118 
FISHES. 
descend to the bottom ; and this would be the un- 
avoidable consequence, if the fish had not in his 
entrails a vessel filled with air, which enables him 
to suspend himself in what part of the water he 
pleases. This vessel swells the fish a little, and 
enlarges his natural dimensions, without making 
any addition to his weight : this is a circumstance 
that deserves a particular consideration; for by these 
means he takes up more space than he could pos- 
sibly fill without the vessel, and this brings him to 
an equilibrium with the mass of water whose place 
he occupies. We will suppose that the fish with- 
out this vessel weighs sixteen ounces, and that the 
water whose place he fills weighs no more than 
fifteen : the fish must in this case infallibly sink. 
But if we then place in the fish a little bag of air, 
which makes no sensible addition to the animal’s 
weight, but only enlarges its body, this will then 
possess more space. If the water, then, whose place 
he takes up should weigh sixteen ounces, the crea- 
ture is in an equilibrium with this quantity of the 
fluid, and will consequently be sustained in any 
part of the river where he may chance to find him- 
self. But it is not sufficient for the animal merely 
to remain suspended ; it is likewise necessary for 
him to rise occasionally to the top of the water, and 
sometimes to descend to the bottom : this he has 
the power of effecting by means of his abdominal 
muscles, with which he can compress the air- 
bladder, and thus diminish the bulk of his body, 
till his specific gravity becomes greater than that 
