180 



HYDRODYNAMICS. 



to c. If the layers happen to be broken in any place 

 below, the water finds its way up through the crevices 

 by the pressure of the head above, and forms springs. 

 If there are no openings through the rocks, deep borings 

 are sometimes made artificially, through wliich the wa- 

 ter is driven up to the siu-face, as at a, forming what 

 are termed Artesian Wells. The head of water which 

 supphes them may be many miles distant, the place 

 of discharge being on a lower level. It has sometimes 

 been found necessary to bore more than a thousand 

 feet downward before obtaining water which will flow 

 out fireely at the surface of the earth. 



SECTION II. 



DETERMINING THE PRESSURE ON GIVEN SURFACES. 



The pressure of liquids upon any given surface is 

 Pig 151 always exactly in proportion 



to the height, no matter what 

 the shape of the vessel may 

 be. If, for instance, the ves- 

 sel a {Fig. 151), be one inch 

 in diameter, and the vessel b 

 be three inches in diameter, 



