448 SCIENCE PROGRESS 



conical head to its widest and deepest part in front of the 

 middle of the trunk. Thence it tapers off gradually, and 

 finishes in a filmy tail fin. 



The tail with its fin is the sole propelling organ of the 

 fish. It usually occupies at least half the length of the body but 

 in some, such as the dogfish, it is proportionally much longer. 

 The mechanics of progression are briefly as follows : starting 

 with the tail already bent to the right, it begins to make a stroke 

 towards the left. The water against which it strikes offers a 

 resistance such as would be offered by a frictionless inclined 

 plane. In fig. 6 the pressure (a) of this plane against the fish 

 has two components (b, c), one of which sends the fish forwards. 

 As the fish glides on, the tail passes over to the left of the 

 line of progress in such a manner (fig. 7) that by the time the 

 stroke is finished it is already in position for the return stroke. 

 These points can be detected more easily in long, lithe types 

 such as the dogfish than in short, stiff ones Jike the goldfish. 



Which way will a fish strike its tail when it is about to 

 turn to the right ? This is not a question which can be 

 answered off-hand, but it can soon be settled by means of a 

 simple experiment. Cut a thin strip of celluloid to the same 

 shape and size as the fish from its head backwards. Weight 

 this strip so that it will neither rise nor sink in water, and 

 then tie it to one side of the body, say the right. The tail is 

 now free to strike only in one direction, viz. towards the left, 

 and it will now be found that the fish can turn only towards 

 the left. This experiment repeated for the other side gives 

 similar results. The stroke of the tail gives the body such a 

 bend as is shown in fig. 8. The swirl produced behind this 

 tends to suck the tail back. For the time being the whole 

 body is kept rigid, with the result that the snout swings round 

 like the bow of a boat. The fish thus makes a sharp turn 

 by rotating on a vertical axis or fulcrum. 



The above description applies more particularly to the bony 

 fishes in which the front part of the body is stiff and practically 

 rigid. In lithe fishes such as the dogfish and sturgeon rapid 

 darts are not indulged in and the fish changes its course by 

 simply gliding round a curve. 



Now we may turn to the consideration of the fins, taking 

 first those which are situated in the middle line. Of these 

 there are usually two above — these are the dorsal fins; one 



