384 



EORKLLI ON SWIMMING. 



heavier than the water, or will float upon its surface, if their weight has 

 been" diminished. 



But such uniform constancy in the gravity of fishes, and in the den- 

 sity of water, cannot be preserved in all places and for any length of 

 time, because fishes themselves add to their weight by food, and lessen 

 it by excretion and transpiration. Besides, water is condensed and 

 rendered heavier by the mixture of salts, by the conturbation of mud, 

 by ambient cold, by the absence and occultation of the solar rays. And 

 the same 'water, on the contrary, is rarefied and made lighter by the 

 mixture of the fresh water of rivers and rains, by subterranean heat, 

 by the ambient air, and sunbeams. And these changes happen at dif- 

 ferent times. But some parts of the water may be gladdened by the 

 rays of the sun, whilst, at the same moment, others being overcast with 

 clouds, or shadowed by rocks, are not rarefied to an equal temperature. 

 Some parts of the sea may be sweetened by rivers, and others not. 

 Moreover, the upper parts of water are lighter than the deeper, because 

 the salts, and other earthy particles, slowly descending, render the 

 water at the bottom more turbid and feculent. 



By all these causes, therefore, the very equilibrium of fishes, which 

 consists in indivisibility, is disturbed ; and hence, provident nature has 

 bestowed on fishes that ready mechanical artifice, by which the inequa- 

 lity of weights may be reduced promptly and easily to the precise 

 equilibrium. As to this matter, among elementary bodies nothing 

 admits of greater expansion, or condensation, than air ; for experience 

 teaches, that five times the quantity of air is conveyed into the pneu- 

 matic receiver by the aid of a piston, and is there condensed, than is 

 usually contained in the same narrow space ; and in the Boylean ma- 



