PROGRESSION ON AND IN THE WATER. 



67 



motion. According to this theory the tail, when flexed or 

 curved to make what is termed the back or non-effective 

 stroke, is forced away from the imaginary line, its curved, 

 concave, or biting surface being directed outwards. When, 

 on the other hand, the tail is extended to make what is termed 

 the effective or forward stroke, it is urged towards the ima- 

 ginary line, its convex or non-biting surface being directed 

 inwards (fig. 31). 



Fig. 31.— Swimming of the Fish.— (After Borelli.) 



When the tail strikes in the direction a % the head of the 

 fish is said to travel in the direction c h. When the tail 

 strikes in the direction g e, the head is said to travel in the 

 direction cb; these movements, when the tail is urged with 

 sufficient velocity, causing the body of the fish to move in 

 the line d c f. The explanation is apparently a satisfactory 

 one ; but a careful analysis of the swimming of the living fish 

 induces me to believe it is incorrect. According to this, the 

 commonly received view, the tail would experience a greater 

 degree of resistance during the back stroke, i.e. when it is 

 flexed and carried away from the axis of motion (d c f) than 

 it would during the forward stroke, or when it is extended 

 and carried towards the axis of motion. This follows, because 

 the concave surface of the tail is applied to the water during 

 what is termed the back or non-effective stroke, and the con- 

 vex surface during what is termed the forward or effective 

 stroke. This is just the opposite of what actually happens, 

 and led Sir John Lubbock to declare that there was a period 

 in which the action of the tail dragged the fish backwards, 

 which, of course, is erroneous. There is this further difficulty. 

 When the tail of the fish is urged in the direction g e, the 



