428 THE MECHANICAL FUNCTIONS. 



of expanding and of closing the rays, like the 

 opening and shutting of a fan, according as their 

 action is required to be effective or the contrary. 

 All these auxiliary instruments are chiefly ser- 

 viceable in modifying the direction, and ad- 

 justing the variations of force derived from the 

 impulse of the tail. They are employed also in 

 suddenly checking or stopping the motion, and 

 giving it a more rapid acceleration. But still 

 the tail is the most powerful of the instruments 

 for progression, being at once a vigorous oar, an 

 accurate rudder, and a formidable weapon of 

 offence. 



Independently of these external instruments 

 of progression, most fishes are provided with in- 

 ternal means of changing their situation in the 

 water. The structure by which this effect is 

 accomplished is one of the most remarkable in- 

 stances that is met with of an express con- 

 trivance for a specific purpose, and of the em- 

 ployment of an agency of a class different from 

 that of the mechanical powers usually resorted 

 to for effecting the same object. We have seen 

 that if the body of a fish were heavier than an 

 equal bulk of water, and if no muscular exertions 

 were made, it must necessarily descend in that 

 fluid. If, on the contrary, it were specifically 

 lighter, it would as necessarily rise to the surface. 

 Were the animal to acquire the power of altering 

 at pleasure its specific gravity, it would then pos- 



