit descends. But you must remember that the
eight of the plane must be less than the radius of
the bas of the cone.
C. I it upon this principle that a cylinder i made
to roll up hill
F. Y es it is, but this can be effected only to a mall
distance. If a cylinder of pasteboard, or very light
wood, ab, having it centre of gravity
at e, be placed on the inclined plane '
de, it will roll down the inc ined
plane, because a line of direction
m that centre lies out of the base. .
If I now fill the little hole a with a Fig. 1.
plug of lead, it will roll up the in.
lined plane, till the lead gets near the base, where
it will lie still: because the centre of gravity, by
means of the lead, is removed from e towards the plug
and therefore is descending, though the cylinder is
Before I put an end to this subject, I will shew
you another experiment, which without understanding
the principle of the centre of gravity cannot be ex-
plained. Upon thi stick a, which,'
ofitself would fall, because its centre
of gravity hangs over the table b, I e t
spend a bucket e, fixing another
tick d, one end'in notch between *
and e, and the other against the in- F. 13
ide of the pil at the bottom. Now ''".
you will see that the boe will, in this potion, be
pported, though fled r water. For the bucket
bng pushed a little out of the prpendicular, by the
stick d. the entire of gravity of the whole in brought
mder the table, ad coeqety ppoted by it.
The knowledge of the pi rpl the centre
gravity in bodice, will enable Oa to explain the
structure of a variety of to which ae put into the

heams of ehildim, such a the auk asper, frpe.
denr, tamebNer, e*.

O am a0w or P .IOM1 .
C. Are ou now ping, pip.s d theo me.
*iiM, hich you call mi m/ehi ml I
F. We mut, I believe, defer tht a day r t
o an ur I have a fnew moe genal Deipl wth
which I wish you piously to be mquied.
E. What an tdhe, papal
F. In the A place, you mut well underst
whet ae denomiMntedt the three ip a laws of mo
tion: the ist of whib is, ths rey hd wj am-
tinue ts ltst if (rut, or if Mufm = e, mail i
is resrped h-~ ier fio ek es its swse. This
eemtiMte wlt is dem.miated the imnrtis, or imae
ty of mtear. And it ml be observd tot, a all
cas, the qanthy of maoe ghned byr o body is
always equl to that l by s other boy.
C. The no diiculty 9of concvi tht a body,
u this ik-sud, i states rest mut always m ma
so, if o external frc be imprem d upou it to ieit
motio. But I know of no example whichwi l-d
me to uppese, that a body once put into motion
would of itself cotine so.
F. You will I think, pM ly d the latter
prt of the asertion as as the T er, although it
cmaot be esablished by expermeat.
E. Ihall be gad to bear how thi i.
F. You will o deny that bell which you strike
roim the trap bh no more Mw either destroy
its motion, or cae any change n its vloky, than t
has to cha i shap.
C. Ctaiy; everthel, in a few Moads aft
I have struck th ill with onl sy for*, it Mls to

YDo oe ind O in the thime that

ashen up before it comei to m r, emn upp g pe
blew same?
C. Ya, if I am playing on the gra, it rolls to a
eas distance than when 1 play on the smooth gravel.
F. You &d a like difference when you ane playing
at mrbles, if you play in the gravel court, or on the
een pavement in the arade.
C. The marbles run so eily on the smooth stone
in the arcade, that we can scarcely shoot with a force
mail enough.
E. And remember Charles and my cousin were
lat winter trying how far they could bsoot their
marbles along the ice in the canal; and they went a
prdigious distance, in companion of that which they
iuld have gone o the gravel, or ee on the pave-
weat in the arcade.
F. Now the instances property applied will coa
vaee you, that a body once put sto moati would
0 on forever, i it were not compelled by m e.
tenal force to change ts as.
C. I perceie what you a Cg toa ny:-it i the
msbbing or friction of the marb aSiut the ground
which do4e the busias, For on t pavement the
e hbwer obstades than on the gravel, nd fwer on
the fe tha on the pavement; and hence y would
lead as to colude, that ifll obtacles wer removed,
they might proceed on for eer. ut what a we to
say of the ball; what top that I
F. Besides rctio, them i another and still more
important circusnamnce to he taken it eaderatiom,
which afcts the ball, marbles, and every body i

C I aderstand you, that is te action of gravitio.
F. It ; for fro what we said when we co-
veed that object, it ppeaed that gravity has a
endecy to bring every bod in motion is the earth;
qumy, in afw emoonds, your ball mut come
to the round by that case alone; but bemad the at.
trctido of grvitatio there i the resistance whi
tbhe ir. th which the ball move make to is

aVws 09 MtbON. I
L That eamthe su, I thk.
Pr with regard t the bhll snrc fArs
ywr b t hm of me great eveMMedatim, be.
camethe elocit but m but inall great ves.
ciis, s- tht ofI hll ftm a mudat or mne, theme
will be amateril dikrnce btw ee th thw sid
rctice, if i be egleted in the ealculati.
mor wamma' ridirglwhip through the ak irlowly,
e oberve mothin o remind you that thtm u this
isin mediu; but Vi you wing it wit conider-
.ble swiba, the nas e whic it ecmuim will it-
rm yoe f th*e reitance it msew with faim some-
thing, wWhi ir thet~ h .
C. If Ino 1 ar d n y, the frc which com-
pdila body medom r a A lof i hreehi ; l.the
ttraction orfraviation ena f th alir;
-em &lM. mi e i m- with fhm ition.
P. Yuase IN right.
C. I bwe ao didalty ef conceing, dtt a
keiy in mion will aot osem to a se of et, till it
i brought to it by a external fre, acting upm it
emen way o ther. I have sm,- gndelmoa, w
sailing n VerT np ice, o a great wy without
ay eMetio bi f, bet whore the ice was reugh
he mod not go half the dLa without making
fbeh debrt.
f. I will memaA uaother abui C or two of thi
law of motion. Put a bim of water atoe our little
itrs wagg, uad when the watr pertly still
amw the wagO ad he water. wei" the miotio
of t"Ih tv l will at t rie up in thedne tio eao
nr to that i which thb veel mosm. I, when tbe
motn mflthe verel i commanicated to the wat oer

gllop, ing tlo t niml step en a ndde, ymM
wdll b l tobhem, the wur, in h~d~
Sa in or mod ri up o the
In No ommmer, K whale ye we sitting glaty an
PUTeres, the Miniml iam fhrwl, Yo wsill in
gof falling or backward; but af, whe you we
galopig a the animal stops a a uddhg
will be I to be thrown forward.

C. This I know er aplmc bt-I was s w en
d the neom l t it i to-day.
F. Ome thn Lnt, and net leon ium st, --
of ct* p dlos of ianI afM |y that they
may7 i aTuld to. and wt. = many of the
We new ees the IM an law do mtiomo whih
hi, dh atl Atmnp of otdi if pirprtimal to Me
feew imor, and i the dirinmts of ~ktfrw."
C. Thim s ,o dieiulty in tb; for ir while
cacbd l is =URy insw afIor If Hay has tro
it I ink it aain, a goe e with ime mi rae -
city, ad that i proprr toe the enlth which I
eert on the if. w; wke i, while is ruling,
I trike it bck Oagai, or giu a as dtilw, I chage
the diction of its co .
P. Inthe sam way, r. ny, d Lthe ei of
the tmosphre, change the dieit er a eaaeao h ll
from itcomes i a ouht line, and bing it to the
Pgrend; tMd the bll goe to afrtfh r r lwdiimei
a tpo oi t her quity of prowde u&
htrW bh 0 *miso tht, to Iwyatiem
eflms h spon maither, reI is paslul ad e.
tnry retim." I sike this table, I communoi
cat to it (wkick J Ieve by the shLking f the
glass) the motion of my hand: and the tA*e r
act, hand, jest mch e my hbad acts
If ye piIes with yowrw one seale ot a balc,
to kep it a sqmufifo wit I pWr wreiht io the
other cale, ye will percetve the pr- ed
by the r am apm it with a Ce equ to as
px wA which thek other @W% cdsmes to
dozens. I all cass, the rsadhy ar mat gi*a
by to bdy is always q t that e by the other
m the sae direction. Thi, if a blh in meti**
tA'e another at ns, the modio com eded to
* httarnwiU be takeM hr the Imer, and the Lo.
sity of the mer will be peeportionally adiminisd.

IAWSI 1oglwN. w
A hao dnrwig a heay load, is u much draw
back b th led a he drme i kfrmrd.
E. Idonotm capre~d bow the cat drw the
F. B3t*e progem oldi herh bipfJdebyiae
lked, whieh b th t ume for the (faee which
lh boe Mirti wdl eMay Om ae ee ditaUe
in tIe mse time, weehe ftmel Bl th emem
of the ead, am, the~dm, mu -as his *M
lb sbort do that diams, so ek i Ib 1i d(.
drawi back by the semi the headed est.
Apim. if yo ad yer bMet er wem a bemI, a
if. mUn f da n, ae ws e alemIpt to daw
Mthr to yw, tdhebeatL whieh you wr uold b
a muck poUd toward the empty kea that WM
be movd to yoa; id, tf wei thke two bea
*em al, y wd meut i a PheP way be.
tween as two.
If ~n rika gla belllh aa Ia him ,
tM blX rwib b eeivel by the hbmmer 4nd
gla ; ad it h iimei whether the -a s he
moved again the bole* at er, or hi boale be
moved agaula th hemnr .as a, yetthe bodle wil
be broke, though the hammr bek s id because
th gm bhlowwhich bi seciW ato i le
ot eufficieot t break Wr isju a lump Of ie.
rm this lw d me y a y lear in what
m ne a bi, b theeb d e wm, is ub
*npp the M tf- ldy.
F. IUr wi kLEh b ite bs th oir below
it i q to the weightof its body, them the ftode
of the a waprudw sliewe al toit; uad.thebd,
being aeedupsM by two SIUSIlM'a Seems-764
w ctior- I wa t-t Ifu the haf ae
atr i et thar is weight, the d ill d~i
witL thke fm ldim to fw es; end. if t
i eke bhe thua i weit. thea ik all k *wi
tih disJfrs.


C. Ae thee laws maoion which yea a aie
ym rday l at import e in an ~se bqapy
F. Ye, tkqry a aud sheld b coimot.
ed to memry. They wen framed by hir I. New-
ta tIe fundmnta plincwplr of mehacbi, and
yea will Lad tem at n hd f meat bmio wrn
ten theme subjects. Fre these aimo we ane ma.
rally led to som other breach of seace, which.
= we can but lightly motion, should not be
w egleted. They are, in eat, but corollri
toth la o motion.
-. What is a corollary, papat
F. It. omthiibng m tian ser tt clearly d4-
decible frm some other truth bean demomrated or
admitted. Thusby thefr olaw od moide a h
must mansr- to "ins In the asrt i"no hiek it i
put, wher it he V" or mayl"m motion insa gtm0g
in: mr e which it follow, a cormllary, that w
we s a body more in a cam lin, it mu be acted
pa by at at two fane.
C. Wen I whirl a st romnd in a sig, what
arethen two for whicact potheone .
F. Thme is the force by which, if yeou t go th
ting, the n will fy of i right ine; and thee
the orc of te hand, which eep itin a cilr
E. Are there any of them circular moi i
F. The moon ad al the plant move by thi
law --t take the moon as a instance. It has
m tuant endency to the umit, by to attraction of
pm aio had at aM t atdecy to pFeMed in a
Si o ie by ti t po tle force impressed pon it
Sthe Creater, in the meaer a the stoM em
fam your hand; now, by the joint action of tbhe
two fore it describes a rcular motion.

E. And whet would be the ceamquae, wpp
the pfMDJ reh e to eCr ?
F. Th mon mt fhll to the mth ; ad if the
fare of grvit we to ea sea r ul a thn moom,
it4 woemldlM oit l4ta e Nw
fae, when applied tothbe'0tM,16isis = 82
trif g o h having c Mdre nmeed or f
from the catre; srMd d oer fdm b rad thi
m ripral Ifor, fr its fIa decy poit a-
C. And all this is ina eequene d the iditky
ofmtter, by wbieh bodim hlu a d cy to om-
tinve in the mMe sate tey na in, whMlA oflt or
F. Y ae right, aad this prie pl, which Er
Irae Ncewtisemod to be in a l Mdi, he called
their b inarli, which buees bek d t before.
C. A fir aoru ag yef sMwed a tht th
ttractof t ar p tbhe meosss 3W0 time
I. tha it s pM hev bedi nmer th death's sta
face. Now ah tb ttno is meaed by the space
fallen through in a gives tme, I hae endeavored to
calulate the pae which te moou wold fal through
in a minute, wre the projeti fore to came.
F. Wall, and brw hav n brouh it out.
C.A body Alls h s fIeeot he snt r m med,
cooe qu~a la ininut, or 0 sroad, it weld hfl
Oti8 m 0-fblet, a dpi d by 16, tht is 8000 feet
which is ko b ltipd by 1; ad as th moon
would fall thour si600 ti = ei ai a give
teal thau b beJ, it wed frimly 16 fi in
As fiMt emate.
F. Yor calculation i accurate. I w rmal to
your mind the seem law, by which it appMe, th
mry eatism, a shgisf muSh., poediesd Sm ated.,
met be pampra tibl t, n.d in th diruminq. d,
fAMarir se. Thmefoe, if- ng body Aces
a iaplMhi te dimetiao of i motn, ie vloea
*Se Oeumvanetia IV.

will be inaese in; the ontrriy detion, 6
* will be diminihed t-but i the fino be m.
im a dirstiM mm bliqu to tt in which it owes,
tn' direction will be bi v that of its fhora
modi, and tha of the am fees imesMd.
C. Tis I haw ho. te bmelmvesi aI ha mad
with m a richtall.
F. By this mcad la r mof do, yes will Mly
adustd, that if a body at r sesm to im-
plmis at the urme time, fom force who dinetions
Sto e i a lie taw i m IbtwnoM ot dincio of
E. Hm T ys a y m u to prove ti M e-
teely to thirsss
F.o Tom a may msuc irnv d by diorat p
sme, dMsioI of which ye wil he n AW i
riu bse the bjes. Bi it is eily an-
desMtod by a fals. If aom iobh l sa
fare be passMd smsa to ae it m --
move with a uaiora eloity to the
point b, in a cod of tima; aid if a --
other bfor be also imp ed a ti bll, Fig. 14.
which alo would make it m to the
point in the sa time t aill by amo of hI
two brcs will dscrilb t line ed, wi is a diago-
M l of tihe whme si, anr o sad a .
C. HOw t is motion uoada"d in the Jireti
4I"ftvi Ascrdin to th Mmed law, it ouht to
a in oe CM i n di uineni as, and in tin 4b
in tlht f o,, where it i that of d.
F. Examine the dfiu a tl6tl atteninely, can
th in yar mind, that for a body to mov ia t
.Me= dinrtion, it is *cw that it should move
Sin th s. lgtlieb; es tt it is sucin t to
mo er inthat lint a aaa or i a pratll to it.
C. I pinve thm that t ll whm arrived at d,
has n e in the d ieedi a hs, b i id is pallel
to a; and also in the direction *, bea ed is
parallel to it.

F. &ad is aes mhe peeb ishamis bedt ide
point bed caMth t be confrmable te dh
awmd IMW dOf m Wh ba mli rm
auve, it Mut be Iemt in id dthe mm b
meuUed adsi d mam el Geeo i Ieb i Nw ASe
seabo w a mn at my pu m tho ihdy w Md
cotimiu be mios a dm i ala ie.

F. If ye e sealkst*o a t wht w d
day am the emosId lan d motim, iye r M .
deas t M' eoew llul (F. d4t)
1. That( Vhe tmo to bwe Ma i t at s igYt
saa am aeter, l Hdesib ed by ,the-hi
il be t diogml l of But Btm all oer
eom, itd ll be th dum a pIm uMobraq
soma inad.
a By vauylg th anml the Ires, ye ermy
tre frmn of your lr
C. Ye, pam, and em another eaMmeme, vi.
that the motism of two (fCes aig com)ointly i th
ay, ar m ot a geat as wh they set seaitely.
F. That is t, ad y e n ed the eluie,
I srppe, aem the Mo0lIM, tht L eey tiagl
anay two AMi takeM taLr ef peter them td W
mainsgide; sd then yo er, ad justly o.
that the motir which the ball mot hae reied,
had the forces been applied s dyl, would have
been equal to as and a*, er, whie h thing,
to and ed, the tw sides the tisale ad, but by
their jomt gimn, the mioa is oely equal to d, the
emn i side ofd t triage.
Hme the ye wil nomember, that i the esr M .
Aime, or aIding agpher f fmss (as thim n aled).
motion is always tet a"d i the rsihioms of Fmy

a M11OhNIM.
one fwr, m a, into two stim, a mad A, madL
C. Well, pp, but bow it it that te heavmly
bodim, the moon far intce, which is lld Vy
two fore. perfonM ber motion in a rcuw e
raod Utm death, uan ot a in a di-enaml hetens be
direction ofthe prjectile fao, mdtit t of the ttrac-
tion of graity to tM eat 1
F. Bc e, in the ce jt mentioned, there was
but the action of a single uls n each direction,
whereas the action of g ity on the oon is con-
ti ul, and cameu acle d moide, -md hba
the line a a curv.
C. DaI ing than, tht a prem nt (l mhe and
a the aute fet throh which it wold fall in a
mimue by the ttrnetion of towy m ds th* earth,
and ab lrer thle ri rm pon it for
the iw wme. If Ob adsacted asu l npleP.
the mooM would u that am describe the diagonal
edt but sooe thes forom ar constandy Seag, and
that f gvity is au accelebng force ao, therefore
listed of thelstaigt lie ad, the mom will be drawn
into the cure line u. Do I uderstand the moter
F. Yea do; Mnd hnme s eIT comnprend
how, by god intruma s a lnul the attre-
tim o th eath apn the mc n wa discovered.
The hird hw of motion, va. m tt aen maJ r
etim are saf a dl in mery diretitns, may be
illtrad by the motion communicate by t pr-
cmesi of lii and s-AiAstis bodiM.
E. What are thek, papat
F. ElAei bodies are those which hae a certain
in, by which their parts upon being pmead in-
wards, by premon, rtrn to thirfomer sta; this
property is evident in a bll of wool or cotte, or i
t compmend. Nn-wsest bodim are thom
"w h w-m O arik other, do Met reabned, bul
move togher after the Ulak.

l&rw OrP Mlo. -l
et two equl ery bab a ad bbe s-
-pdd by timd; a be draws a tl
eat ei the p ea let p 4
aill bta mtdint bwy
e hih will b driMv to a ditame e
emsal to that thdkg whekh a l; al
hiooa kt appears tt thel e. m of bu w s
equal to te tiao of. i.
E. Bt dorth pr orthd ivery lnbW U by th
iaob, or, M yo call h, by ptrmiM
F. Thleyo f. kri laylk~i t om i an lHt
k t- bk, it make bta nmall spec ap ik;
wut if itjW sp*a the wWI Wfl 1muh Il ;r,
whle prowe tct anM bauk em e, ad at a
lihl hollow, or diat, was made ah by coelli
sim. If ow to equal ft bll of clay,or li
putt, which am oaumdebi, amt e other wkth
rqual vdodti, they womhM op ad ae L toehr at
th plce of their a their cio
detros ech other.
C. have sometime ht my while ally ageial
other mubl o plnply, that the marble IM
at as switly tu t ak l oy mMached a tLht r.
fmaaid in dt place thy rUhleo. An arub
thro, Ma wd inwy, lei v
F. They u.-lf thir el-lie ball ,
, *, be hang from adjoiningcentre,ad I
a be drawnlik ot d te prpWMdi -
lIr, and let fll po a them will a d /1
Sbecome maioary, aad will be drive L i
to d, the distea tluroh which afa l -)
upon 6. Fig. Il
If ym hag ay u m mbr of balls, a
ax, eiht, &e. o as to to ch ech other, mad if y
draw the ouMid oe away to a tlt diuac and
eha lt is fall n tho i t thi bdll u th pea
ili will drive erf, wbile the* P r
aiory, equally i th cdi and rmiwe of
the Matioary bill d-ivile them. 1a th
me m maner if two am drawsm ide mad sa

to fdll olo h m, th eoposiom two will y of, an
the cthe ama oatonauy.
There i e ther de cuu o*c d iqasdng uo
t4 satin ud rnctie of bodia atd also apo
the Ih imnrds of atr, weoth &odic*g a by soM
umnes yiu will l d im l ~ra .
If I itrik a bi wlt kah' i kmW,.
li. and a tion baW d n il strib e the
lauer a fa hcibly th ew sikt the avl.
If tahe vbile larc maun, I might ky it as my
beast, cod safe r yeto op iok itw* a hemmer
aim all your tamgth ritho ai orI for the
to i6Wri of the anl noI tfore of the bliw.
Bt i tLhe enil wer beta pad or o t a wigh t
ye blow would probably i sm.


C. Will you now, pa., emplai the mchaacal
F. I will, "d I hw7 he ot lspotea wrhrt
theM uumeu ofi a body u.
C. No, it is th fe ofa moving bhdy, which
force is to be iated by the weight, multplied into
its velocity.
F. Then a smil ubodyy mLs am equal m me-
tum with one much klrr t
C. Yes, provided the smaller body moe muck
nwifr than the larr one, as dA weight of the letter
is gmmr than that of tho former.
F. Whbat do you mam when pyo y that m
body mo m wter, or hs a grmter veloity, thb
C. Thtit k over a pers pa i the nme
ti. Yaour wach will la b meaWi: a the
miust-hu d t rel rmad th dmilate am m
hour, but the hour-hd takes twelv hrn to per
it Coue in, cosequly, the velocity of -the Uam

nunamu tal Um. A
lhed a tldWi tdi g reatr thnm ILth of dthe hbLr
hand; beem., te ma t timn, vl. twelv kheoa
by the Mkr-had.
F. Blt lm ena bw Oely tU .ea uthe mia
that th two doa alm eMual. I myr i, the
lnew-hud I blor th t t her, aLtka aea
aatly, ti eel dr to i d b f bher rtha thtr
deab yd by th k-tnd.
C. I t am once that aTm Ir a o l gthe
It th e rwh t te has are q].
F. Thm ir, bmowr, pIuNhr t th
Iame haMn, of wheh it fmayd *=ilwkh te sriedatm
tru that it has excdy twelve tm the Toelo of
the trmity ofathor isocr.
C. TIat the int at whck, if the reader
were cut of, the two bamw wdM blemul. And,
in fact mwy mIt pmlat f the habd dbmri
difilhrt mhin thme in t.
F. Te lie t hot am wrrh the tewo hudas
to move (frw y am nalIy mee yT dient
vro, one within another) may b thee asmw
md eM, whk is a Ied pdiat;: letkh glMr the
isl the g sr is th opMo decribd.
C. The eteitr off T d a wd iM l,
W*he tiwy ie v ry bet, ar ete dlmia.
guiihabl, t sf Mieate pUrt, newrit mill,
are sdiay e d; tI M is oWa to the vlift of
ther ezimaiti bming o ch greater thuhite of
the other partl
E. Did not the wl of tdeobalmoboi whi
we aw a the air, depen m the -m pMHlie* vib.
thelnth ofiDthe polr ap ith thesMwa ised
f. YO; t rert Ihe d i e atI rwiMb th6
seats wt pwd1 frheo the catm of mats, the
greawtr dte spUte h the it byi uld girt
trnveltdltikrtit rh y.
L Then dtoe is the Msnoad rew. hb a hd
ride fir tb' meir y thi thes a tth end of th

5 MaoRAIW .
F. Ys, hter as to pce, but the am as I
Ia. In the me way, whea you and Chad go
juad the vel-walk fr half am hour's exercise i
e run while you walk, e will, perhaps, have goe
si r eight times round, i the sm ie that you
hasv been but this or fau times; now, a to tm,
your enrei has been equal, bat he may Ihve pm
ver double the nae i te suie time.
C. How do th apply to the explnation of the
moclhaical pow
F. You ill find the application very easy:-
withbot clear ides of what ia meant by tim and
qea, it were in vain to expect you to comprehnd
jw6inociples of mechanics.
n am siz mechanical power. TIh lever;
the wheel ad ale; the pulley; the inclined plae;
th wedge; and the crew.
E. Why a tb allied mechaaal power
F. Becaume, by other wm e reenabled ms.
eAssiasrl to raise weh, mve heavy bodies, and
overc- e r, l 'n which, without their siutce,
could not be does.
C. But i tUre no limit to the a tce gained by
these power? for I remember read of Archime-
do, who aid, that with a place for hs lcrm he
would mve the earth itself.
F. Haman power, with all the smitance which
art ean gire very oon limited, and upon this p-
c s e.t gsin a. pMsr, aes ar g diu. That
i your own united strength, you are ble to
mir y poMud to a certain distance in one minute,
adifb tb h help ofmachiery, ou wish to rai five
h d pouds to the ame height, you will require
ten miu-ito pformit in; thus youi mrme your
power tenfold, bu it is at the upese of time. Or
ia other words, yo are enabled to do that with one
effort in tm minute, which yu could have dow in
tn sprat efrt in the sam tine.
T impaUpera n of me mim, the, is an a
ery coiderable as e, a t bt iht would n.

UimanUm a1 -S -My

a 1M froa t-e are inU able If there an
F. be me y s L t inlesm 4

arsenal amil weigh, magbl by umsan tr th,
to be raid st aL ima ih it y be fell as on.
taniest to elevate them oe by one, a to take the ad-
vantage of the mechanical power, in ing them all
at once. Because, a we he owa, the lame te
will be neceary in both cases. But suppose you
have a large block of te of a too wigt to cany
awa or a weight gra, ht is tobe doew
I dil not thiak of tht.
F. Bodies of thi kind cannot be sepaated ino
pats o~io mble to the human wtreth
mense t abo or, perhaps, without rdei tea m
unit or those purpose for which they a to e ap
plied. Hence t"hyoupeeie the gmt F np we
of the mehnical powrs, by te m of a man
s able with ea to manage a weight many asne
greater than himself.
C, I have, indeed, seen a few men, by means of
pulleys, and eeminly with no very great aeron,
ras an enormous into a timbercarriag, in ader
to covey it to the dock-ad.
F. A very ezcelent antace; for if the tre mad
'oen cut into suc piece as could have been ma-
aged by the natural regth of then men, it wmld
not have beea worth carrying to Dsptfo or Chat-
ham for the purpose of hp-bilding .
E. I akewldge my eror .-what i a falcrs,
I. It is Jasd pei, or prop, round which to
other pa~t ia sahi moe.
C. The pir an which the hands of d r w h
move, is a fleme the
F. It is. ad yea member we called it als the
-ns of mai- y riP of thdt isuar s al
alm ad also te center of miom.


E I tat a t aed point. rpp
F. Certaily its a ed Oin 0a it de thea
tw parts f Jia Ksciesrs; for th always sa
in t puirio, wbile the aher pars mew
aba eit. T these stdMirte ; mew t
rt df lme bar O wrdI dm PdMr iAm s a BknM
-fr tl pa r mo v wes it M sMm.

or "as LSTMv.
F. We will now comidr the Zrer, whibh s
n ly called the fAt mecaical power.
Tbe r.w b Iuany m sf bl3 bu of d, im e.&c.
wHcH or to rUa weigh, while it i sppeted a
a point by a prop or flerem, e wic, the actm
Smotdio, a toh other put trnm. o
wil repM aleverad thepowt .
uriAlcrmaorcenBtnolfmin Now m ij
k i evident, if the ler tur n on its
cenm of motion e, so that b comes
nothepoitio d a; att he sam tie i. .
m t come to the position e. If both the arm of
dt ver be equal, that is, ife i qual to k, thb
s o advantage gained by f r tdy p a ove equl
speamin the sae time; d, aoording to the .n-
dameantl principle already lid down, (p. 46,) uas
advantage or power a g md, t amt be lost:"
therefore, Do tie being lo t byalw of this kind,
iere can be no pwer gnd.
C. Why then it called m kebil power
F. Strictly speakig, perhaps, it ouht not to he
ambsed u ae. But it usually fesnad mog
them haei the tleiam b -twem e we d the
of the It kind. And wCe It rumhe is exactly
tlhe mil poa hetws the wI *Iad pwer it m
the arsames balance: to wack, i b l e sa

piaded at a ad lb. ik lied for weighing all sorm of
a mediies.
E. You ay t is a verof t e kind; ae the
reveal ert of levers I
F. Then are three sort; me peno rech
fear the fourth, however, is but a beaded ea oi the
fnt kind. A lever of tbe.rt kind l the fulrum
between the weit and power.

Fig. 18. Fig. 19.
The eend kind of liver has the fulceum at one
aed, the power at the other, a the weight between

f&----, .h-4
)g. 20. Fig. 21.
In the third kind, the power i between the ful.
crum and the weight.
Let us take the lever of the fit kind, (Fig. 18.)
which if it be moved into the position ed, by turning
on its folcrum it is evident that while buh tra-
velled over the short space O, ha travelled over
the greater space d, which pae ar to one another
exacla proportismto to the of the ma aand
f, now, ye apply y hand fire to the point
a, and afterwards to n order to move the ever
into the peitio ed, using the me velocity in both
ceam~ you will fnd, that the time spent in moving the
lever when the hand i at b, will be as much greater,
as that spent when the hand is at au the arm h is
leger than the arm as; but then the exetien re-
quied will, in the m proportion, be l at u than
at .
C. The am he appears to be for times the length
of s.

F. Then it a lever which gais peer il the
propor ,c of fout to one. That is, a sngle pond
eight appd to the end of the arm i, u at p, will
luace four pounds suspended at a, a w.
C. I have mn workmen move luge piee of tim-
ber to vey sll distance, by mans a long bar
of wood or iron; is that a lever
F. It is; they force one end of the bar under the
timber, and then place a block of wood, stone, &c.
beneath, and as near the ame end of the lever as
possible, for a fulcrum, applying their own strength
to the other: and power gained in proportion as
the distance from the fulcrum to the part where the
men apply their stgth, i greater than the distance
from the flcrum tothat end under the timber. Hand-
spikes are lever of this kind, and by these the hea-
viest cannon ar moved, as well a other heavy
C. It mut be very considerable, for I have see
two or three men move a tree in this way, of several
tons' weight, I should think.
F. That i not difficult; for suppoin a lever to
gain the advantage of twenty to one, and a man by
his natural sbength i able to move but a hundred
weight, he will fnd that by a lever ofth sort e can
move twenty hundred weight, or a ton; but, for
single eertion, a strong man can put forth a much
greater power than that which is sLfient to remove
a hundred weight; and lever re al frequently
used, the advantage gained by which s ill morn
considerable than twenty to one.
C. I think you id, the other day, that the coms
mon selyard made use of by the butcher is a. lever
F. I did; the short arm so (Fig. 19.) i, by an
increase in isn, made to balance the longer o b,
and from the cenre of motion, the divihsi mst
commence. Now if b be divided into as many pars
u it will contain, each equal to ac, a single weight,
u a pound, p, will serve for weighing any thing as
heavy as itself, or as many times heavier u there are

01 TUB L lWL. I
dieos thle m If the wight p played at
the diWm. I in the rm A it wll blauce me
pod in the aels aet ; i it be sieve to 3. or
, I wil balaee or 7 p o e scale; for
them divom being mrpetivdly 3, 5. r 7 time d
dotnes from the Oel of motis e, tht is, it bo.
e" a lver, which aim "d stM, i tido points,
in the popeaaf 3, 6, nd 7. now, the iair.
al betweea the dhvismo a the lage arm k sRb.
dived into halve, quares, &e. say weight may b
acerately ascertied, to halves, qu rte of pemds,
or yas LuYZn.
E. What advantage has the slyt i, which you
desibed in ou last omvertioa, ovra par
F. It may be b sms nadily removed km
place to place; it require o as peee, ad only a
ingle weight for ll the purpms toi which it ea be
applied.- ometiom the are not o equal
weight. In that cUe th wil p must be moved
long tbe srm b6, till it btly mlue the other
anr without a weight, and in that point a notch mat
be ade, markig over it a cyper 0, be whence
the divisions mus comments.
C. b. three aot requitd spet arean y i Io me-
afactere of ih mets of ths kind
F. Ye; o such imported i it to the pbei
that thm, should be as ermr or bald y mans of
as weights, or alse balances, that it is the rmis
of certain public oiousa to mamie at aed seasons
the wgigfts, meaU Le, Ie. ef ery i is
the lud. Y1 it is t be fond tia r, 1 p
caution, ieeh fied is palised pea & -wa
o.-o le day sumer be ht, as I ,ppoeed
a pound of cherries at the door; bt Charle thik.

g there wa not a pound, we tried them in ye
sales, and found but twelve ounces, or three quar
ter, instead of a pound, sad yet the sale went dow
, if the man had given me fll weight. How was
tat managed
F. It migt be done man ways: by short
weights ,-w by the scale i which the fruit was put
ben heavier than the other;-but frud may be
prctied with good weight and even cals, by
makin the ar of the bance on which the weights
hang aborter than the other, for then a pound weight
will be balanced by a much lees fruit than a pound
as that am is shorter than the other; this wa pro.
bebly the method by which you were cheated.
E. By what method could I have discovered this
F. The scales when empty a exactly balanced,
but when loaded, toh till in equilibrio, the weihts
a unequal, and the deceit is instantly dicovred b
changig the weights to the contrary cal. I wi
ie you a rule to find the true weight of any body
by uch a alse balance; the reason of the rule you
will understand hereafter: find the sightu f h
body by Ath seeks, si*iply theus together, the
Ado the squan e t of the product, whio the t rhe w
Let me ee if I undetand the rule :-uppse
a body weigh 16 once in one sale, and in the other
12 ounce an a quarter, I multiply 16 by 12 and a
quarter, and I get the product 196, the square oot
of which is 14; for 1 multiplied into itself gives
196; therefore the tree weight of the body a 14
F. Tht isjut what I meant.-To the lever of the
rst kind may be rSnaed many emmao instruments,
uech as simes, ineers, i maeu &e. which an
made of two lever, acting contrary to e another.
E. The rivet is the fulcrum, or centre of motion,
the hand the power eed, and whatever to be cut i
the ristance to be overcome.

C. A poker dfu ing the io in alo a leve, for th
ar is the fulcrum, the hand the power, ad the ceal
the resistance to be overcome.
F. We now proceed to leves of the ecod kind,
ua which the flcrm a (Fi. 20.) is at oe end, the
power p applied at the and the weight to
be ried at ,, somewhere between the fulrau and
C. And how is the advantage gained to be m-
mated in this lever
F. By looking at the Ig you will d that
power or dvntage gained in proportion the di-
ance of the pr atr than the distance of
the weight from the fom m.
C. TheM if the weight hbug at one inch AmI the
fulernm, and te power acts at Ave inch from it,
the power gained i Ave to one, or pound ap
will balance 6ive at
f. It will; for you perceive that the power
over Ave time au pgeat a spee the it, or
while the point in the lever moves over ea inch,
the point I will move over five inches.
. What things in common e are to be a ferred
to the lever of the second kind
F. The most common and useful of all thinp;
every door, for instance, which turns n hinges s a
lever of this ort. The hinges may be conidered as
the fulcrum, or center of motion, the whole door i
the weight to be moved, and the power is applied to
that "ide on which the lock is usually fisd.
E. Now I ee the meam why them is considerable
diculty i pushing open a heavy door, if the head is
applied to the part net tdhe i althoul it my
be epee" with the groom me a the Mi d etd.
C. This me, with ier up= it, nepraeti a ver
of the second kind.
F. Certainly; it while ie is iling it, i the
middle, you rn e oe end, while the ether remai
ixd aI a prop r fulcrum. To thi kind of lwer
ma be also reduced not-.acken; eanr; dde of

ships thma ouatin g aim which hIoe se ed 8sed
ia blo, meh s are ued fr outing cha, drug,
wood for pattens, &c.
A. I do at se how am and nedde e lovers of
this sMt.
F. The bet ib weight toe beo d, the wuer
i the falen., and toh w arama at the he.dle the
power. The mast of ships are als ever of the
NsOed kind, fr the bottom of the vee the ful-
crum, the ship the weigt, and the wid acting
against the ail s the w power.
The knowledge of this pciple ma be efal in
many siuatism and circumstae of lif:--f two
men unequal in strength hae a heav barde to
carry e a pole between them, the ability of each
may be ceoalted, by placing the brdea as mach
eaer to the stronger ass, us ais t th i gat
than that of his partner.
Which would you call the p in this ae t
F. The stronger ma, for the wght i nearet to
him; ad the the weaker must be ooaiderd a the
power. Again, two boes may be s yoed to a
eari that each shal draw a pat opoil to
his strength, by dividing the beam in an a maaer
that the point of t ein, or drawing, may be as
aeek meane to the staer hone than to theweaer,
as the seogth of the rmer exceeds that of the
We will now describe the tdi kind d lev. Ia
this the pr or flerum e (Fig. S1.) i at one ead,
the iht at the other, d the peer is applied
at 6, somewhere between the rop aad- t.
C. In this the weight bo iae fme the
Oae# aM io than the power, mut pm thIMgh
mee mae thJn it.
F. And what is the coequence of dt t
C. That w powmr t be gatru thb the ht,
and as meah greater m the ditane of the weiht
from the prop eareeds the ditaee of the power fto
t, that is, to balance a weight of three pounds at ,

tere will require the exertion of a power p, acting at
t, equal to ive pounds.
F. Since, them, a lever of thi kind i a diadvan-
ta to the moving power, it is but seldom ead, and
ony i case of necesity; such uin that of a lad-
der, which b ei ed at one f against a wall, or
otser obstacle, U by the ategof a man's am
raised into perpendiculr suatio. But Die mat
important application of ththird kind of lever, is
manft is u stuctare of the limb of aimals, par-
ticularly in the of man; to take the arm an m-
tauce: whea we lift weight by the hand, i s
defied by meam, of musces coming from the
shoulder-blade, and terminating about one-toeth a
far below tbe elbow a the had is: now the elbow
being the centre of motion round which the lower
eat of the am turns, according to the principle jut
laid down, the muscles muitexert a force ten times
as great as the weight that is raised. At first view
this may appear a disadvantage, but what is lost in
power : gained in velocity, and thus the human
re i better adapted to the various function it has
to perfom.

F. Well, Emma, do you understand the principle
of the lever, which we discussed so mch at large
YL lele gains advantage in proportion to the
pace p d thru b the acting power; that is, if
te to be rmed be at the distance of one inch
from fnlro m ad the power is plind e
inches distant from it, then it is a lever gi
advantage nne to one, ecamse the ope pased
throu~ge y the power is nine times greater than that
paend through by the weight; and, therefeot, what

Slost in time, by passing through a greater space, i
gained in power.
F. You recollect also what the different had& o
.ever are, I hope
E. 1 shall neveUee the ire stirred without think.
g of a simple lseWr of the first kind; my scimu
will frequently remind me of a combination of two
lever of the same sort. The opening and shutting
of the door will prevent me from forgetting the na-
ture of the lever of the second kind; d I am sure
that I shall never see a workman 'raie a ladder
against a house without recollecting the third sort of
lever. Beid, I believe a par of tongs is a lever of
this kind
F. You are right; for the fulcrum i at the joint,
and the power i applied between that and the parts
used in taking up coals, &c.-Can you, Chares, tell
us how the principle of momentum applied to tei
lever 1
C. The momentum of a body is estimated by its
weight multiplied into its velocity; and the velocity
must be calculated by the pace passed through in a
given time. Now, if I examine the lever (Fig. 18.
20.) and consider itu an infexible bar tunin on a
centre of motion, i is evident that the same tame
used for the motion both of the weight and the power,
but the paces passed over a very different; that
which the power pase through being as much
greater than that pased by the weiht, a the length
of the distance of the power from the prop i great
than the dance of the weight from tbe prop; and
the velocities being a the spaces pased in the same
time, must be greater in the same proportion. Con.
sequently, the velocity of p, the power, multiplied
nto its weight, will be equal to the mailer velocity
of multiplied into its weight, and thus their mo-
ments being equal, they will lance one another.
F. This applied to the first and second kind of
lever; what do you may to the third?
C. In the third. lsh velocity u. power p, (Fig.

11.) Iidg lIe than t o the weight w i is e.
dest, i wan that their mometa may e equal, that
to weight acting at p ms be as michL g r than
that of w uas e ie I than t and than t will b
in equilihem.
F 1eT med meshim l plqr i the WAWl e
Aisu, which gm power popoLrtion the r c rl o-
fenm~ of the wheel i rMat t thaLt of the as
this mchim may be mfemed to
the primipl e of th l ye
is th wheel, ed its si and if
th eircumferece of the w el --
be eight ~timm eap t a that
ofthe azis. thea n mg p*d,
p, will balance a waght, ef Ig .

it b an instrument of thi kind that water i
drwn from those deep walls m comon in muny
pons of the country
F. It i; but a in mst cases d th" kid a
sile bucket i mind at oeac, td n irsbut
little po n t ah opertio, and thenfe, imad
of a l wheel, as an inon badle ased at is
mde ie of, which, you know, by i cireolur me-
on, awer the purpo t a wheel.
C. I eace ame watir br a meaine of thi
lad, and I &mad that a, tho b scea ded ma User
the op the diAlculty inceased.
F. That mat always tbe cas. wlme the walls
re so deep m to ceo e, in the aicet, te op to aeil .
moe than oe the le gth of the ais, become the
advatage ained is in popertion s t eircumfe-
ene of the wheel i geer than that of the aiB;
so that if the e falmes of the wheel be I time
n mthuan t of the L, me pond a atplied the
a will balmnc mtwelv heagig t tihe im it
by the oiingi tf O mnidthe aM, the d*Mr-
cw between t Xi-e a of tbe wheel ed that
of the aiuk eotimauUy dminises; conequeetly the
Iulya mIe gamed is m vary time a ew d i


rep i wound on the wol leath of the axia: this
explais why the u dicutyo drawing the wuer, or
any other weight, incMae it Mends neaer the
C. Then by diminishing tbe U or by i ng
the l th o d headle, odv(a e is gaed
F. Yew, by eiher of themo Ieod you may gip
i but t i very evideat t the as cernst be
niihed beyond a certain limit, withol mdering
it too weak to sustain the weight; anr can t headle
be managed, if it be coustrucud on a se al meh
la r tha what is commonly urd.
C. We must, then, have course tohe wheel
with spikes standing oat d it, at certain ditancee
from each other, to erve as levels.
F. You may by this means increase yr power
aceedina to your wish, bu t must be at the espeme
of time, far ym know that a simple handle maybe
turned several times, while you ase puatl the wheel
resd mce.-To the prinscple tf te uMad ot
may be rsafeed the capstan, widh1a, ad all tbose
numerous kinds of cranes, which are to be een at
the dier At wherL on the backs of the Thames.
C. I have *een a erame, which ca t of a wheel
large enouh for a an to walk in.
In ti the weight of the man, or en (or
there am meties two or these is the mray
power; for, a the man sep f wme, he pat upo
which he des hbecer the hea& i, and coee-
qet desenids tall is br e e low Ob the same
prmeipe. y may ee at the door Oft =MY id.cage
mase, a biKd, by its w M gin a wker eae a
idreduar otion; now, ift were ae ll weight
sespdod to the axis of the ca the bid by its o.
tie would daw it up, fee, as ai p feem the bottom
lar to the Maet, its uea meu tho descead;
and th tihe adm is pOp*med, both with regard
to the cage, and to those large mram which you
Laer i t.
E. Is there so danger if the man happens to lipl 1

F. If the weht be wy re i lip with the
may be ateded with very dau eoM o. equMncs.
To pIoent which, them is gsl Smfly d at one nd
of te as a little wheel,, (F. 22.) called a
racketwhel, with a seth, t, tal into it tseth
this will, at any tim, apport the weight in casr o
W acc leut. S8omet d i of s waiog
wit the -eat w el. e m t md is o tbe
ouaide, ad a small ta=l to work in
the co, and to be tred by a winh.
C. re these meat tite (o crams, i which all
danaeris avoid t
F. The eras is a mahi ie d mh ipute t
the commerce ina of dhis eay, t ew i
ventions of it are cetinulty et a th p ie:
I wi, when we to th Mhrw =e in 1e
10th ol. of the TI;aMdm thie s ocy ir ho
Emewageest of Mas ad BSeleses, an emgavh
of a s, e d, 1 beism t emselt cems. It
s iested by a fim f mf M. r.Ja e Wi e,
who poassessed a ma adefimary pm. fhr m
C. Yo id that this mecaical power migkt %a
considered as a lever of the nt kind.
F. did; if yecn eiv the wheel and axi
to W ct sbhMI he middle in te- di-
ection b, f j wis a setio /
of it. ei ta irwhe e wente d mo .(- --
tio is; *Iea eig r, t tdby he
ape aU, a ulied at the ua M
the adi othe aiu; ad thepor w .
ed th in dirtfin *ip vapplidat rig
the drtua the rdim of te wheal .. 3
therefore, according to the piciple of the lever, the
owr will hala- s the t hi it is as much
tee th the w bt as diuome is g eater th
the diutaI the weih e.


F. The third mechaical power, the Pautay, may
Slikewuie eplaised on the pinple of the lve
The line a may be eied to be a
lever, whose a a and be ar equal,
audethe fuleram, or centre of mooa.
If now two equal weights, w and p, be
hang oa the card paying oer the palley,
they will balaune o another, ad the
fulerus will setainh bth.
C. Doe this lley, them, slke te com- Fig. 24,
mon balance, gie no advntag
F. From the digle js plley n m animal
advantage is derived: it, nvrthel of git im-
portance in chagig the directive of a power, ad is
very much ed in buildings for drawing up mull
weigs, it being much eaer for a man to raise sch
burtbes by ans of a inle plley, than to carry
them up looM leader.
E. Why is t called a mehancal power
F. Though a i gle ixed pulley gves no advan-
tage, yet when it i not fsd, or when two or moe are
combined into what i called a system of pulleys
they thea posss all the properties of the other me-
chancal powea. Thus in e is the fit.
cram; therefore a power p acting at b,
wil sustain a double weight aetn at
for b is doable the distance ofco
the fulrulem
Again, it i eridet, in the prsea ease,
that the whole weight is nai the
cord dp, and whatever sais bal the
cord, sustain also half the weight; but Fig. 26.
one half i stained by the ixed book
consequently the power at p has only the other

half to tai, or, in other wod, say in apewe
at p will ke inm ilibrio a demble = at w.
C. I. the vlcity ofp double edt of
F. Undoubta d if i yo copa t the opa ed
through by the hed at p ib mat witd by >, y
will &d tht tw fonur i jua donblo d tlte rt,
md therefore tk MeN of the power ad wei t,
Sin the lever, a equal.
C. I think I me the m n of thi for if th
weidet nid an inch, or fat, beth ides of the
od must hbe ried a isch, o foot, but thi
cuaoo hab withet that pet of the aod at p
ps thin two inche or two dt of .
F. Yo wiU now Weily ifr, frM whin
has beo almdy Ihran of thbif M-
abl pulled, that in ra sysmo =p y h
gar asd mut be inted by dea-
Ui te nuber of plleys i the lower
or movable bleek. Be that whe the
fi d block a contain tw pulley which
only turn om their Uam, a thbe wer
block b cotain also two plleys, which
not oaly turn their aus, but aso rise
with the weight, the advunta is as for;
that is, a sinle pend at p will stin four
C. In the pdi t indamo, a Iper- Fig. 6.
ecive, that by ranl w n iach, there a
for rope srteed eah as iLch, and the ef the
hand m hba lMd thrTou fow inch.e of spre
inaenig the wita single ie; which establihes
the mim that whet gied a pow r lat im
spM But, p you hat edly talkd of the
power balsacig or tni dI we ht; something
mer mat, I Msppe, o ddd t& re it.
F. Them mst; comciderabl alowease mut lie-
was made hfr the fietia of the cards, ad of the
piM or am, Om whihe the pulle turn. la t6
mechanical powernin genea, on-tld of power
mut be added for the loa sustained by friction, ad

for the imperfct manuar in which machine aso cem
only ooaltrutd. Thun, if by shol youa ga a
power of 000, in prW t you mut reckon only upon
400. l those pulleys which we have been deerib-
, wies he taken notice of thbe thingL, which
t much from the general advantage ad cove-
nieace of pulled as a mechanical power. The Far
is, that the diameters of the ae bear a grent prepr-
ton to their own diameters. The seed is, that in
working they are apt to rub agaiMnt one another, or
against the deof the block. And te td died-
vantage N the uilam of the re tat ge over and
under dthm.
The two amt bjectim have bees, ia a
great deasg removed by the concentric
pley imvented by Mr. Jams White:
Si a solid block of brasa, in which
gromv ar cut, in the prportio o 1,
3, 7, 9, &c. and a is mneer'lhek of
the mine imd, whom groevem e in the
proportion of 8, 4,6, 10, &e. and teund
thee groove a cord is poed, by w6ich
mean th asoner the ppse of so
maiy diauct pulleys, every p-it of
which moving with the velocity of thr
trig in contact with it, the whole friction
is moved so the two eeatr of meoun Fig..
of the blocks a and L; beside, it is of mo adl
drantae, thatthe ptlleys being all of am peso
thee ia no bbing ag agemt the other.
E. Do you calculate tho r gained by thi pl.
ley in the mm method a wahthe common pralley
F. Ye, forpully of ever kind the rile iae
ral; th -admotageu pi i ul I b douling the
nurmbw ofthp i t doe lower bocksin that
bef e them are s gNoves, which awer to a
many disiet plleys, ud coamqueatly the power
gained i twelve, or one peoud at p will balance
twelve pem l at w.


F. We may nO describe the inclined plan,
which i. the fourth mechanical power.
C. You will not be dble, I think, to reduce thiL
aso to the prhiniple of the bver.
F. No, it is a idict principle, and mome wrin
on thee subjects reduce at once the si mechanical
power to wo, vis. d lever and the inclined ple.
E. How do oe stimte the advantage gained ib
this mec hnical p 1
F. The metdis very easy, for jut as much a
the length of the plane exceed perpendicular
height, o much is the advantage
gained. Supper ab is a plane -
g on the table, and ti another
plane inclined to it if the length
ad be three times r than the
papndicular hei t, tn the c- Fi gB.
lender will be mpported up" tbe
plane by a wit equal to the third pat of its
own weight.
E. Could I then draw p a weight on uch a plan
with a third part of the strgth that I mudt exert i
lifting t up at the oed
F. Ceait y you m~ght; allowance, however,
must be made far oerming the fiction; but then
you perceive, in the other mechanical powm, that
you will have three time the p to pass over, er
that you gin poer ou will M time.
C. Now 1 an the reason why sometime
there are two or three mIpdt m laid from de
street to the groueadoer making theet-
wit an inced plm, on which eavy peckages are
rad or lowered.
F. The inclined plane i chiefly ued for naing
heavy weight to mall heights, for i warehouse i.

sted in the upper prt of buildings, cranes and pulley
ar better adapted for the purpose.
C. 1 have sometimes, papa, amused myself by ob-
jerving the difference of time which one marble has
taken to roll down a smooth board, and another
which has flen by its own gravity without any sup-
F. And if it were a long plank, nd yo took care
to let both marbles drop fbo the had at the sme
instant, I dare say you found the dierence very
C. I did, and now you have enabled me to account
for it very atisctorly, by hewing me that as much
more time is pent in ailing a body along a inclined
pl, tha in lifting it up at the d, as that plane is
loner than its perpendicular height. For 1 take it
for granted that the rule holds in the descent as well
u in the ascent.
F. If you have any doubt remaining, a ow words
will make every thin clear. Suppoe our marbles
placed on a pl perfectly horontal, a on this
table, they ill remain at ret wherever they ar
placed: now it o elevated the plane insucb a
manner that is height should be equal to half the
length of the plane, it is evident from what has been
shewn before, tha the marbles would require a force
equal to half their weight to sustain them in my par.
ticular position: suppose them thepla prpeo u-
hlr to the table, the marbles will deceri their
whole weight, for now the plane contributes in no
respect to support them, cosequently thy would re-
amep mwer ua to their who weight to keep
C. And the swiftneo with which a bdy ls is to
be estimated by t force with which it i actd upon
F. Certnly for yo an now sufeiatly ac-
ouainted with plki y to know that th eect must
be estimated om the cause. Supp an inclined
plane is thirty-wo feet long, and is perpediclar
Leiaht is siteen feet, what time will a marble take la

flling dow th pla, amd lo in deS ding from
the uWto tho earth by th e breie gi t
C. B the sattibo of gmaitto, a body fbl
s U in a S a nd tbshref the marble will be
ao ocad in fllz ppedielalY to the ground;
ad a h ort tpl dobl e it bl
the mabl mst take two scond to edll down it.
F. I will try ou with another amp. If there
be a pla 64 feet perpediclar height, and 8 times
64, or 19 feet long, tell what tim a marble will
take in falling to the earth by the attrctio of gravity,
and how long it will be a desceding down the
C. By the attraction of gravity it will in two
seconds; because, by maudlyiag the lteea fet
which it lls in the fit M nd, by e square of two
seconds (the time), or fear, I got ity-r, the
heigt of the plane. But the pi n being throw time
as mg a it i pmrpdicurly Wig, it mut be three
tie a many second i ing down the ple, a
it was in demanding feely by thfarce of gravity,
that is, siZx M nds.
E. Pry, papa, what common instruments ar to
be r to this mechanical power, i the se
way as sem rs pi rs, &c. are revered t the lever
F. Cisels, hatchets and whatever other harp in-
struent which are chamfrd, or sloped down to an
dge on e ide onl, may be referred to the princi.
Sof tho ieliedpa.

or niu wsoo.
F. The next mechanical power is the wedge. which
i mad u of the twoM clned plans
da a ned joi ether at their ba Sr
do is the whole thickness of the
at its back aeed, where the power
i anpid, and df and of an the eth
a i is; now there wall e ao equili.
brum between the power impelling the ag. 9.

wedge downward, and the resistance of the wood, or
other substance acting giant its side, when tie
thickness do of the wedge i to the length of the two
ids, or, which is the sme thing, when half the
thickner de of the wedge at its back is to the length
of f one of its sides, u he power is to the reitance.
C.Thi is the principle of the inclined plane.
F. It is, and notwithtandingall the disputes which
the methods of calculating the advantage gained by
the wedge have occasioned, I see no reason to depart
from the opinion of those who consider the wedge as
a double inclined plane.
E. I have een people cleaving wood with wedges
but they seem to have no efect, unless great force
and great velocity are alo used.
F. No, the power of the attraction of cohesion, by
which the parts of wood ick together, is so great as
to require a considerable momIn uU to separate them.
Did you observe nothing else in the operation worthy
of your attention 1
C. Yes, I also took notice that the wood generally
oplit a little below the place to which the wedge
F. This happens in cleaving most kinds of wood,
end then the advantage gained by this mechanical
power, must be in proporto as the length of the
ide of the cleft i the wood is greater than the
length of the whole back of the wedge. There are
other varieties in the action of the wedge, but at
present it is not neceamry to refer to them.
E. Since you said that all instruments which sloped
ol to an ed n one side only, were to be explained
by the principle of the inclined pline; so, I suppose,
that those which decline to an edge on both sides,
mut be referred to the principle of the wedg.
F. They must, which is the cae with many
chisl, and almost all sorts of azes. &c.
C. Is the wedge much used as a mechanical
F. It is of great importance in a vat vaty of
cesa, in which the other mechamcal pows ane of

01 THI 5023W. W'
o avail; ad this aisesm e the momentum of the
blow, which is greater beyond comprion thn the
application of any dead wight or prems, ech a is
employed in the other mechanical power. Hece it
is d in splitting wood, rocke, &c. and even the
largest ship may be nied to a small height by driving
a wedge below it. It also sed for racing up the
beam of a house when the floor give way, by reason
of too great a burden being laid upon it. It is ual
also in separating large mill-stoe from the iliious
sand-rocks n some prts of Derbyshire, to bore bor-
nmtal holes under them acirle, ad fll thee with
pep or wedges made of dry wood, which gradually
swell by the moisture of the earth, and in a day or
two lift up the mill-stooe without breaking it.

or TaR o01w.
F. Let us now examine the properties of the
sixth and lat mechanical power, the sreo; which.
however, cannot be called
a simple mechanical power,
nce it never ed with- "
out the stance of a lever
or winch; by which it be- .
comes a compound engine, b
f at power in preng
together, or in ram -
ng great weigh is Fig. 30.
the reprsentation of one,
together with the lever A.
F. You aid just now,pap, thatall the mechanic
power were reducible either to the lever o ineln
plane; how canthe scrw be erred to either I
F. The screw is composed of two parts, one of
which a is called the crew, and cote of a spiral
protuberace, called the tread, which may bIW,
seed to he wapt onad a cylie; the other p ,

called the aut, i perforated to the dimension of the
cylinder; and in the internal cavity is also a s al
groove adopted to receive the thread. Now ifJ y cut
a slip of riting-pper in the form of a inclined
plaue od, and thn wrap it round a cylinder of wood,
you will lnd that it makes a spiral answering to the
nal pert of the screw; moreover, if ou consider
e ascent of the screw, it will be evident that it i
pecsely the ascent of an inclined plane.
C. By what means do you calculate the advantage
gained by the screw
F. There ae, at first eight, evidently two things to
be taken into consideration ; the first the distance
between the threads of the screw;-end the secod i
the length of the lever.
C. Now I compnro d pretty cleary how it i an
inclined plane, and that its ascent i more or les
easy a the threds of the spiral.ar earer or farther
dtant from each other.
F. Well, then, let me examine by a question
whether your oceoia. he accurate; uppose two
screws, the circummence of whore cylnde. are
equal to one another; but in one, the distance of the
threads to be an inch apart; and that of the thread o
the other only one-third of an inch; what will be
the difference of the advantage gained by one o
the screw over the other I
C. The one whoee thread are three times earer
than those of the other, must, I hold think, gve
three time the mot advantage.
F. Give me the reason for what you art.
C. Because, from the piiplof the inclined
ple, I leart that if the Wtdof two plaes were
tho sme, but the length of one twice, thrice, or four
times gater tha that of the other, the mechanical
advantage l ed by the longer plane would be two,
three, or tim more than than t gined by the
shorter. Now in the present cae, the height gained
in both mes is the sme, one inch, but the space
psed in that, thee of whose threads go t an inch,

Or THB saORW. p
must be three time as great a the pace pased
in the other; therefore, as space passed, or ti
lot, just in proportion to the advantage gained, I
infer that three tues more advantage a pined by
the screw, the thread of which are onethird of an
inch apart, than by that whose threads are an inch
F. Your inference lut, and natural follows
from an accurate knowledge of the prii of th
inclined plane But we hae id nothing about the
C. This seemed hardly necesary, it being so obvious
to any one, who will think a moment, that power in
gained by that as in levers of the first kind, according
to the length k from the nut.
F. Let us now calculate the advantage ined by
a screw, the threads of which are half an inch dis.
tance from one another, and the lever 7 feet long.
C. I think you once told me, that if the radis of
circle were given, in order to nd the circumference I
must multiply that radius by 6.
F. I did; for though that is not quite enough, yet
it will answer all common prp till you are a
little more expert in the use of decimas.
C. Well, then, the circumference of the circle
made by the revolution of the lever will be 7 feet,
multiplied by 6, which is 42 feet, or 504 inches;
but, during this revolution, the screw is raised only
half an inch, therefore the space passed by the moving
power. will be 1006 times greater than that gone
through by the weight, conequently the advantage
gained i 1006, or one pound applied to the lever
will balance 100 pounds acting against the screw.
F. You perceive that it follow as a corollary
from what you have been ayig, that there are two
methods by which you may incease the mechanical
advantage of the screw.
C. I do; it may be done ether by taking a longer
lever, or by diminihing the distance of the three et
the rsrew.

F. Tell me the result then, supposing the threads
of the screw so fe as to stand at the distance of but
one quarter of an inch asunder; and that the length
of the lever were 8 fet instead of 7.
C. The cirrumference of the circle made by the
lever will be 8 multiplied by e, equal to 48 feet or
676 inches, or 2304 quarter mches, and as the eleva-
tion of the screw is but one quarter of an inch, the
space pared by the power will, therefore, be 2304
tes greater than that passed by the weight, which is
the advantage gained in this instance.
F. A child, then, capable of moving the lever suf-
iciently to overcome the friction, with the addition of
a power equal to one pound, will be able to raise
2304 pounds or something more than 20 hundred
weight and a half. The strength of a powerful man
would be able to da 20 or 30 times a much more.
C. But I have een at Mr. Wilmot's paper-mills,
to which I once went, six or eight men use all their
strength in turning a screw, in crder to press out the
water of the newly-made paper. The power applied
in that cue must have been very great indeed.
F. It was; but I dare say that you are aware that
It cannot be estimated by multiplying the power of
one man by the number of men employed.
C. That is, because the men standing by the side of
one another, the lever is shorter to every man the
nearer he stands to the screw, consequently, though
be may exert the same strength, yet it is not so effe-
tual in moving the machine, as the exertion 6f him
who stands nearer to the extremity of the lever.
F. The true method therefore of calculating the
power of this machine, aided by the strength of these
men, would be to estimate accurately the power of
each man according to his position, and then to add
all these separate advantages together for the total
power gained.
E. A machine of this kind i, I believe, used by
bookbinders, to press the leaves of the books together
before they are stitched I

F. Yes. it is found in every bookbinder's work-
shop, and i pnaticula uful whemn pesons an
desirous of having mall books reduced to a till
smaller sie for the pocket. It is alo the principal
machine used for coining money ; for taking off cop
per-plate prints; and for printing in general. Mr.
loulton invented a magnihcent apparatus for coining:
the whole machinery is worked by an improved team.
.. une, which rolls the copper for half-pence; works
th screw-presses for cutting out the circular pieces
of copper, and coine both the hem and edges of the
money at the same time and sine the circulation of
the new half-pence, we ane all acquainted with the
superior excellence of the workmanhip. By this
machinery four boys, of tea or twelve years old, a
capable of striking 30,000 guanes in an hour, and
the machine itself keeps an unerring account of the
pieces struck.
E. And I have een the cyder-pres in Kent, which
consists of the same kind of machine.
. F. It would, my dear, be an almost endless task,
were we to attempt to enumerate all the purposes to
which the ncew u applied in the mechanical arts of
life; it will, perhaps, be sufcient to tell you, that
wherever great pressure is required, the power
of the screw is uniformly employed.

E. I have been s delighted with the Converse
tions you have permitted us to have with you, my
dear ap, that I can scarcely think of any thing
but what you have been explmning to u; and ow
when I me a machine of any kind, Ibegin to emne
the various combination of levers, whe oand pulleys.
F. I am ver glad to nd that my explaation of
the meanical powers have excited your curiosty
so mueh; and I shall always feel a pleasure in com-
ununicating any thing I know.
E. I am very much obliged to you, papa. Whei

Charges and I were down stain, we were enmiug
the kitchen clock; we saw wheels d axl ler ve
screws, pulleys, &c. but neither of us knw what to
all the pendulum. Is that a mechanical power
F. It t not called a mechanical power, beeae i
does not convey any mechanical drantage, but the
Sof the pendulum depends on that of the in
E. ht is meant b the term pMdalums f
F. The ame u s appld to any body o suspended
at t it may wing frely bckwar and forward, of
which the pea law *, iat its oseillion a arlway
performed equal time; and it is this maukbl
property which make it a timekeeper. A common
pendulum consists of a bell,
suspended by a rod
from a fixed point, at b, and
made to swmig badirrds
sad forwards under this
point The ball being ',
raised to e, and then mt at
liberty, falls back to s,
with aa a rntinmotio, F. ,1.
like a bll rolling down a
ope (or inclined e); ad when arived th ,
it has jst acquirel force enoh to l tod Iat
an equal election on the other ide; hom this it
fl bck again, again to rie, and would so continue
on for ever, if thee was no impediment either from
the air or friction.
C. Ae the laws which regulate these moremnts
so simple that we can understand them I
F. I think they ae; the most important of them
ae thee :-1. If a pendulum vibrates in very mall
ircle, the time of vibration may be considered as
'qual, whatever be the prpornio of the cirele-
2. Pendulums which are of the ame l t vibrant
in the same manner, whatever be the ropio of the
V@.ght of the bob.-3. The velocity the bob, or
Sall in the lowest point, will be a the length of the
tird of the arch which it describe in its dMcent.-

4. The tim of vibration of dilerent pendulum is
similar arches are proportional to the sq roou of
their different lengths-6. Hence the lengths of pen-
dulums ar as the square of the times of vibration.
-6. In the latitude of Lodon, a simple pendulum
will depart owM in a second in a mall arch,if its
length be 39J inches.
C. Doe the length of a pendulum influence the
time of its vibrations?
F. Yes; long pendulums depart more slowly than
sort ons; because,in ca ending m,or patoi, the
bob r ball of the large pnlm a a greater jour-
ney to perform, with a steeper He of decent.
C. If I underand yo rightly, a pedlum which
describes second i he in th, and the
length ae a the squareoot of the time; therefore,
Sgthlengthof a a blfecond pendulum is 9 inches,
and the length of a quarter pedalu will be
2A inchb.
F. You are quite correct.
E. But, if you whed the pendulum to beat longer
than one second, how could that be done?
F. As a body fall four te asfast in two seconds
as m one, a pendulum mut be four times as long to
beat once in two secods as to beat every eond.
E. How i it ade to dentethe time on the cloes
F. A common clock is merely a pendulum with
wheel-work attached to it to recol the number of
vibration. The wheels hew how many swing of
the pendulum have taken place, because at every
beat a tooth of the last wheel is allowed to pO;
now, if this wheel have 00 teeth, it will just turn
round once for 60 beat of the pendulum, or seconds,
and hand id o theai poectins g through the
dial-plate will be the Mcond-hand of the clock. The
ether wheel a so arranged, and their teeth so pro-
portioned, that one turns 60 times slower than the
first, to fit its axi to carry a minute-hand, and
another, by moving twelve times slower till, is ftted
to carry an hour-hand.


CHARLIs. The delay occasioned by our unusually
long walk has afforded us one of the most brilliant
view of the heavens that I ever saw.
Jamen. It is uncommonly clear, and the longer I
keep my eyes fixed upwards the more stan seem to
appear: how is it possible to number these stars and
yet I have heard that they are numbered, and even
arranged in catalogues according to their apparent
magnitudes. Pray, sir, explain to us how this busi-
ness was performed.
Tutor. This I will do, with great pleasure, some
time hence; but at present I must tell you, that in
viewing the heavens with the naked eye, we are very
much deceived as to the supposed number of tats
that are at any time visible. It is generally admitted,
and on good authority too, that there are never more
than one thousand stars visible to the sight, unassisted
by glases, at any one time, and in one place.
J. What I can I ee no more than a thousand
star if I look all around the heavens? I should sup.
pose there were millions.
T. This number is certainly the limit of what you
can at present behold; and that which leads you,
and persons in general, to conjecture that the number
is so much linger, is owing to an optical deception.
J. Are we frequently liable to be deceived by our
senses ?
T. We are, if we depend on them siagl : but
where we have an opportunity of calling in the assist-
ance of one sense to the aid of another, we are seldom
subject to this inconvenience.

C. Do you not know, that if you place a i
marble in the ptlm of the left hand, and then croa
the cond fnger of the right hand over the first, and
in that poitio, with your eyes shut, move the marble
with those parts of the two fngers at once which are
not accustomed to come into contact with any object
at the same time, that the one marble will appear to
the touch as two In this instance, without the a-
sistance of our eye, we should be deceived by the
ene of feeling.
T. This s to the point, and ahows that the judg-
ment formed by means of a single sense a not always
to be depended upon.
J. I recollect the experiment very well; we had
it from papa, a great while ago. But that has no-
thing to do with the false judgment which we are said
to form about the number of stars.
T. You are right; it doe not immediately concer
thb subject before us, but it may be useful a af-
fording a lemon of modesty, by instructing u that
we ought not to close our minds against new evi.
dence that may be offered upon any topic, notwith-
standing the opinions we may have already formed.
You ay that you see millions of stars, where the
ablest astronomers assert, that with the naked eye you
cannot at one time see so many as a thousand.
C. I should indeed have thought with my brother,
had you not asserted the contrary; and I am anxious
to know how the deception happens, for I am sure
there must be a grt deception somewhere, if 1 do
not at this time behold very many thousands of tars
in the heavens.
T. You know that we see objects only by means
of the rays of light which proceed fro them in every
direction. And you must, for the present, give me
credit, when I tell you that the distance of the fiaed
tars from us is immensely great; consequently the
rays of light have to travel this distance, in the course
of which, specilly in their page through our at-
moaphere, they are ubiect to numberless nreitioe

%ad r/nractimon. By means of these, other rays of
ht come to the eye, every one of which, perhaps,
impreses upon the mind the idea of so many sepa-
rate sta. Hence aries that optical fallacy by which
we are led to believe the tars which we behold are
J. I should like to see an experiment to confirm
T. I have no objection:-in every case you ought
to require the but evidence that the subject will ad-
mit of. I will shew you two experiments, which will
go a good way to remove the diiculty. But, for this
purpose, we must step into the houe.
ere are two common looking-gluses, which, phi.
oeeophically speaking, are planr miwrro. I place
them in such a manner on the table that they up-
port one another from falling by meeting at the
top I now place this half-crown between them, on
a book, to raise it a little above the table. Tell me
bow many pieces of money you would suppose
there were, if you did not know-that I had used but
J. There e e several in the glasses.
T. I will alter the position of the glasses a little.
by making them almost parallel to one another; non
*look into them, and say what you see.
J. There are more half-crowns now than them
were before.
T. It is evident, then, that by rejfctig only, a
single object, for I have made use of but one half
crown, will give you the idea of a vast number.
C. If a little contrivance had been used to conceal
the method of making the experiment, I should not
have believed but that there had been several half-
crowns instead of one.
T. Bring me your multiplying glans; look through
it at the candle: how many do you se 1 or rather
how many candle should you supple there were,
did you not know that there was but on the

OF TRB F11nt STARr 77
J. A great many; and a petty eight is.
C. Let me we;-yes, thee are: but I can emay
count them; there are *ixte
T. There will be just as many imag of the
candle, or any other object at which you look, a
there ar difrent surfaces on your gl. For by
the principle of freidon, the image the candle
seen ia many differet places a the glia has r-
face; coequently, if instead of 16 there had been
0, or,if they could have been ct and polished s
mall, 000, than the il candle would hae given
you the ide o 0, or 0. Whatthink you now about
the sans
J. Since I ha se me that ntflt and rfr tin
will each, singly, afford such opical deceptions. I
can n longer doubt but that, ifboth these cae
ae combined, as you a they am, with expect m
the rays of iht comu m the ted tas, a thou-
uad eal lum ri may bae the power of exciting
i my mind the ide of million.
T. I will mmtio another experiment, for which y-
maybe prepared against the net clear taright night.
Get a long narrow tube, the longer and narrower the
better, poded it weight does not reader it unma.
n ble: examine through it any o of the largest
sied tars, which called tan f the .f~t magu
tde, ad you ill ind that, thbogh the tube take i
at much aky a would contain many uch stas, yet
that the ge one at which you an looking a
scarcely vible, by the few rays which come iraett
from it: th i another proof that the brilliany o
the hea much more owing to rfip d ad re-
frtad light, than to the direct rays owing from the

or Tas nxn srTAR
C. Another beautiful evening presents itself; shall

we take the advantage which it offers of going L
with our astronomical lectures I
T. I have no objection, for we do not always enjoy
such opportunities as the brightness of the present
evening affords.
J. 1 wish very much to know how to distinguish
the stars, and to be able to call them by their proper
T. This you may very soon learn; a few evenings,
well unproved, will enable you to distinguish all the
mtar of the first magnitude which are visible, and all
the relative positions of the different constellations.
J. What are constellations, sirt
T. The ancients, that they might the better distn-
guish and describe the star, with regard to their stu-
stion in the heavens, divided them into constellations,
that is, systems of stars, each system consiatug of
such star as were near to each other, giving tem
the names of such men or things as they fancied the
space which they occued in the heavens represented.
C. Is it then perfectly arbitrary, that one collection
is called the great bear, another the dra.o, a third
Herevue, and so on?
T. It is; and though there have been additions to
the number of stars in each constellation, and various
new constellations invented by modern astronomers,
yet the original division of the stars into these collec-
tions was one of those few arbitrary inventions which
have descended without alteration, otherwise than by
addition, from the days of Ptolemy down to the pre-
sent time.--Do you know how to find the four car-
dinal points, as they are usually called, the North,
South, West, and East I
J. 0 yes, I know that if I-look at the sun a
twelve o'clock at noon, I am also looking to the
South, where he then is; my back is towards the
North; the West is on my right haqd, and the East
on my left.
T. But you must learn to find these points without

the suaeance of the sun, if you wh to be a young
C. I have often heard of the north- po er; that
will perhaps answer the purpose of the son, when he
has left us.
T. You are right; do you see those en star
which are in the constellation of the Grest Bre r
some people have upped their potion will aptly
represent a pugh; others say, that the are more
like a a gges ad horse ;-the four
stars representing the body of a wag- ,
gon, and the other three the houses,
and hence they are called by some
the plough, and by others they are
called Carlei's wain or waggon.
Here is a drawing of it; d re-/ s
present the four stars, and es the "
other three. Fig. 1.
C. What is the lsaru
T. That represents the polar tar to which you just
now alluded; and you oeerve, that if a line were
drawn through the sta r and a, and produced far
enough, it would nearly touch it.
J. Let me look at the hearens for it by this guide.
There it is, I soppo; it hines with a steady and
rather dead kind of iht, and it appears to me that it
would be a little t the right of the line posing
through the stan 6 and a.
T. It would, and these stars are generally known
by the name of the poinr, because they point to p,
the north pole, which is stated a little more than
two degrees from the star .
C. is that star always in the same part of the
T. It may be considered u uniformly maintaining
its position, while the other star seem to move round
Sas a centre. We hall have occasio to refer to
this star ag ; at present, have directed your tten
ton to it, as a proper meb of Sadin the cardinal
points by tarigbt.

J. Yes, I understand now, that if I look to the
north, by standing with my face to that star, the suth
is at my back, on my right hand i the east, and the
west on my left.
T. This is one important step in our astronomical
studies; and we can make use ofthee tars a kind
of standard, in order to discover the names and
position of others in the heavens.
C. In what way must we proceed in this business
T. I will give you an example or two: conceive a
line drawn from the star (Fig. 1.) leaning aa little
to the left, and it will pas through that very brilliant
star near the horizon towards the wet.
J. I lee the star, but how am I to know its name?
T. Look on the celetial globe for the star s, and
suppose the line drawn on the globe, as we conceived
it done in the Been, and you will find the star, and
its name.
C. Here it is;-its name is Arcturs.
T. Take the figure, (Fig. 1.) and place Arctur
at A, which is its relative postio, in respect to the
constellation of the Great Bear. Now, if you con-
ceive a line drawn through the star g and b, and ex-
tended a good wy to the right, it will pas jut above
another very brilliant sr. Examine the globe as
before, ad nd its name. *
C. It i CqpiUa, the got.
T. Now, whenever you see any of these star, e
ll knowwhereto look forth others without hesitatoa.
J. But do they never move from their places
T. With respect to us, they eem to moe together
with the whole heaves. But they always remain a
the ame relative poison, with respect to each other.
Hence they are called fre stars, m opposition to the
pt ssM, which, like our earth, are contmually chang
tg their places, bo witwih regard to the fixed tar
and to themselves also.
C. I now understand pretty well the method of
acquiring a knowledge of the name and places of the

T. Aad with this, w wil put as aed ear

or trs nmo STAiR, AnD icurrc .
T. I d say that you will ave s diahy i
fiGding the o plar star as m weip ib the
open air.
J. I "hllt ne haow wbo to look r that ud
the odtr tma wich yu out las sigt, if
they have mt ehand dir $me.
2. ThwlwmsEhel lrthemeimposiiem, withi
to Mch other, their situtio, with nlj lto
tib hie, will h Wtt at diAeet sem s of
the year, d in dieset hebeoa of the nigh. Let
g ias the gaudem.
C. The man a all ia tlh sea plae as we lk
them la evening. Now, Sir, if we comnave a
might line dmaw tbrosg the two stars in the
plou, which, in your Are, (Fig. 1.) ae maued
4 1 g, and to tend a good wy down, it will pim
or nearly pe through a very igt str, though
met bright Aortwam or CqL ; what is tat
T. It i a starof th .eom d mmg'it, sad if ye
mrefer tsh olmial 0" is the nr wy M you
wn instraeLlha t fi%. I will fad it is called
aqpdsw, or Car L0i6, the In's Htart. By thi
methd y my qicUy 4sioer the names of a
tho epad*l ss, and:d atOrward, with little
pdti yes will esil diriguish the othm which
C. But tbe7 hau not all .ames; how a they
T. If y look agohe h, m will aber, that
the1 d asg, 4hs b letter s the
G I s h at n ; c**tel at*i- ias
whiabhe em awe ( ded a mprenotntMgle

dh l igst ia a lpha, d* eat is em b ta, dh
third r gamma, the farth dela. and o sh.
J. is th any particular "uo for thbi
T. The adopo th cha m ctr of the Grek
alphabet, nth than y oher, was perftly arbi.
trary; it is, khow r, o great importance t the
sam charatrs should be und in gnral by tro-
aemeM eacoMuisr for by thi mans the scice
D in pe i of a o t of sulvral laguag
c. wil you pl in ow thi is
T. Sepp ae a-- soemer North Amrie Asa,
or ay o rt of th earth, obhm ra mo ia th
put of trh beans ler the counllaio of the
Pd i situatd, and bewidmhto dMscibe it to his
ft i Onut Brikmi, i order that he may know,
wheer it wan em by tie inhabitants of hiL Hilnd.
For dni purpem, be bha only to meatiao tbh time
whea he dicovered it its posii, a nmst to
same one of the Sent, caling it by the kttr by
which it i designated; dthe core whih it t
frm one sla toward another. Thus e eight y,
that om ebha time he saw a mcoet Uear in tod
Great Bea, and tat is coe was directd Afi
to or any other iit happen.
C. Then, if hi ftied here had n a comot at the
me tbme, he would, by thi" means, know wheer
wa te e or a diemat emaeti
T. Cetinly; ad hence ye pNesive of wat in.
psrtace it i tha utn amenm a dilerot eonbls
hold ae to mak tih ram srm, ad sysims o
sa, by the Mme chum tes. But to nm to atha
Star, to which v jst edu my a ttemd, de COr
Lm is it is ot ly a emaai h Str, bat it
S is aso imarhable: i it itrated i the
J. What thatir
T. The~ d an im oag ry eide in the
Meavews. w d s ppuo s to dos be dth
enomre of a yr. If yU lek a the ed 8 0,
you will se it marked wit a red ite, pwhap n

or "ar Saia. W
blesofd ism t bet c IwouS S by
that bdly.
J. d Mu tis to buys ahd mobs h
T. Itawu. Anm d CAM im "baumtAWi4
or day mah, wich i s m-y hew ne.
It apear b aveu in As emu l ens a
frnwr i sbm d by ini itahmod "e
whe eMaoim but hsmw doh s fth is a mvy
mmorin AShmllt. at MMa ma 6fathNad in
60 sevmmu ithe Wad; but hmhusrldgist od
1wv ms he the mtk of pike bmsumm.
C. AMvwu isthe N"sW" iuemns it It
T. It Cislmdtlm the ,tb Is eM la s

camcrd, a Ithe -mL9 i e u s
c" TUdheiI W l ot f Au i i
poducd to ftrp of tif AWA a it Willi
ad ~ ~~ev eslh eh oi siA~ II
"muar, or "Yiuitwi, which Nil adt *6 .SO hI
two pub; amod whih We agomft am* uv
the CUMr mabmut 24 dogma.
J. Can we ums the s e of as alow a *
heaem ?
T. It amy he de with Isl sth y Wto
Rd-, a; = ISesed ofb
And IbaM to own is il cm inis
oh. Thm Mead aeled is by ubumq do

C. Itbummm so" miayahtr bs ecildm
T. Not ainedy so; butit i aLa oiburhi do
melpeb, m fLth iye dogma Uls aiki ni amu
Ad a the adr. The pleasio, b I
IMMIR CVV SRA, Book eigh, IKm &ad
mthe. the lims amh thu "MftMde.
I7. HeW an we fas WOe by hIp* m

B. Dy ampag do do in thu hemam uts

wich may ilbe y sequised, a yoes I e m. I
will mnioa to you the am of those sta, and you
aykr A"6 i n th 6a globs, aMd hn Lr to a
many o them an u e now ibh ia th ea-veI.
'Ite At is the R's hore, called a Arid abhut
tm4pem as the eart o dte liprie; tih mseed is
the tar O idek ua thi Be ts eye, a dopses stA
C. The if at y time I e thee two man, I
kww that th ecdie rie bebtw them, sad smr
s Aldebase thm ta tt in th Rm's bei.
T. Yes mncarry yer sy eatwMie adime
somew er fpC rm Aldsbr, dthm that is et of
SAirti, Md you will percei two bra st a at
miu diemfe oa eM other, calld Ctarr oud
Ietws; ikte ow ^rMd e t ie, i is Iote shbil
lit is Pellu, sr ve degra s ea north aide of t
aliptie. allowing the me tret, yo will com to
luIuor t Car Leis, which I a already ob-
Mrdiis a theli mof theoliptie. Beyoed this, ad
oly two depees asnth of thatl, will d tih
beautiful tar i the virgin's head d Si ir ris
Yar them rit IAnters, r the &sJm's eart,
Aw depres on the same sde of the ecliptic. After-
werdi will lad I A'i, which ismaid early
dtiwr g .me of the eoiptio; ud arther m. a
the aeir fLii in te Ash's mthl.abot as mmy
(ses th oftha ltime. The faith and last ul
the tas is Pegus, in the wi of the 4iW
hoem, which is t of the cliptc may welty
pe whot aCc ua theme m aim par.
T. Tbo selectd am mth fmek pi ta
ner tmh* am' oi ed m r cmiend I- ppaer
sateioon, fi e which the mea's diNi is ldula
frewiry ths howes of tims a hoe b um Ce-
straid thse tables i the Nuetial Almerm, by
,ua.of which muvigptn i their amt diftst
ep w ae abled to eimale, em d tr ackli

or rn mmIkMi. '*
sea, the pmahaul oji the o "I@a wkibhey
are. '
C. What moa ab thl e Nautl Ailmpe
T. It is bdd of I f Ali iatd.d
chily forth e of pldauiver t. n ty
oc I. It wa begap I the yeur W, bfDr.
MaukelyM, tbSAItreaerB EIl; u"and ibd
by antcipati for ~ al yea beemnhd, for the
moeeutae..4 ships (-e .t up hegrmagb.
Thli work be b iria = Mdr
ree rst tid he a rmi" t-W e*wM kg
dheasres, ead Is hbty ael te .3 gaqaei

or Tug WmEu333*.
C. Your m d am d e*chg the a
me a ftho mitioi of dth plhMa wll yo es.
duat notw
T. will; mi to nder e perLldy li
frori e tben n the a ill
endise to the oupOM Of .X-i- the w -
Whbear ife e iaii bk i phisle
annually,. ld whri is geemry a e.p f
J. M~twmdeteld Ul" toe waudy tedo l
T. You mt r amM ether book 1o the i
kind, it y weMUt id a. tch bet ad
ratio l ldwhm ym knew th s df
this eakwhi6 ye wiN mplely wikUf fa
hm% asntems, has newonre o adas uar
i 4d 9l p le fle at at dayd dl
yea, than to tunm a thas dibe I elllhs d
e at l' b be ir o thu I -m dfi
Iw ih n n du ..pmau.e
C. Here tihe aiobeiLur t -mma .
T. The we aofl 1 t qypaMM the

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