G R A 
G It A 
8/0 
weight required in the same manner to sink 
it in distilled water. Divide the first sum 
by the latter, and the quotient will be the 
specific gravity of the fluid in question.” See 
Specific Gravity, Hydrostatics, and 
H YDROMETER. 
GRA'i ING, in the sea language, is bring- 
ing a ship aground, and then burning off with 
furze, reed, or broom, all the tilth and foul- 
ness that sticks to her bottom withoutboard, 
j n order to pay her anew. 
GRAVITATION. See Attraction, 
and Gravity. 
Gravitation, lazes of, are as follows: 
1. It is common to ail bodies, and mutual be- 
tween them. 2. It is proportional to the 
quantity of matter in bodies. 3. It is exerted 
every way from the centre of the attracting 
body in right-lined directions. 4. It de- 
creases as the squares of the distances in- 
crease : thus, if a body at A on the earth’s 
surface, distant one semidiameter from the 
centre C, weighs 36.00 pounds, it will, at 
the distance of 2, 3, 4, 5, 6, semidiameters, 
weigh 9.00, 4.00, 2.25, 1.45, 1.00 pounds, 
which numbers decrease as the squares of 
the distances increase. The triflh of this 
proposition is not to be had from experi- 
ments; the utmost distance we convey bo- 
dies to, from the surface of the earth, bear- 
ing no proportion to their distance from its 
centre, but is sufficiently clear from the mo- 
tions observed by the heavenly bodies. 
Hence we learn, that all bodies have gravity, 
or are heavy, and that there is no such thing 
as absolute levity in nature; and by the se- 
cond law, the gravitation of all bodies is pro- 
portional to the quantity of matter they con- 
tain : and hence, since bodies of equal bulk 
are found to have unequal quantities of mat- 
ter, it evidently follows, that a vacuum, or so- 
lid void of matter, must necessarily exist, and 
that an absolute plenum is a doctrine unphi- 
losophical, and equally false and absurd. 
Also from the third law it follows, that ail 
bodies descending freely by their gravity, 
tend towards the earth in right lines perpen- 
dicular to its surface, and with equal veloci- 
ties, abating for the resistance of the air; as 
is evident from the second law above. 
Again, since the gravitation is always as 
the quantity of matter, and inversely a’s the 
square of the distance, it follows, that were 
the internal parts of the earth a perfect void, 
or hollow concavity, a body placed any where 
therein would be absolutely light, or 'void of 
gravity ; but supposing the earth a solid body 
.throughout, the gravitation from the surface 
to the centre will decrease with the distance, 
or it will be directly proportional to tire dis- 
tance from the centre. 
Gravitation being found by many experi- 
ments and observations to affect all 'the mat- 
ter of bodies equally, we have hence more 
reason, says Mr. Maclaurin, to conclude its 
universality, since it appears to be a power 
that acts not only at the surfaces of bodies, 
and on such bodies as are removed at a dis- 
tance from them, but to penetrate into their 
substances, and into that of all other bodies 
even to their centres, to affect their internal 
parts with the same force as t-lie external, to 
be obstructed in its action by no intervening 
body or obstacle, and to admit of no kind of 
variation in the same matter, but from its dif- 
ferent distances only from that to which it 
gravitates. 
This action of gravity on bodies arises 
from its action on their parts, and is the ag- 
gregate of these actions ; so the gravitations 
of bodies must arise from the gravity of all 
their particles towards each other.” The 
weight of a body towards the earth arises 
from the g-avity of the parts of that body: 
the gravity of a mountain towards the earth 
arises from the gravitation of all the parts oi 
the mountain towards it ; the gravitation of 
the northern hemisphere towards the southern 
arises from the gravitation of all its parts to- 
wards it ; and if we suppose the earth divided 
into two unequal segments, the gravitation of 
the greater towards the lesser arises from the 
gravitation of all the parts of the greater to- 
wards the lesser. In the same manner the 
gravity of the whole earth, one particle being 
excepted, toward that particle, must arise 
from the quantity of gravitation of all the 
other particles of the earth towards that par- 
ticle; every particle, therefore, of the earth 
gravitates towards every other particle : 
and for the same reason every particle in the 
solar system gravitates towards every other 
particle in it. 
GRAVITY, in physiology, the natural 
tendency of bodies towards a centre. 
Gravity may be distinguished into particu- 
lar and general. 
Particular Gravity, is that which re- 
spects the earth, or by which bodies descend, 
or tend towards the centre of the earth ; the 
phenomena or properties of which are as fol- 
low : 
1. All circumterrestrial bodies tend towards 
a point, which is either accurately or very 
nearly the centre of magnitude of the terra- 
queous globe. Not that it is meant that 
there is really any virtue or charm in the 
point called the centre, by which it attracts 
bodies ; but because this is the result of the 
gravitation of bodies towards all the parts of 
which the eartii consists. 
2. This point or centre is fixed within the 
earth, or at least has been so considered as 
far as any authentic history reaches. For a 
Consequence of its shifting, though ever so 
little, would be the overflowing of the low 
lands on that side of the globe towards which 
it should approach. Dr. Halley suggests, that 
it would well account for the universal deluge, 
to have the centre of gravitation removed tor 
a time towards the middle of the then in- 
habited world ; for the change of its place 
but the 2000th part of the radius of the earth, 
or about two miles, would be sufficient to lay 
the tops of the highest hills underwater. 
3. In all places equidistant from the centre 
of the earth, the force of gravity is nearly 
equal. Indeed all parts ot the earth’s sur- 
face are not at equal distances from the cen- 
tre, because, the equatorial parts are higher 
than the polar parts by about 17 miles; as 
has been proved by the necessity of making 
the pendulum shorter in those places, before 
it will swing seconds. In the New Peters- 
burgh ’Transactions, vol. 6 and 7, M. Krafft 
gives a formula for the proportion of gravity 
in different latitudes on the earth’s surface, 
which is this-; 
y = (1 -f- 0.0052848 sine A) g ; 
where g denotes the gravity at the equator, 
and y the gravity under any other latitude A. 
4. Gravity equally affects all bodies, with- 
out regard either to their bulk, figure, or 
matter : so that, abstracting from the resist- 
ance of the medium, the most compact and 
G R A 
loose, the greatest and smalled bodies, would 
all descend through an equal space in the 
same tune; as appears from the quick de- 
scent of very light bodies in an exhausted 
receiver. The space which bodies do ac- 
tually fall in vacuo, is 16 ^- feet m the first 
second of time, in the latitude ot London ; 
and tor other times, e.ther greater or less 
titan that, the spaces descended from rest 
are directly proportional to the squares of the 
times, while the falling body is not far from 
the earth’s surface. 
5. 1 his power is the greatest at the earth’s 
surface, whence it decreases both upwards and 
downwards, but not botli ways in the same 
proportion ; for upwards the force of gravity 
is, as we have seen, lea , or decreases, as the 
square of the distance from the centre, above 
the surface, the force would be only one- 
fourth of what it is at the surface ; but below 
the surface the power decreases in such a 
manner, that its intensity is in the direct 
ratio of the distance from the centre ; so that 
at the distance of had a semidiameter from 
the centre, the force would be but half what 
it is at the surface; at one-third of a semidia- 
mctei the force would be one-third, and so on, 
6. As all bodies gravitate towards the 
earth, so does the earth equally gravitate to- 
wards all bodies ; as well as all bodies towards 
particular parts ot the earth, as hills. See. 
which has been proved by the attraction a hill 
has upon a plumb-line, insensibly drawing it 
aside. Hence the gravitating force of entire 
bodies consists ot that ot all their parts : for 
by adding or taking away any part of the 
matter of a body, its gravity is increased or 
decreased in the proportion of the quantity 
of such proportion to the whole mass. Hence 
also the gravitating powers of bodies, at the 
same distance from the centre, are propor- 
tional to the quantities of matter in the bodies. 
General or universal Gravity , is that by 
y. hu h all the planets tend to one another, and 
indeed by which all the b xlies and particles 
ol matter in the universe tend towards one 
another, 
1 he existence of the same principle of gra- 
v .lation in the superior regions of the heavens, 
as on t lie earth, is one of the great disco- 
veries ol Newton, who made the proof of it 
as easy as that on the earth. At first it would 
seem that this was only conjecture with him: 
he observed that all bodies near the earth, and 
in its atmosphere, had the property of lend- 
ing directly towards it ; he soon conjectured 
that h probably extended much higher than 
any distance to .w hich we could reach, or 
make experiments ; and so on, from one dis- 
tance to another, till he at length saw no rea- 
son why it might not extend as far as to the 
moon, by means of which she might be re- 
tained in her orbit as a stone in a sling is re- 
tained by the hand ; and it so, he next in- 
ferred, why might not a similar principle exist 
in tiie other great bodies in the universe, the 
sun and all the other planets, both primary 
and secondary, which might all be retained 
in their orbits, and perform their revolutions, 
by means of the same universal principle of 
gravitation. 
I hese conjectures he soon realized and 
verified by.mathematical proofs. Kepler had 
found out, by contemplating the motions of 
the planets about the sun, that the area de- 
scribed by a line connecting the sun and 
planet, as this revolved in its orbit, was al- 
• 
