GRA 
GRAVING. See Engraving. In sea 
sffairs the word graving is used for the act 
of cleaning a ship’s bottom, when she is 
laid aground during the recfess of the tide. 
See Breaming and Careening. 
GRAVITY, a term used by physical 
writers to denote the cause by which all 
bodies move toward each other, unless pre- 
vented by some other force or obstacle. 
The most familiar effect, and that which 
continually obtrudes itself on our notice, is 
the weight of bodies, or their tendency to- 
ward the centre of the earth. It has not 
been ascertained, or rendered probable, that 
gravity is a secondary property of matter ; 
that is to say, that it flows from any of the 
other known original properties. Sir Isaac 
Newton, however, was of opinion, that our 
reasonings on the subject might be simpli- 
fied, by supposing it to depend on a pro- 
digiously elastic and rare fluid, by him called 
ether, and assumed to possess an increasing 
degree of condensation, in parts of space 
more and more remote from the various 
masses of matter. According to this doc- 
trine, a falling body moves, because it is 
pressed toward the rarer parts of this ex- 
tended fluid. We shall leave this theory to 
its merits, as being neither very perspi- 
cuous, nor much related to our subject. 
Bergman, and others, have considered the 
chemical and cohesive attractions to be one 
and the same with the attraction of gravity, 
but modified in its laws, by variations in the 
masses, densities, and distances of the par- 
ticles of bodies. Many difficulties appear 
at first sight to offer themselves against this 
supposition. But in truth it cannot be exa- 
mined at first sight; and requires to be 
submitted to the rigour of mathematical 
investigation, which has not yet been done. 
The phenomena of particular gravity, or 
that which respects the earth, or by which 
bodies descend or tend towards the centre 
of the earth, are as follows : 
1. All circumterrestrial bodies do here- 
by tend towards a point, which is either ac- 
curately, or very nearly, the centre of the 
magnitude of the terraqueous globe. ,Not 
that it is meant, that there is 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 parts of which the earth consists. 
2. In all places equidistant from the cen- 
tre of the earth, the force of gravity is 
nearly equal. Indeed all parts at the earth’s 
surface are not at equal distances from the 
earth’s centre, because the equatorial parts 
VOL. III. 
GRA 
are higher than the polar parts by afjouG 
seventeen miles ; as has been proved by 
the necessity of making the pendulum 
shorter in those places, before it will swihg 
seconds. In the new “ Petersburg Trans- 
actions,” vol. 6 and 7. M. Itrafft gives a 
formula for the proportion of gravity in dif- 
ferent latitudes On the earth’s surface, 
which is this : 
y = (i -j- 0.0052848 sine 2 xj g ; 
where g denotes the gravity fit the equator, 
and y the gravity under the other lati- 
tude x. 
3. Gravity equally affects all bodies, with- 
out regard either to their bulk, figure, or 
matter : so that abstracting from the resis- 
tance of the medium, the most compact and 
the most loose, the greatest and the small- 
est bodies would all descend through aa 
equal space in the same time, as appears 
from the quick descent of every light body 
in an exhausted receiver. The space which 
bodies do actually fall in vacuo, is 16,’j feet 
in the first second of time, in the latitude of 
London; and for other times, either greater 
or less than that, the spaces descended from 
rest, are directly proportional to the squares 
of the times, while the failing body is not 
far from the earth’s surface. 
4. This power is the greatest at the 
earth’s surface, from whence it decreases 
both upwards and downwards ; but not 
both ways in the same proportion ; for up- 
wards, the force of gravity is less, or de- 
creases as the square of the distance from 
the centre increases ; so that at a double dis- 
tance 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 sort, that its 
intensity is in the direct ratio of the dis- 
tance from the centre; so that at The dis- 
tance of half a semi-diameter from the cen- 
tre, the force would be but half what it is 
at the surface ; at one-third of a semi- dia- 
meter the force would be but one-third, 
and so on. 
5. As all bodies gravitate towards die 
earth, so does the earth gravitate towards 
all bodies; as well as all bodies towards 
particular parts of the earth, as hills, &c. 
which has been proved by the attraction a 
hill has upon a plumb line, insensibly draw- 
ing it aside. Hence the gravitating force 
of entire bodies, consists of those of ail 
their parts ; for, by adding or taking away 
any part of the matter of a body, its gra- 
vity is increased or decreased, in the pro- 
