BORELLI ON SWIMMING. 383 



of water it displaces. It must be admitted, that fishes are of the same 

 specific gravity as water, since they are observed to remain at rest in 

 any depth without any exertion made with the tail or fins. Hence it 

 happens, that fishes are supported better and easier in water than they 

 are upon land. The under parts, likewise, of fishes are not pressed by 

 the back and shoulder-blade above ; that is, they are not fatigued by 

 supporting their proper weight, as I have shown when treating of the 

 motions depending upon gravity, showing, in the first place, they do 

 not require feet, as land animals and birds do ; secondly, they are not 

 fatigued, nor do they feel any uneasiness, while stationary, because 

 their limbs being of the same equilibrium as water do not gravitate nor 

 press upon their under parts ; thirdly, their bodies can be formed of 

 greater bulk than land animals, as Galileo taught, because fishes are 

 not under the necessity of supporting their weight, and they employ no 

 pressive force on account of their specific gravity. But the manner in 

 which they preserve their equilibrium in water, depends upon the laws 

 of hydrostatics ; for nature has formed in the belly of fishes an air- 

 vessel, that the air it contains might counterbalance the excess of 

 weight of the muscles and bones ; by this resource the whole mass of 

 the fish, compounded of solids and the included air, is reduced to the 

 same specific gravity as the same volume of water. Besides the air 

 contained in the vessel, nature does not employ any instrument of 

 quicker effect ; for we observe, that oysters, and other shell fish, which 

 live always at the bottom of the sea, do not require such an air-vessel. 

 We have observed, in the Medical Experimental Academy, that a fish, 

 whose air-bag had been broken in a Torricellian vacuum, could not rise 

 in the water for a whole month, the time it was in the pond, but 

 crept along the bottom like a serpent. Fishes being of the same 

 specific gravity as water, can move in it in any direction, upwards and 

 downwards, and sideways, the water by its density and stillness oppo- 

 sing the impulse, and they move forwards the head and rest of the 

 body by an effort of the tail, and the force of the muscles connected 

 with it. Since they balance themselves in the water, provided their 

 whole mass is immersed, they can stop and rest in any place in the 

 water, whether above or below. 



As the equality of the specific gravity of fishes and the water in 

 which they swim is preserved, the weight and body of fishes must 

 necessarily remain in precisely the same degree, and the density and 

 gravity of the water also must not be changed, for otherwise the 

 equilibrium, which consists in indivisibility, would be destroyed, and 

 so the fishes will either sink to the bottom, if they have been rendered 



