HTDROSTA.TICS. 



125 



What great law 

 constitutes the 

 basis uf all the 

 mechanical 

 phenomena of 

 liquids ? 



290. Liquids transmit pressure equally in 

 all directions. 



This remarkable property constitutes a very characteristic; 

 distinction between sohds and liquids ; since solids transmit 

 pressure only in one direction, viz., in the line of the direction 



of the force acting upon them, while hquids press equally in all directions, 



upward, downward, and sideways. 



„. ,, In order to obtain a clear 



Ulustr.ite the 



eciuality of understandmg of the pnnci- 



Sid&"'^'° "^" P^® '^^ *^® equality of pressure 

 in liquids, let us suppose a 



vessel, Fig. 103, of any form, in the sides of 



■which are several tubular openings, ABC 



D E, each closed by a movable piston. If 



now we exert upon the top of the piston at 



A, a downward pressure of 20 pounds, this 



pressure will be communicated to the water, 



which will transmit it equally to the internal 



flice of all the other pistons, each of which 



will be forced outward with a pressure equal to 20 pounds, provided their 



surfaces in contact with the water are each equal to that of the first piston. 



But the same pressure exerted on the pistons is equally exerted upon all parts 



of the sides of the vessel, and therefore a pressure of 20 pounds upon a square 



inch of the surface of the piston A, will produce a pressure of 20 pounds upon 



every square inch of the interior of the surface of the vessel containing the 



liquid. 



Fig. 104. The same principle may 



also be shown by another 

 experiment. Suppose a 

 cylinder. Fig. 104, in which 

 a piston is fitted, to termi- 

 nate in a globe, upon the 

 sides of which are littlo 

 tubular openings. If tho 

 globe and the cylinder aro 

 filled with water, and tho 

 piston pressed down, tho 



liquid will jet out equally from all the orifices, and not solely from the ono 



■which is in a direct line with, and opposite to the piston. 



291. This property of transmitting pressure equally and 



ner^iay a^liq- freely in every direction, is one in virtue of which a liquid 



uid act as a becomes a machine, and can be made to receive, distribute, 



machine? , ^ i . i 



and apply power. Thus, if water be confined m a vessel, 



and a mechanical force exerted on any portion of it, this force wiU be at once 



transmitted throughout the entire mass of hquid. 



