33 MAXWELL'S LAW 67 



ciple of the parallelogram into play, or, what comes to the 

 same thing, to subtracting the components of both velocities 

 from each other. 



The most general case can be at once deduced from this 

 very simple one. If the gaseous mass so moves that the 

 molar velocity is not everywhere the same, but in different 

 places is different in magnitude and direction, we have at 

 each particular place to subtract the velocity of flow at 

 that place (which is the same thing as the molar velocity), 

 and the remaining molecular motion will satisfy Maxwej^^ F 



law. s 



UNIVFBSIT 



34. Pressure of a G-as in Motion. 



If such a distribution of molecular velocities exists, the 

 different directions can no longer be looked upon as having 

 no distinction. The pressure, too, of a streaming gas will, 

 therefore, no longer be equally great in all directions. In 

 the direction of the flow the velocity and pressure will 

 be greater than in any other direction ; the pressure is 

 increased by the stress which the gas by its motion exerts 

 on a surface in its way. 



It is easy to calculate this increase of pressure if we 

 remember that, according to the kinetic theory, the pressure 

 consists *in a transference of momentum. In a gas at rest 

 this transference is effected by the to-and-fro motion of the 

 molecules. In a gas in motion there is an additional cause 

 in the velocity of flow by which not only momentum but 

 also mass is transferred. Through a surface F at right 

 angles to the direction of flow there passes in unit time a 

 volume Fa and a mass pFa, if a denotes the velocity. 

 This mass possesses the momentum 



pFa*. 



In consequence of the flow, therefore, the momentum 

 transferred in the direction of the flow increases in unit 

 time by 



Since, now, according to our theory, the pressure is 



