118 



HYDROSTATICS. 



FIG. 66. 



pressure is distributed among the 1000 molecules upon which it 

 presses. Owing to this freedom of 

 motion, these molecules will transmit 

 this pressure to those adjacent, and 

 these to those beyond, until every 

 molecule of water in the vessel exerts 

 a pressure equal to that exerted upon 

 any one of the molecules upon which 

 the pressure was originally exerted, 

 i. e.j every thousand molecules in the 

 vessel will exert a force of one pound. 

 Then will the 2000 molecules at 2 

 exert a force of two pounds and the 



3000 molecules at 3 will exert a force of three pounds. 



<, 219, An Important Principle. The foregoing 

 argument may be summed up as follows: When fluids 

 are subjected to pressure, the pressure sustained by 

 any part of the restraining surface is proportional 

 to its area. 



22O. Experimental Proof. The above principle, 

 which we deduced from Pascal's law, may be verified by ex- 

 periment. Provide two com- 

 municating tubes of unequal 

 sectional area. When water is 

 poured into these, it will stand 

 at the same height in both 

 tubes. If by means of a piston 

 the water in the smaller tube 

 be subjected to pressure, the 

 pressure will force the water 

 back into the larger tube and 

 raise its level there. To prevent 

 this result, a piston must be ' 



fitted to the larger tube and held there with a force as 

 many times greater than the force acting upon the other 



FIG. 67. 



