1 88 



HE. 



FLUIDS AT EEST AND IN MOTION: THE 

 MECHANICS OF THE CIRCULATION. 



CHAPTER XIX. 



HYDROSTATICS. 



THE department of physics which has regard to 

 the laws of force as applied to fluids is termed HYDRO- 

 DYNAMICS. This has two subdivisions, one of which 

 considers the laws applicable to fluids at rest and is 

 termed HYDROSTATICS, the other considers fluids in 

 motion and is termed HYDRO-KINETICS. 



A liquid is a body whose molecules attract one 

 another so feebly that a slight force suffices to displace 

 them relatively, to one another. Their cohesion, that is 

 to say, is slight Gases are also fluid, but differ from 

 liquids in this, that however the molecules of a liquid 

 be displaced relatively to one another, the distance 

 between the various molecules is always the same, 

 consequently the liquid does not expand, and main- 

 tains, as a rule, a constant volume, while the mole- 

 cules of gas vary in their distance from one another. 

 Gas is, therefore, expansible, and alters its volume 

 with every alteration of pressure. A liquid, therefore, 

 owing to the easy displacement of its molecules, alters 

 its form to suit any vessel in which it may be con- 

 tained. Experiments have shown that liquids are 

 almost, but not quite, incompressible. On removing 

 the pressure, however, a liquid returns to its original 

 volume and is thus perfectly elastic. The instrument 



