380 



BORELLT ON SWIMMING. 



We observe that all terrestrial animals while alive, likewise birds, 

 and fishes which breathe and have lungs, are not wholly immerged 

 while in the water, but a part of them at pleasure remains above the 

 water without any effort of the muscles, these are of less specific gravity 

 than water, and they move in it as animals do upon land. But since 

 these animals are supported by their specific gravity, not by the hard- 

 ness of the water, these animals' motion in the water is performed 

 by the rowing of feet and arms, as ships or boats by oars. In this 

 manner terrestrial animals, birds, frogs, and tortoises, swim by one 

 large oar or tail violently moved sidewise as boats and fishes of the 

 whale species move directly upon the water ; or by impelling the 

 water upwards and downwards by the tail extended like a shovel, as 

 dolphins swim by rising and sinking in the water. 



All fishes, except oysters and a few other shell-fish, are of the same 

 specific gravity as water, and therefore can remain at any depth, and 

 move in any direction, whether upwards or downwards, sidewards or 

 backwards, as we shall afterwards explain. 



That we may be enabled to proceed further,, we premise the following 

 explanations. 



Any foreign body, whether at rest or in motion in a fluid, will 

 dispose itself so that its centre of gravity will occupy the lowest 

 place ; that is, will fall nearest the bottom. 



Fig. 1. 



v 











/ ¥ 



e /\ 

















1/ fL - 





\ 



. k 





Let a body, Fig. 1. be composed of lead pih and of thin and light 

 wood lift, whose lineal centre is b, and c the,; centre of gravity, be 

 immersed in water rst. First let the whole compound be of the 

 same specific gravity as water. It is evident that when covered wholly 

 with water, so as not to reach the bottom, it will remain at rest where 

 it is placed. I say, however, that it will revolve in such a situation, 

 until the heavier portion p, occupy the lowest place. 



