Chap, xix.] PASCAL'S LAW. 189 



by which the compressibility is measured is called a 

 piezometer. 



Transmission of pressure by liquids. 



Pascal's law. 



The law or principle, first enunciated by Pascal, is, 

 that in a liquid, pressure exerted upon any point of its 

 mass is transmitted equally in all directions ; and the 

 pressure is at all times perpendicular to the surface 

 on which it is exercised. Thus, suppose a mass of 

 liquid, pressed upon by a piston at A (Fig. 92). Suppose, 

 also, in the interior of the mass of liquid a 

 molecule M, one of the infinite number of 

 molecules of which, it may be conceived, 

 the liquid consists ; then if the molecule 

 M retains its equilibrium when pressure 

 is exerted at A, it must be because 

 the tendency of M to move is resisted in Fig. 92. 



T ,- -i .1 -i Transmission 



every direction by the pressure exerted O f Pressure, 

 upon it by the surrounding molecules. The 

 piston presses upon the molecules of the mass of liquid 

 in immediate contact with it; these, in turn, press 

 upon the neighbouring molecules, and thus the pres- 

 sure is transmitted to the walls of the vessel, which 

 re-act upon the molecules with a force equal to their 

 own. Thus the pressure exerted at A is transmitted 

 equally in all directions throughout the fluid, and 

 each molecule of the fluid is equally pressed in all 

 directions. 



An important application of this principle is illus- 

 trated in Fig. 93. A closed vessel of water ABCD has, 

 in its upper wall, an opening pq, in which is fitted a 

 piston P. A piston of the same size p' is fitted on one 

 side, and one of double the size p" on the other side. If 

 a force be exerted at p, by the law already announced 

 it is transmitted equally in all directions, and will, 

 consequently, act with undiminished strength upon p' 

 and P". If, therefore, P be pushed in with a given 



