92 



THE SEAS 



B 





Shape 



To the average person the word fish summons up in the 

 imagination a silvery, wriggling, slippery animal of a certain 

 definite torpedo shape. 



This characteristic spindle or torpedo shape is admirably 

 suited to the habits of most fishes. If we take, for example, 

 a fish hke the mackerel, we realize that not only is it beauti- 

 ful and elegant, but that it is ef&cient ; the fish is made to 

 swim and nature has made no mistake. Just as man has 

 learnt many lessons from the forms of birds to aid him in the 



designing of the most 

 ef&cient aeroplanes, so 

 can much be learned 

 from the mackerel about 

 the best shapes for rapid 

 motion through the 

 water. Its shape is 

 " stream-lined " ; that 

 is, it is adapted so 

 that in its progression 

 through the water the 

 least possible friction is 

 set up and it can cleave 

 its way without hindrance. If a square block of wood is 

 placed in a flowing stream there will be considerable 

 resistance on the front surface and at the same time many 

 eddies will be created just behind it, which act as a suction 

 and impede its passage through the water (Fig. 14). If 

 now the front surface be rounded off the water becomes 

 parted with a minimum of resistance ; the water can now be 

 seen to flow round the block in two streams which meet 

 again some little way behind. Between the back surface 

 of the block, the two surfaces of the divided stream, and 



Fig. 14. — A, B, and C, are diagrams showing 



the successive building up of an oblong block 



into a stream-line shape to reduce resistance 



caused by eddies. 



