374 Prof. Challis's Theoretical Explanations of 



charged body instead of in the discharging wire ; for, taking the 

 model of a condenser of capacity C (= (««:)" *), and discharging it 

 through «i circuit of resistance p, the restoring force, at any instant 

 during the discharge when the cord is displaced / and is flying back 

 with velocity u, is 



there being of course no time for any slip between cord and but- 

 tons. Writing this thus, with A for the total mass of all the dis- 

 placed particles, 



and noticing that f=q, and that when t=0 #=Q and u = 0, we 

 obtain at once Thomson's fundamental equation, (5) p. 395, from 

 which all his results follow. 



In this paper I have abstained from mentioning current-induc- 

 tion, because I have not yet read Maxwell's second volume ; but 

 the model suggests ideas as to the nature of the process concerned 

 in producing the extra-current &c, which I expect will turn out 

 useful. 



XLY. Theoretical Explanations of Additional Phenomena of 

 the Radiometer. By Professor Challis, M.A., F.R.S., 

 F.R.A.S* 



AT the end of the "Theory of the Radiometer" which I 

 proposed in the Philosophical Magazine for May 1876, 

 I stated that I gave with reservation the explanation of the 

 rotation of the glass globe when it is floating in water, not 

 having then seen Mr. Crookes's communication " On the Move- 

 ment of the Glass Case of a Radiometer " contained in the 

 ' Proceedings of the Royal Society/ No. 168, p. 409. On 

 reading that communication, I found experimental proof that 

 the movement of the glass globe was indicative, as I had 

 inferred from theoretical considerations, of " friction be- 

 tween the glass support and the point on which the system 

 of vanes turns." This explanation has recently been con- 

 firmed by an experiment made by M. Jeannel (see Phil. Mag. 

 for October 1876, p. 320), who observed that the rate of rota- 

 tion was influenced by musical vibrations excited in the sur- 

 rounding air, and reasonably attributes this effect to momen- 

 tary suspensions of the friction between the pivot and its sup- 

 port caused by oscillations impressed on the instrument by the 

 aerial vibrations. 



The main purpose of the present communication is to take 

 into theoretical consideration some additional experiments of 

 * Communicated by the Author. 



