GRAVITY. 
in her orbit, and prevented from running 
off in tangents, is the very power of gravity 
of the earth extended thither. See “ New- 
ton’s Principia,” lib. i. prop. -15, cor. 2, 
and lib. iii. prop. 3; where the numeral 
calculation may be seen at full length. 
The moon, therefore, gravitates towards 
the earth, and reciprocally the earth to- 
wards the moon, and this is also farther 
confirmed by the phenomena of the tides. 
The like reasoning may also be applied 
to the other planets. For as the revolu- 
tions of the primary planets round the sun, 
and those of the satellites of Jupiter and 
Saturn round their primaries, are phe- 
nomena of the same kind with the revolu- 
tion of the moon about the earth ; and as 
the centripetal .powers of tire primary are 
directed towards the centre of the sun, 
and those of the satellites towards the cen- 
tres of their primaries; and, lastly, as all 
these powers are reciprocally as the squares 
of the distances froni the centres, it may 
safely be concluded, that the power and 
cause are the same in all. Therefore, as 
the moon gravitates towards the earth, and 
the earth towards the moon, so do all 
the secondaries to their primaries, and 
these to the secondaries; and so also do the 
primaries to the sun, and the sun to the 
primaries. Newton’s Princip. lib. iii, pro p. 
4, 5, 6 ; Greg. Astron. lib. i. sect. 7, prop. 
46 and 47. 
The laws of universal gravity are the 
same as those of bodies gravitating towards 
the earth, before laid down. See Astro- 
nomy, Attraction, Geography. 
Gravity, specific. Boyle is among the 
first of our philosophers, who suggested the 
advantage that chemistry and mineralogy 
might derive from an attention to the spe- 
cific gravities of bodies. Much advantage 
may indeed be derived from this property 
in the general determination of the classes 
of minerals, and the purity of some metallic 
bodies ; and it is very probable, that an at- 
tention to the specific gravities, capacities 
for heat, fusibilities, volatilities, laws of 
crystallization, elasticity, hardness, tenacity, 
malleability, and some other obvious spe- 
cific properties of bodies, may produce a 
more intimate acquaintance with the mu- 
tual actions of their particles, than any we 
have hitherto acquired. 
Annexed to this article is a table of spe- 
cific gravities, from various authors. It 
appeared useless to carry it to more than 
four places of figures, as the temperatuues 
are not noted ; and the various specimens 
of the same substance often differ in the 
third figure. Besides this, it is remarked 
by Nicholson, in his “ Chemical Diction- 
ary,” that the fifth figure changes in water 
at every three degrees of Fahrenheit’s ther- 
mometer ; that lead, tin, and probably all 
other metals, though cast out of the same 
fusion, will vary in their specific gravities 
in the third figure, from circumstances not 
yet determined, but most likely from the 
cooling, as is seen in the hardening of steel ; 
that salts, and other artificial preparations, 
retain more or less of the solvent they were 
separated from, according to the tempera- 
ture at which the crystallization was ef- 
fected; and that all parts of organized sub- 
stances not only differ according to the 
place of their production, their age, and 
other circumstances, but likewise from their 
dryness, moisture, and manner of preserva- 
tion. 
The specific gravity of solids is determin- 
ed by weighing them first in air, and then 
in water. The loss of weight, arising from 
the action of the water, is equal to that of 
a mass of the fluid possessing the same di- 
mensions as the solid itself. Whence it is 
easy to construct a general table of specific 
gravities, by reducing the proportion of the 
absolute w'eight to the loss sustained by im- 
mersion, into terms of which that express- 
ing water shall be unity. If the solid be so 
light as to float upon water, it is convenient 
to attach to it a heavier body sufficient to 
cause it to sink, but the weight of which in 
water must be added in computing the loss. 
The specific gravity of fluids is ascertained 
by weighing a known body immersed in 
them. For the loss by immersion will ac- 
curately show the weight of the same bulk 
of the fluid; and, consequently, the propor- 
tion of these several quantities to the loss the 
same solid sustained in water, being reduced 
as in the other case to the common standard 
of unity, will exhibit the specific gravity. 
Other methods are likewise used in experi- 
ments with fluids. Thus equal bulks of dif- 
ferent fluids may be weighed by filling a 
small bottle with a ground stopper with each 
respectively, and from their several weights 
the weight of the bottle and stopper must 
be deducted. Or otherwise, the instrument 
called the hydrometer may be used. See 
Hydrometer. This possesses the advan- 
tage ot portability, speed, and a degree of 
accuracy not easily obtained by the use of 
ordinary balances. 
C c 2 
