neighbourhood of strongly Illuminated Bodies, 237 



a bombardment up towards the maximum- velocity layer from 

 either side. 



Calling the three components of the pressure p x , p y p z) it 

 can be shown that 



Px=pu 2 , 



p 3 =pv*, 



and p 2 z=pw 2 =p, the proper pressure of the gas. 



Hence 



p x = P (T-cf) 



dp x 



and -p?_ which determines the intensity of the differential 



dx 7 



molecular bombardment per unit distance along the normal 

 to the surface, is proportional to the original pressure and to 



the rate of change of ^-. 



Remember that (p is the up-streaming velocity, and T the 

 temperature, of the different layers. In a region where (j> is 

 increasing and T is decreasing, the differential bombardment is 

 considerable, and it acts in the direction of increasing <£. In 

 a region where <£ and T are both increasing or both decreasing 

 the bombardment is likely to be feeble, and may be nil if <j> 

 varies with the root of T. 



Now outside a warm plate we have near the surface (j> in- 

 creasing and T decreasing as you go outwards : consequently 

 here is an outward bombardment. Further on both are de- 

 creasing, so the bombardment is feeble or nil, and one cannot 

 say without further consideration which way, if any, it ought 

 to act. 



Close to the surface of a cold body proceeding outwards cf> 

 is increasing, but T is also increasing : consequently there is 

 here feeble bombardment. Further on </> is decreasing and T 

 still increasing ; consequently here is a bombardment inward 

 towards the body. 



Representing these things diagrammatically, the arrow re- 

 presents the direction of the bombardment when it is decided, 

 the dotted line the maximum-velocity layers : — 



