The Air-Bladder 12 S 



billzer — an organ which would permit the fish to change 

 depth without trouble, and to remain at any level without 

 exertion. If the fish is to be at rest in the water, its weight 

 must be the same as that of the volume of water it occupies. 

 Technically speaking, it must have the same specific gravity, 

 the same weight-to-volume ratio, as the surrounding water. 

 It will then neither rise nor sink. Suppose that a fish is so 

 adjusted that this condition is met at a depth of, let us say, 

 twenty feet. If the fish rises ten feet, the pressure of water 

 on its exterior will decrease, and its body will, according to 

 their theory, expand. It will take up more space. The volume 

 of water it occupies increases. Since its weight is unchanged, 

 it will now weigh less than that water, and it will be unable 

 to stop rising until it floats at the surface. To avert this dan- 

 ger, the fish decreases the size of its air-bladder as it rises, 

 and thus keeps its volume constant in spite of the diminishing 

 pressure. The ratio of its weight to its volume remains un- 

 changed, and it is still stable at ten feet depth. 



When it descends to thirty feet, the reverse takes place. 

 The pressure of the water on its exterior increases as it goes 

 down, crushes it into a smaller space, diminishes its volume. 

 It takes up less space, but its weight is unchanged. It there- 

 fore weighs more than the volume of water it occupies, and 

 it would sink down and down with ever-increasing speed 

 until it struck bottom, if it did not inflate its air-bladder suffi- 

 ciently to keep its volume constant and thus maintain itself 

 in equilibrium with the water. Some scientists even went 

 further, and asserted that the fish could make active use of 

 the air-bladder to raise or lower itself. If it wanted to rise, 

 it pumped up its air-bladder, and up it popped. If it wanted 

 to sink, it deflated its air-bladder, and down it went. 



This theory is engaging, and has the merit of harboring 

 some truth. Certainly in fish in which there is an air-bladder 

 it must have a hydrostatic effect. In some this effect is far 



