434 THE MECHANICAL FUNCTIONS. 



If we consider the density of the medium 

 which fishes have to traverse, the velocity with 

 which they move will appear surprising. They 

 dart through the water with apparently as much 

 ease and rapidity as a bird flies through the 

 air. Although this may partly be accounted for 

 by the size of their muscles, and the advan- 

 tageous mode of their insertion, yet these ad- 

 vantages would avail but little, were it not 

 for the sudden manner in which their power 

 is exerted. Where the great length and flexi- 

 bility of the spine tend to impair the force with 

 which the tail strikes the water, the resulting 

 motion is slow and desultory, as is the case with 

 eels, and other fishes of the same elongated 

 construction.* Most fishes, however, move with 

 the utmost rapidity, and with scarcely any 

 visible effort; and perform long journeys without 

 apparent fatigue. The Salmon has been known 

 to travel at the rate of sixteen miles an hour for 

 many days together. Sharks often follow ships 

 across the Atlantic, not only outstripping them 

 in their swiftest sailing, but playing round them 

 on every side, just as if the vessel were at rest. 



sequel, jhe remarkable conformation of the respiratory organs of 

 these and other fishes, which enable them to live for a time 

 out of their natural element. 



* Carlisle, Phil. Trans, for 1806, p. 9. 



