’GRAVITY. 
S/2 
motion to bodies., which in all bodies shall 
exactly follow the proportion of the quantity 
of matter in them? 
Mr. Cotes ^oes yet farther. Giving a view 
of Newton’s philosophy, he asserts that gra- 
vity is to be ranked among the primary qua- 
lities of all bodies; and deemed equally es- 
sential to matter as extension, mobility, or 
impenetrability. Praefat. ad Newt. Princip. 
But Newton himself disclaims this notion; 
and to shew that he does not take gravity to 
be essent al to bodies, he declares liis opi- 
nion of the cause ; choosing to propose it 
by way of query, not being yet sufficiently 
satisfied about its experiments. Thus, after 
having shewn that there is a medium in na- 
ture vastly more subtle than air, by whose 
vibrations sound -is propagated, by which 
light communicates heat to bodies, and by 
the different densities of which the refraction 
and reflection of light are performed ; he 
proceeds to enquire : “ Is not this medium 
much rarer within the dense bodies of the 
sun, stars, planets, and comets, than in the 
empty celestial spaces between them ? And 
in passing from them to greater distances, 
-doth it not grow denser and denser perpe- 
tually, and thereby cause the gravity of those 
great bodies towards one another, and of 
their parts towards the bodies ; every body 
♦endeavouring to recede from the denser parts 
of the medium towards the rarer? 
“ For if this medium be supposed rarer 
within the Sun’s body than at its surface, and 
rarer there than at the hundredth part of an 
inch from his body, and rarer there than at 
the fiftieth part of an inch from his body, 
and rarer there than at the orb of Saturn ; I 
-see no reason why the increase of density 
.should stop any where, and not rather be 
continued through all distances from the 
Sun to Saturn, and beyond. 
“ And though this increase of density may 
at great distances be exceeding slow ; yet if 
-the "elastic force of this medium be exceeding 
great, it may suffice to impel bodies from 
the denser parts of the medium towards the 
rarer with all that power which we call gra- 
vity. 
“ And that the elastic force of this medium 
is exceeding great, may be gathered from the 
swiftness of its vibrations. Sounds move 
about 1140 English feet in a second of time, 
and in seven' or eight minutes of time they 
move about 100 English miles: light moves 
from the Sun to us in about seven or eight 
minutes of time ; which distance is about 
70,000,000 English miles, supposing the ho- 
rizontal parallax of the Sun to be about 
twelve seconds ; and the vibrations, or pulses 
of this medium, that they may cause the al- 
ternate lits of easy transmission, and easy re- 
flection, must be swifter than light, and by 
-consequence above 7000000 times swifter 
than sounds ; and therefore the elastic force 
of this medium, in proportion to its density, 
anust be above 7000000 X 7000000 (that 
is, above 490,000,000,000) times greater than 
the elastic force of the air is in proportion to 
its density : for the velocities of the pulses of 
elastic mediums are in a subduplicate ratio of 
the elasticities and the rarities of the me- 
diums taken together. 
“ As magnetism is stronger in small load- 
stones than in great ones, in proportion to 
-their bulk ; and gravity is stronger on the 
surface of small planets than those of great 
ones, in proportion to their bulk; and small 
bodies are agitated much more by electric 
attraction than great ones: so the smallness 
of the rays of light may contribute very 
much to the power of the agent by which 
they are refracted ; and if any one should 
suppose that aether (like our air) may con- 
tain particles which endeavour to recede 
from one another (for I do not know what 
this aether is), and that its particles are ex- 
ceedingly smaller than those of air, or even 
than those of light ; the exceeding smallness 
of such particles may contribute to the great- 
ness of the force by which they recede from 
one another, and thereby make that medium 
exceedingly more rare and elastic than air, 
and of consequence exceedingly less able to 
resist the motions of projectiles, and exceed- 
ingly more able to press upon gross bodies 
by endeavouring to expand itself.” Optics, 
p. 32 5, &cc. 
Gravity;, in mechanics, denotes the tend- 
ency of bodies towards the centre of the 
earth. That part of mechanics which con- 
siders the equilibrium, or motion of bodies 
arising from gravity or weight, is particularly 
called statics. 
Gravity in this view is distinguished into 
absolute and relative. 
Absolute Gravity is that with which a 
body descends freely and perpendicularly 
through an unresisting medium. See Me- 
chanics. 
Relative Gravity is that with which a 
body descends on an inclined plane, or 
through a resisting medium, or as opposed by 
some other resistance. See Mechanics. 
Gravity, in hydrostatics. The laws of 
bodies gravitating in fluids make the business 
of hydrostatics. 
Gravity is here divided into absolute and 
specific. 
Absolute, or true Gravity, is the whole 
force with which the body tends downwards. 
Specific Gravity, is the relative, compa- 
rative, or apparent gravity in any body, in 
respect of that of an equal balk or magnitude 
of another body ; denoting that gravity or 
weight which is peculiar to each species or 
kind of body, and by which it is distinguished 
from all other kinds. 
In this sense a body is said to be specifi- 
cally heavier than another, when under the 
same bulk it contains a greater weight than 
that other ; and reciprocally the latter is said 
to be specifically lighter than the former. 
Thus, if there are two equal spheres, each 
one foot in diameter ; the one of lead, and 
the other of wood: since the leaden one is 
found heavier than the wooden one, it is said 
to be specifically, or in specie, heavier ; and 
the wooden one specifically lighter. 
This kind of gravity is by some called re- 
lative; in opposition to absolute gravity, 
which increases in proportion to the quantity 
or mass of the body. 
Laws of the specific gravity of bodies. 
I. If two bodies are equal in bulk, their spe- 
cific gravities are to each other as their 
weights, dr as their densities. 
II. If fwo bodies are of the same specific 
gravity ojr density, their absolute weights will 
be as their magnitudes or bulks. 
III. In .bodies of the same weight, the 
specific gravities are reciprocally as their 
bulks. 
IV. The specific gravities of all bodies are 
in a ralio compounded of the direct ratio of 
their weights and reciprocal ratio of their 
magnitudes. And hence again the specific 
gravities are as the densities. 
V. The absolute gravities or weights of 
bodies are in the compound ratio of their 
specific gravities and magnitudes or bulks. 
VI. The magnitudes of bodies are directly 
as their weights, and reciprocally as their 
specific gravities. 
VII. A body specifically heavier than a 
fluid, loses as much of its weight when im- 
mersed in it, as is equal to the weight of a 
quantity of the fluid of the same bulk or 
magnitude. 
Hence, since the specific gravities are as 
the absolute gravities under the same bulk ; 
the specific gravity of the fluid, will be to that 
of the body immerged, as the part of the 
weight lost by the solid, is to the whole 
weight.. 
And hence the specific gravities of fluids 
are as the weights lost by the same solid im- 
merged in them. 
VIII. To find the specific gravity of a fluid 
or of a solid. On one arm of a balance suspend 
a globe of lead by a fine thread, and to the 
other fasten an equal weight, which may just 
balance it in the open air. lmmerge the 
globe into the fluid, and observe what weight 
balances it then, and consequently what 
weight is lost, which is proportional to the 
specific gravity as above. And thus the pro- 
portion of the specific gravity of one fluid to 
another js determined by ’ immersing the 
globe successively in all the fluids, and ob- 
serving the weights lost in each, which will 
be the proportions ®f the spedlic gravities of 
the fluids sought. 
This same operation determines also the 
specific gravity of the solid immerged, whe- 
ther it is a globe, or of any other shape or 
bulk, supposing that .of the fluid known, for 
the specilic gravity of the fluid is to that of 
the solid, as the weight lost is to the whole 
weight. 
Hence also may be found the specific gra- 
vity of a body that is lighter than the fluid, 
as follows: 
IX. To find the specific gravity of & solid 
that is lighter than the fluid, as water , in 
which it is put. Annex to the lighter body 
another that is much heavier than the fluid, 
so that the compound mass may sink in the 
fluid. Weigh the heavier body and the 
compound mass separately, both in water 
and out of it ; then find how much each loses 
in water, by subtracting its weight in water 
from its weight in air ; and subtract the less 
of these remainders from the greater. 
Then, As this last remainder. 
Is to the weight of the light body in air. 
So is the specific gravity of the fluid. 
To the specific gravity of that body. 
X. The specific gravities of bodies of 
equal weight, are reciprocally proportional to 
the quantities of weight lost in the same fluid. 
And hence is found the ratio of the specilic 
gravities of solids, by weighing in the same 
fluids, masses of them that weigh equally in 
air, and noting the weights lost by each. 
The specific gravities of many kinds of bo- 
dies, both solid and fluid, have been deter- 
mined by various authors. It will be sufTi- 
