382 



BOEELLI ON SWIMMING. 



were suspended from its centre b by a strong nail. Then the cord- 

 pendulum be rises, because the whole force of gravity pi is employed 

 and rests at the point c. 



The pendulum be, as its nature requires, will turn about the fixed 

 centre b, until the centre of gravity of the whole arrive at the lowest 

 point d perpendicular to the horizon. 



Hence it plainly follows, that in any foreign body, either at rest or 

 in motion in a fluid, the common centre of gravity is in the lowest part 

 of it, nearest the centre of the earth. 



As to the situation of the centre of gravity in animals swimming 

 upon the surface of water, since all land animals and birds, while 

 living, are of less specific gravity than water, it is evident that they 

 can swim in water ; and if wholly immerged in it, can rise at 

 pleasure (to the surface), so that some parts of the body will remain 

 above the surface. This takes place without any effort of the animal, 

 even although the feet are not moved by impelling the water under it. 



Provided we knew what part of the animal is the weightier, and in 

 what situation of its bulk the centre of gravity is, we should be certain 

 of the posture of the animal while it swims, because quadrupeds and 

 birds, in whatever manner they fall into the water, or are immerged in 

 it, or in whatever posture they are in it, yet always rise and emerge 

 from it with the belly downwards, and with the back and head extended 

 upwards. 



Therefore, their centre of gravity is in the middle of the belly, and 

 summit of the breast ; the back and head are of less specific gravity 

 than the rest of the body. But this is by no means the case in man, 

 for the head is of much greater weight, which, when under the water, 

 is raised with difficulty to the surface, unless the body be constantly 

 impelled by the hands and feet, which is performed by quadrupeds 

 without any effort ; their head rises above the water of its own accord. 

 Fishes of the whale species seem to have the centre of gravity in the 

 belly, and for this reason, they always swim, rest and sleep, upon the 

 water with the belly downwards and back upwards. Not so with sea 

 tortoises ; they sleep in the water with their belly upwards. Therefore, 

 their centre of gravity is situated in the back, the magnitude of the 

 shield with which it is covered leading to this belief. 



As to the manner in which fishes preserve their equilibrium in deep 

 water, it was the opinion of Archimedes, that no body under the sur- 

 face of water, and not supported by the bottom, can remain at rest, 

 unless of the same specific gravity as water ; namely, unless the abso- 

 lute weight of the body immersed be equal to the weight of the volume 



