DIRECTION OF PRESSURE. 3 



Fluids are divided into two classes, incompressible and 

 compressible. The former are sometimes called inelastic 

 and the latter elastic fluids. 



Incompressible fluids are those which retain the same 

 volume under a variable pressure. Compressible fluids are 

 those in which the volume is diminished as the pressure 

 upon it is increased, and increased as the pressure upon it 

 is diminished. 



The term incompressible cannot strictly be applied to 

 any body in nature, all being more or less compressible. 

 But on account of the enormous power required to change, 

 in any sensible degree, the volumes of liquids, they are 

 treated in most of the researches in hydrostatics as incom- 

 pressible or inelastic fluids. It was shown by Canton, in 

 1761, that water under a pressure of one atmosphere, i. e., 

 of about one ton on each square foot of surface, undergoes 

 a diminution of forty-four millionths of its total volume.* 

 All liquids are therefore regarded as incompressible. Water, 

 mercury, wine, etc., are generally ranged under this class. 

 The c/as'es are highly compressible, such as air and the dif- 

 ferent vapors. 



4. The Direction of the Pressure of a Fluid on 

 a Surface^ If an indefinitely thin plate be made to di- 

 vide a fluid in any direction, no resistance will be offered 

 to the motion of the plate in the direction of its plane, i. e., 

 there will be no tangential resistance of the nature of fric- 

 tion, such, for instance, as would be exerted if the plate were 

 pushed between two flat boards held close to each other. 

 Hence the following fundamental property of a fluid is 

 obtained from its definition : 



TJie pressure of a fluid is always normal to any sur- 

 face it'itlt irliich it is in contact. 



* Galbrai Ill's Hydrostatics; Gregory's Hydrostatics. 



The compressibility of water per atmosphere at 8 C., as given in Everett's 

 Units and Physical Constants, is 48.1 millionth*. Ency. Brit., Vol. XIL, p. 439. 



