68 Royal Society : — 



example, and particularly in regard to the difficulties incident to 

 the treatment of " Vibrating Systems in general." He has done 

 this by his elaborate discussion of the case of a vibrating system 

 having one degree of freedom, which, as already mentioned, forms 

 the subject of Chapter 3, as well as by the discussion of the system 

 having two degrees of freedom at the end of Chapter 5. 



The complaint has often been made, and with regard to widely 

 different subjects, that, with a few conspicuous exceptions, our 

 best scholars and ablest men of science do not write — that they 

 content themselves with the pleasant task of acquiring knowledge, 

 and possibly of adding to it by means of brief memoirs which are 

 apt to be lost in the waste sea of the literature of Memoirs and 

 Periodicals. The last step between acquiring the knowledge, and 

 drawing up a formal statement of it for the benefit of others in- 

 volves labour which they decline to take*. The present volume is 

 a striking exception to the common practice. Its noble author 

 might well have considered himself absolved from the irksome labour 

 of writing a book, a task which he might have regarded as falling 

 more properly to the lot of the professional mathematician. "We 

 do not doubt that this consideration will add to the gratitude of 

 students, who will find in the work before us a means by which their 

 labours in this branch of science will be most materially lightened. 



XII. Proceedings of Learned Societies, 



ROYAL SOCIETY. 



[Continued from vol. iv. p. 395.] 



April 26, 1877.— Dr. J. Dalton Hooker, C.B., President, in the 



Chair. 



^HE following paper was read : — 



- 1 - " On Repulsion resulting from Radiation. — Preliminary Note 



on the Otheoscope." By William Crookes, F.R.S. &c. 



I communicated to the Royal Society in November last an 

 account of some radiometers which I had made with the object 

 of putting to experimental proof the " molecular pressure " theory 

 of the repulsion resulting from radiation. Continuing these re- 

 searches, I have constructed other instruments, in which a movable 



fly is caused to rotate by the molecular pressure generated on 



fixed parts of the apparatus. 



In the radiometer, the surface which produces the molecular 



disturbance is mounted on a fly, and is driven backwards by the 



excess of pressure between it and the sides of the containing vessel. 



Regarding the radiometer as a heat-engine, it is seen to be im- 



* "Not the least of the many benefits which he conferred -was the example 

 be set of unceasing labour ; for this was a permanent rebuke to that indolence 

 which is the besetting failing of the place — not the grosser form of aimless waste 

 of tiin'e, but the more seductive error which consists in the mere acquisition of 

 knowledge, which is never reproduced for the benefit of others." — Todhunter's 

 Account of the Writings of W. WhewelL vol. i. pp. 415, 416. 



