546 Prof. G. G. Stokes on certain [Dec. 20, 



Let (o and r be the original and present angular velocity, and p the 

 equatorial component which is destroyed by friction. Then 



P°= tanl°; 



But 



therefore 



•=1-00015234. 



Hence the original length of the day was 

 86400 



1-00015234 



= 86386-8 seconds. 



Hence the formation of Asia and Europe had the effect of lengthening 

 the day by 13*2 seconds, no matter at what rate the formation took place. 



III. " On certain Movements of Radiometers." By G. G. Stokes, 

 M.A., Sec. R.S. Received November 1, 1877. 



Nearly two years ago Mr. Crookes was so good as to present me with 

 two of his beautiful radiometers of different constructions, the disks of one 

 being made of pith, and those of the other of roasted mica, in each case 

 blackened with lampblack on one face. With these I was enabled to make 

 some experiments, having relation to their apparently anomalous move- 

 ments under certain circumstances, w^hich were very interesting to myself, 

 although the facts are only such as have already presented themselves to 

 Mr. Crookes, either in the actual form in which I witnessed them, or in 

 one closely analogous, and have mostly been described by him. Although 

 it will be necessary for me to describe the actual experiments, which have- 

 all been repeated over and over again so as to make sure of the results, I 

 do not bring forward the facts as new. My object is rather to endeavour 

 to coordinate them, and point to the conclusions to which they appear to 

 lead. 



I do not pretend that these conclusions are established ; I am well 

 aware that they need to be further confronted with observation ; but as I 

 have not leisure to engage in a series of experiments which would demand 

 the expenditure of a good deal of time, and have lately been urged by a 

 friend to publish my views, I venture to lay them before the Royal Society, 

 in hopes that they may be of some use, even if only in the way of stimu- 

 lating inquiry. 



In describing my experiments I will designate that direction of rotation 

 in which the white face precedes as positive, and the reverse as negative. 



