388 FISHES 



at any depth when the fish is alive and in normal condition is 

 to render the weight of the fish equal to that of the water which 

 it displaces : in these circumstances the fish has no tendency 

 either to rise or sink, and can therefore keep at the same level 

 in the water without any muscular exertion. A shark or dog- 

 fish, on the other hand, having no bladder is heavier than the 

 water and always has a tendency to sink ; it can only keep up 

 by the action of its tail and fins. If, however, the fish with an 

 air-bladder swims to a higher level, the gas in the bladder 

 expands, the fish as a whole becomes larger and therefore 

 lighter than the water it displaces, and has a tendency to rise 

 more and more rapidly till it reaches the surface. Con- 

 versely, if a fish with an air-bladder swims to a slightly greater 

 depth the bladder is compressed, the fish as a whole becomes 

 smaller and heavier than the water displaced, and tends to 

 sink with increasing velocity to the bottom. Such a fish in 

 fact is in the condition of the scientific toy known as the Car- 

 tesian diver. This consists of a hollow figure containing a 

 bubble of air which cannot escape because the aperture is at 

 the lower end. The figure is placed in a tall jar of water over 

 the mouth of which is fastened an air-tight cover of india- 

 rubber or other flexible membrane. If the figure is so adjusted 

 that it floats just at the surface of the jar, very slight pressure 

 on the cover causes it to sink to the bottom and when the 

 pressure is removed the diver rises again. It is easy by adjust- 

 ing the pressure of the finger on the membrane to keep the 

 figure in the middle of the depth of water. The pressure is 

 transmitted to the air within the figure and compresses it so 

 that it displaces less water and therefore the figure and the air 

 together become heavier than the water. It is evident then 

 that the air-bladder confines the fish to a very restricted range 

 of depth, and it would be liable to float to the surface or to 

 sink to the bottom at the slightest movement if it were not 

 able to counteract the effect of changes of pressure by muscular 

 compression or relaxation of the bladder or by increasing or 

 diminishing the amount of gas in the bladder. This power, 

 however, it can only exercise within narrow limits, and if sud- 

 denly brought to the surface it is quite unable to descend again. 

 Fish caught to be kept alive in an aquarium are often, though 

 otherwise uninjured, in this condition and float helpless at the 



