Theory of Crookes's Force. 21 



dent of cj), as the effect is evidently symmetrical with regard 

 to X. Then we get 



b 2 = b 3 = b±=b 5 = = c 2 = c 3 , 



and there are no tangential forces, while all the heat is trans- 

 ferred in the direction X, and onr pressures become 







while the heat transferred is 

 Qs=§MN»fr. 



The excess of pressure in X over that in the normal directions 

 is 



and this has been called Crookes's force. 



That it depends wholly upon \ can be seen by the following 

 simple method, mentioned to me by Mr. Stoney. 



Our expressions for P^ and Y yy are 



P^=M2mry ; 



P yy =MW(l-/A 2 )sin 2 0; 



so that, calling 



n= j- Id/ubdcf), 



when I depends upon the distribution of numbers only, we can 

 write the pressures 



p »= S Ifa'd -^) ***** #• 



We can integrate them with respect to <£ ; for we know that 

 Iv 2 is independent of <£ in the case we are considering; 



