HYDROSTATICS. 
the two planks together ; but if the smallest 
opening be given, the pressure of the at- 
mosphere will urge the fluid between them, 
and, by confining it to act as a wedge, force 
the upper one to the surface. The compa- 
rative, weights of fluids are ascertained by 
the Hydrometer, which see. 
The comparative weight of fluids is given 
with the table of specific gravities, (see 
Gravity, specific) ; but it may. be as well 
to point out in this place, that a gallon of 
proof spirit weighs 7 lb. 12 oz. avoirdu- 
poise. 
If a vessel contain two immiscible fluids 
(such as water and mercury), and a solid of 
some intermediate gravity be immersed un- 
der the surface of the lighter fluid, and 
float on the heavier, the part of the solid 
immersed in the latter will be to the whole 
solid as the difference between the specific 
gravities of the solid and of the lighter fluid 
is to the difference between the specific 
gravities of the two fluids. For a body im- 
mersed in a fluid will, when left to itself, 
sink, if its specific gravity be greater than 
that of the fluid; if less it will rise to the 
surface : if the gravities be equal, the body 
will remain in whatever part of the fluid it 
may be placed. But in the case adverted 
to, the one fluid being heavier and the other 
lighter than the body immersed, it is neces- 
sary to combine their gravities by the mode 
above shown. 
Balloons are properly hydrostatic ma- 
chines, and derive their property of ascend- 
ing from the earth into the upper part of 
our atmosphere entirely to the difference 
between the specific gravity of the air, or 
gas, with which they are filled, and the exte- 
rior, or atmospheric, air in which they float. 
The weight of the materials must be taken 
into consideration; for unless the specific 
gravity of the interior be so much less than 
that of the exterior air, as to allow for the 
weight of the materials as a counterpoise, 
the balloon cannot be made to float even in 
a stationary manner; but when liberated 
will fall to the ground. The contents of 
the balloon being ascertained in cubic feet, 
it will be easy to ascertain what weight the 
balloon can lift when filled with ratified 
air, according as that may have been ren- 
dered more light than the atmospheric air: 
if filled with gas, the interior will be at least 
seven times lighter than an equal quantity 
of atmospheric air. From this it will be 
seen, that to bear up a weight of 300 tb. the 
balloon must be large, and the specific gra- 
vity of its contents be adequate to overcome 
the resistance of that impediment. As the 
air of the upper part of our atmosphere be- 
comes gradually more rare, and consequent- 
ly lighter, according to its distance from 
the earth’s surface, we may conclude, that 
there is a point in its altitude beyond which 
a balloon could not soar; because its own 
weight, even if nothing were appended, 
would at such a point perfectly equipoise 
the difference between the confined gas 
and the surrounding atmosphere. And this 
is the more perfectly to be admitted, from 
the knowledge we have acquired of the dif- 
ficulty with which balloons are made to 
reach certain heights; and of their ascent 
being shown (by the slower fall of the mer- 
cury within the barometer) to be far slower 
in the upper regions when they approach 
that state of equipoise. Were it not for the 
opposition offered by the superior air, a 
balloon would rise instantaneously from the 
moment of its liberation, in a most rapid 
manner, to that height where its equipoise 
should be found. We have said thus much 
in explanation of the nature of the balloon, 
as appertaining to the laws of hydrostatics, 
referring the reader to the article Aeros- 
tation, for whatever appertains to the 
practical experience we have had of that 
science, which at first seemed to promise 
the most important aid to various others, 
but in which it has completely failed: the 
whole of the principles on which aerostation 
depends have been long understood. 
We shall now speak of the diving-bell, 
which also depends on hydrostatic prin- 
ciples, though, like the balloon, it has a 
close connection with pneumatics. The 
upper part of a diving-bell is always made 
to contain a certain quantity of air, more or 
less compressed in proportion to the depth 
to which the bell sinks. Thus, if we invert 
a small tumbler into a vessel nearly filled 
with water, and allow it to descend per- 
pendicularly, so that no air may be allowed 
to escape, the water will rise a very little 
way within it. If the tumbler be but par- 
tially immersed, the water could at the ut- 
most but rise to its own level ; but if im- 
mersed so deep as to exceed its own inte- 
rior, and that the bottom edge of the tumbler 
does not touch the bottom of the vessel, the 
water will, in consequence of its own greater 
weight at a greater depth, rise rather, 
though scarce perceptibly, higher in the 
tumbler, and occasion the air to be com- 
pressed into a smaller space. But the quan- 
tity of vital principle in the compressed air 
will be equal to that quantity of air in the 
