14 



a loch in Scotland, and in these the fan of the tail is 

 little developed, yet they are strong swimmers. We must 

 consider, then, that a great portion of the after-part of a 

 salmon is part of the propeller, though the actual caudal 

 double fin is itself the best form imaginable for a propeller. 

 Mechanical difficulties arise which have so far made it 

 impossible to apply this form of propeller to ships ; the 

 screw, so far, has proved the most effectual, but it has the 

 fault of carrying round with itself a certain amount of water 

 which consequently creates a loss of power. The fish's 

 propeller does not do this, and consequently there is no slip 

 or loss of power. 



The framework of this marvellous structure consists of 

 a series of rays, tough and thick at the root, and split into 

 finer rays half-way. Over these is stretched a membrane at 

 once tough, pliant, and extremely thin, so that at its edge it 

 must be of a fineness that is difficult to conceive. This 

 fineness causes the tail to taper away into the body of the 

 water until it would be impossible to say where tail ended 

 and water began. And, further, over this membrane, as 

 over every part of the fish, a curious mucous coating is 

 supplied from glands in the head and along the lateral line 

 which makes the whole creature almost a part of the water 

 itself, especially when we know that the fish is about the 

 same weight as the water when alive, and lighter when 

 dead. 



A dead fish floats. 



The sinuous motion of the after-part of the fish is 

 scarcely noticeahle when a salmon or trout is rushing 

 at full speed through the water, but without it the 

 power necessary could not be transmitted. When moving 

 slowly it can easily be followed, but when speed must be 

 gained the nearer the body assumes a straight line the less 

 will the resistance be to forward progress. The tail's action 

 is not only lateral, but there is also a diagonal motion 

 imparted to the upper and lower ray which again gives 

 a wavy movement to the extreme edge of the covering 

 membrane. A salmon fighting for its life on a line exerts 

 an amount of muscular power that is amazing, for being 

 the same weight as the water in which it is swimming its 

 resistance by weight cannot be what, from its weight out of 

 water, it might be thought. 



