THE SWIMMING OF PISHES. 53 



to the quick strokes of the tail-fin behind, progressive 

 motion is wholly caused by the extension of the cur- 

 vature of the body, or, in other words, by the unbend- 

 ing of the bow, including, of course, the bending of the 

 tail as a portion thereof. The fore part of each cur- 

 vature, or bending, having more weight and propor- 

 tionally less surface than the hinder part, turns round 

 the point of support furnished by the last. 



The order of the motion is this : the fish extends the 

 tail towards g, so that while b, c, is turning round the 

 centre /;, it is not straight like the rays l of a circle, but 

 makes a sort of wavy sweep, the first d, e> being at the 

 same time contracted. When arrived at g, it pushes 

 backwards against the water, and is of course pushed 

 on in the direction of b, a ; by which motion it is 

 brought straight again at b, c,f; after which a similar 

 stroke is made on the other side at h. 



The fins of the breast and belly assist the fish in 

 maintaining the balance or level position of the body. 



(1) In Latin, Radii. 



