536 THE POPULAR SCIENCE MONTHLY. 



follow this curved motion in the swimming of aquatic animals. The il- 

 lustration of Borelli (Fig. 8), shows only a single curve of the body. 



Prof. Owen, commenting on this figure, says the tail of the ani- 

 mal moving from a to i causes the centre of gravity to move forward, 

 and turns the head of the fish in the direction c to h. This implies 

 but a single curve of the body, whereas there are two, the one com- 

 plementary of the other, as is shown to be the case by Dr. Petti- 

 grew. He says : " Observation and experiment have convinced me 

 that when a fish swims it never throws its body into a single curve, 

 as represented in Borelli's figure, but always in a double or figure-of-8 

 curve, as shown in Fig. 9. 



Fig. 9. 



V^*£ 



Swimming op the Sturgeon. 



In swimming, the body of the fish describes a waved track along 

 a medial line. The two curves of the body act as fulcra for each 

 other, as occurs in the movement of a snake, by its usual lateral un- 

 dulations when swimming. In fishes and swimming reptiles of great 

 length, there may occur more than two curves, as four, but never 

 three, each curve having its complementary one. "The fish lashes its 

 tail from side to side, and a figure-of-8 track is formed by the move- 

 ment," both strokes propelling, but with unequal power during the 

 course of the stroke. There is a feature in this movement which is 

 equally obvious in the movement of the wings of birds in the air, that 

 is, a current is formed, as the tail is carried to one side, against which 

 the return-stroke is given ; thus, as the author observes, "the tail may 

 be said to work without slip, and to produce the precise kind of cur- 

 rents which afford it the greatest leverage." 



And this is true, whether the swing of the tail in swimming is 

 sideways, as with the salmon, or vertically, as occurs with the whale 

 or the porpoise. The fins act as balancers of the body, but the equi- 

 poise is impaired if they are injured ; and the removal of the tail, as 

 Owen observes, destroys the power of locomotion. 



The specific gravity of aquatic animals is nearly that of the water 

 they inhabit, or is made so in many cases by the gaseous contents of 

 their air-bladders. Nevertheless, momentum is an important element 

 in swimming. It originates in the movement of the fins and tail, and 

 not from throwing the body forward , as occurs in the initial move- 

 ment of walking. The momentum and velocity attained by some 



