40 PHYSICS. 
which is attached a solid c ibe, fitting exactly in the first one. Place the 
one in the other, and bring the balance: to a state of equilibrium by loading 
the opposite scale with weights ; suspend the solid cube beneath the hollow 
one, and allow the former to be immersed in the water, equilibrium will be 
disturbed, and the weight scale will sink; fill the hollow cube with water, 
and equilibrium will again be restored. 
A perfectly homogeneous body floats in a fluid when its weight is equal 
to that of the fluid displaced, and it may then assume any position ; if, how- 
ever, its centre of gravity do not coincide with that of the fluid displaced, 
it only floats when the two centres lie in one and the same vertical line ; 
the position, however, is fixed, only when the centre of gravity of the body 
is the lower of the two. Thus fishes float in water when they weigh as 
much as the water displaced; the equilibrium of their position, or the 
inferior situation of their belly, depends upon the air-bladder, and is so 
plaeed that the upper part of the fish is lighter than the lower. By means 
of the air-bladder, the fish can rise or sink in the water, floating at pleasure 
at any height, by its simple compression or expansion. As the fish cannot 
inspire air at pleasure, like an air-breathing animal, the bladder must con- 
tain a certain quantity of gas (consisting in most fishes of ;% oxygen and 
zy nitrogen), which is compressed more by the muscles than by the sur- 
rounding fluid. This muscular compression is, of course, voluntary on the 
part of the fish, and the compression or expansion of the bladder stands in 
intimate connexion with it. The apparatus (pl. 18, fig. 11) known as the 
Cartesian Devils, illustrates this condition of things. The devil is a hollow 
glass figure, b, in which there is a very small opening, generally in the point 
of the tail. The figure is filled with water just enough to make it float in a 
vessel filled with water. Cover the vessel with a bladder, and place it in. 
verted upon the stand, in which is placed a strong spring, e; then by the 
pressure of the spring, the air in the vessel is compressed, and the water 
driven into the inside of the figure, compressing the air already contained 
therein. The weight thus increased, the figure necessarily sinks to the 
bottom. Relax the pressure of the spring, and the air in the figure ex- 
panding again, forces out part of the water, thus allowing it to rise. Here 
the figure represents the bladder of the fish, and the pressure of the spring 
the muscular contractions exerted upon that organ. The gas in the blad- 
ders of fish, taken at a depth of about 3000 feet below the surface, sustains 
a pressure of almost 100 atmospheres. The expansion, when the fish rises 
to the top, is so great as sometimes to force the viscera out at the 
mouth. 
The determination of the specific gravity of bodies is a very important 
application of the law of Archimedes. Various forms of apparatus have 
been devised for this purpose ; a few only can here be mentioned. 
The hydrostatic balance ( pl. 18, fig. 12) used for this purpose, is a very 
accurate balance, such as is employed in chemical manipulation, and as will 
be described more fully under the head of chemistry. Any chemical 
balance may be employed for this purpose, by removing one scale-pan and 
substituting another, which, although of the same weight, is hung much 
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