GRAVITY. 
m2 
line defcribed on a plane, and wliicli, by radii draw n to 
any certain point, defcribe areas about tbe point pro¬ 
portional to tbe times, are impelled or adled on by 
fome power tending towards that point. Conlequently 
the power by which all thefe planets revolve, and are 
retained in their orbits, is direded to the centre about 
which they move, viz. the primary planets to the lun, 
and the Catelliies to their feveral primaries.—See the 
article Astronomy, vol. ii. p. 341. 
Various theories hav'e been advanced by philofo- 
phers of different ages, to account for tlte principles of 
gravity. The ancients, who were only acquainted 
with the tendency oi lisblunar bodies towards the eaith, 
aimed no farther than a fyllem that might anfwer tJie 
more obvious phenomena of it. Yet fome hints are 
found concerning the gravitation of celeftial bodies in 
the account given of the dodrine of 1 hales and his 
fuccellbrs; and it is fuppofed that Pythagoras had a 
view to it in what he taught concerning the harmony 
of the Ipheres. Kepler, in his preface to the commen¬ 
taries concerning tlie planet Mars, (peaks of it as of a 
power that was mutual between bodies ; and fays that 
the earth and moon tend towards each other, and would 
meet in a point fo many times nearer to the earth tha§ 
to the moon, as the eartii is greater than the meon, if 
their motions did not hinder it. lie adds, that the tides 
arife from the gravity of the waters towards the moon. 
To him vve alfoowe the important difeovery of the ana- 
loo-y between the diitances of the feveral planets from 
the fun, and the periods in which they complete their 
revolutions, viz. that the fquares of their periodic 
times are always in the fame proportion as the cubes 
of their mean diflances from the fun. However, Kep¬ 
ler, Gallendi, Gilbert, and others, aferibe gravity to a 
certain magnetic attradlion of the earth; conceiving 
the earth to be one great magnet continually emitting 
effluvia, which take.hold of all bodies, and draw them 
towards the earth. But this is inconfiflent with the fe- 
veral phenomena. 
Dr. Clarke, from, a view of the feveral properties of 
rrravity, concludes that it is no adventitious effefi; of 
any motion, or fubtle matter, but an original and gene¬ 
ral lawimprefled by the Creator on all matter, and pre- 
ferved in it by fome efficient power penetrating the 
very folid and intiptate fubftance of it; being found al¬ 
ways proportional, not to the fui faces of bodies or cor- 
pulcles, but to their folid quantity and content. It 
fhouid therefore be no more inquired why bodies gravi¬ 
tate, tlian how they came to be firft put in motion. 
See Annot. in Rohault. Phyf. part. i. cap. ii. 
Of specific GRAVITY. 
Specific gravity is the relative, comparative, or ap¬ 
parent, gravity in any body, in refpebt of that of an 
equal bulk or magnitude of anotlier body 5 denoting 
that oravity or weight which is peculiar to each fpecies 
or kind of body, and by which it is diftinguiffied from 
all other kinds. The fpecific gravity of folids is deter¬ 
mined by weighing them firfl in the air, and then in 
water A body is faid to be fpecifically heavier than 
another, when under the fame bulk it contains a greater 
weight than that other; and reciprocally the latter is 
faid to be fpecifically lighter than the former. If 
there be two equal fpheres, eacli one foot in diameter, 
the one of lead, and the other of wood, fmee the leaden 
one is found heavier than the wooden one, it is faid to 
be fpecifically, or in fpecie, heavier ; and the wooden 
one fpecifically lighter. The laws by which the fpecific 
gravities of bodies appear to be governed, are as follow ; 
I If tw'o bodies be equal in bulk, their fpecific gra- 
viti'es are to each other as their weights, or as their den- 
fities. 2. If two bodies be of the lame fpecific gravity 
or denfity, their ablolute weights will be as tfieir mag¬ 
nitudes or bulks. 3. In bodies of the fame weight, the 
fpecific gravities are reciprocally as their bulks. 4. 
^'he fpecific gravities of all bodies aie in a latio com- 
VoL. VIII. No. 546, 
pounded in the direfl ratio of tlieir weights and the 
reciprocal ratio of their magnitudes. And hence again 
the fpecific gravities are as the denfities. 5. 'i'hc alifo. 
lute gravities or v\eights of bodies are in tlie compound 
ratio of their fpecific gravities and magnitudes 01 bulks. 
6. The magnitudes of bodies are diredtly as their 
weights, and reciprocaliy as their fpecific gravities. 7. 
A body fpecifically heavier than a fluid, lofes as mucii 
of its weight when iinineiTcd in it, as is equal to tlie 
weiglit of a quantity of the tiuid of the lame bulk or 
magnitude. Hence, fince the fpecific gravities . re as 
the abfoliite gravities under the I ime bulk ; tlie fpeci- 
fic gravity of the fluid, will be 10 that of the body 
immerged, as the prart of the weight iolf by tbe folid 
is to the whole weight : and hence the fpecific gravi¬ 
ties of fluids are as the weights Icfi by tlie fame folid 
immerged in tlicm, 8. The fpecific gravities of bodies 
of equal weight, are-repiprocully proportion.il to the 
quantities of weiglit loft in tlie fame fluid. And lienee 
is found the ratio of tlie fpecific gravities of (olids, 
by weighing in the fame fluids, inalfes of them that 
weigh equally in air, and noting tlie weights loft by 
each—See the aiticle Mechanics, 
Much advantage has been, and may ftill be, derived 
from a careful iiiveftigation of tliefe laws, and their ap. 
plication ; fince it is very probable, that by an atten¬ 
tion to tlie fpecific gravities, capacities for heat, fufibi- 
lities, volatilities, laws ofcryftallization, elafticity, hard- 
nefs, tenacity, malleability, and other obvious fpecific 
properties'of bodies, we may yet dilcover at move inti¬ 
mate acquaintance with the mutual actions of their par¬ 
ticles, than any that has hitherto been acquired. 
From the foregoing data it is eafy to conftrnrt a ge¬ 
neral table of fpecific gravities, by reducing the pro¬ 
portion of the ablolute weight to the lols liiftained by 
immerfion, into terms of which that exprefliiig water 
fliall be unity. If the folid be fo light as to float upon 
water, it is convenient to attach to it another iieavier 
body fufficient to caufe it to link, but whofe weight in 
water muft be added in computing the lofs. And fince 
the lofs by immerfion will accurately (hew the weig'iu 
of the fame bulk of the fluid ; (b, confequently, the 
proportion of tliefe feveral quantities to the lols tlie 
lame folid liiftained in water, being reduced as in the 
other cafe to the common ftandard of unity, will exhi¬ 
bit the Ipecific gravity. Other methods are likewife 
ufed in experiments with fluids. 'I has equal bulks of 
different finids may be weighed by filling a Iniall bottle 
witli a ground (topper with each refpedlively, and from 
their feveral weiglits the weight of the bottle and flop, 
per nnift be deducted. Or otherwile, tlie inftrument 
called the hydrometer may be ufed, which polfelfes the 
advantage of portability, fpeed, and a degree of accu¬ 
racy not ealily obtained by the ufe of ordinary balances. 
—See the article Hydrometer. 
Tlie following arrangement will fliew the fpecific gra¬ 
vity of metals and other bodies to rain water, and the 
weiglit of a cubic inch of each, in parts of a pound 
averdupoife. 
Pure gold caft — 
Ditto liainmered —- 
Standard gold caft — 
Ditto hammered — 
Pure filver caft — 
Ditto hammered — 
Standard filver in coin — 
Crude platina in grains 
Platina purified and fufed 
The lame hammered —• 
The lame drawn into wire 
The fame laminated — 
Mercury — — 
Lead fuled — — 
Copper fufed — —- 
Dmo drawn into wire - 
9U 
Sp. Gravity. 
V/. ib, Averd/ 
19253 
0-71036 
19362 
0 
C; 
0 
0 
17486 
0-63250 
I 75 !i 9 
0-63618 
104/4 
0 ' 3779 <' 
10511 
0-38017 
10391 
0-37580 
15602 
19500 
0-70530 
20377 
0 ’ 735 i 7 
21042 
0-/61O7 
22069 
0-79821 
I 3 , 56 !f 
0-49074 
*1352 
0-40965 
7788 
O-281U8 
8878 
0 • 3 2 111- 
Biafs 
