HYDROSTATICS. 



INTRODUCTORY REMARKS. 



400. Hydrostatics is that part of Mechanics which treats of 

 the equilibrium of fluids, and that of solids immersed in them. 



A fluid is a collection of material particles, so constituted as 

 to yield to the smallest force employed to separate them. The 

 fluids with which nature presents us, approach more or less tp 

 this state of perfect fluidity. The adhesion which exists among 

 the particles of several of these substances, and which gives 

 rise to what is called viscidity, opposes itself to the separation 

 of the particles ; but in the theory which we are about to un- 

 fold, no account is taken of this adhesion, and we have reference 

 only to the perfect fluids. 



We distinguish two kinds of fluids ; the one incompressible, 

 or nearly so,t as water, mercury, alcohol, and liquids gener- 

 ally. These are capable of taking an infinite variety of forms 

 without any sensible change of bulk. The second kind of 

 fluids comprehends atmospheric air, the gases generally, and 

 vapours. These are in an eminent degree compressible ; they are 

 also endued with a perfect elasticity, and are thus capable of 

 changing at the same time their form and bulk, upon being com- 

 pressed, and of recovering their figure again, when the compress- 

 ing force is removed. Vapours differ from air and the gases, 

 by losing the form of elastic fluids and returning to the state 

 of liquids, when compressed to a certain degree, or when their 



t See note subjoined to this treatise on the compressibility of 

 water. 

 Mech. 37 



