30ti Mr. G. J. Stoney on Crookes's Radiometer. 



hope, be intelligible to persons who have not made a ipeettl 

 study of the dynamical theory of gases ; and as the subject is 

 one not easily illustrated in this way, I hope the result of my 

 attempt will be deemed of sufficient permanent interest to de- 

 serve a place in your columns. 



I am, Gentlemen, 



Yours faithfully, 



G. Johnstone Stoney. 



[The numbering of the paragraphs is continued from the 

 paper on the same subject in the March Number of the Phi* 

 losophical Magazine. 



Readers of that article are requested to make the following 

 corrections in it : — 



In paragraph 3 (p. 178), instead of " I will assume that the 

 pressure within the chamber is 0*1 of a millim." read " not 

 much short of 0*1 of a millim." 



In paragraph 4, instead of "u\, ir 2 ," read " w x — w,' 2 ." 



In paragraph 7 (p. 180), instead of " ^L^ of an atmosphere," 



read " about ro^ooo of an atmosphere." 



In paragraph 9 (p. 181), instead of " We may further re- 

 gard" read " We may further without risk of serious error 

 regard."] 



10. When light is allowed to fall on a Crookes's radiome- 

 ter, there are two stages of the phenomenon to be noted and 

 carefully distinguished. 



11. The first is a brief period of adjustment which arises 

 when the light first falls on the blackened face of the disk. 

 During this stage the different parts of the attenuated air 

 within the little chamber assume the conditions which corre- 

 spond to the presence of the warm body. This adjustment is 

 made with great rapidity, and may occupy perhaps the 2 sVo 

 of a second. This is about the time that would be required if 

 this adjustment is made at a speed comparable with the velo- 

 city of sound ; and this is likely, since the velocity of sound is 

 the speed with which a somewhat analogous adjustment is 

 propagated in air. 



12. The second stage of the phenomenon is the permanent 

 state which supervenes when the period of adjustment is 

 over. And here there are two cases to be distinguished. 

 The first is that which occurs when the exhaustion is insuffi- 

 cient. In this case (leaving gravity, and consequently con- 

 vection currents, out of consideration for the present) a thin 

 layer of air in contact with the disk has been both warmed 

 and rarefied during the period of adjustment ; while beyonc 



