Mr. W. Crookes on Repulsion resulting from Radiation. 231 



oblique flow is instigated by the abatement of pressure, or lower- 

 ing of free level, in the water along the inner bank produced by 

 centrifugal force in the way already explained. 



It may now be remarked that the considerations which have 

 in the present paper been adduced in respect to the mode of flow of 

 water round a bend of a river, by bringing under notice, conjointly, 

 the lowering of free level of the water at and near the inner bank, 

 and the raising of free level of the water at and near the outer bank 

 relatively to the free level of the water at middle of the stream, 

 and the effect of retardation of velocity in the layer flowing along 

 the bed of the channel in diminishing the centrifugal force in the 

 layer retarded, and so causing that retarded water, and also fric- 

 tion ally retarded water, even in a straight channel of approach to 

 the bend, to flow obliquely towards the inner bank, tend very mate- 

 rially to elucidate the subject of the mode of flow of water round 

 bends in pipes, and the manner in which bends cause augmentation 

 of frictional resistance in pipes, a subject in regard to which I believe 

 no good exposition has hitherto been published in any printed books 

 or papers ; but about which various views, mostly crude and mis- 

 leading, have been published from time to time, and are now often 

 repeated, but which, almost entirely, ought to be at once rejected. 



June 15. — Dr. J. Dalton Hooker, C.B., President, in the Chair. 



The following communication was read : — 



" On .Repulsion resulting from Radiation. Influence of the Resi- 

 dual Gas." — (Preliminary Notice.) By William Crookes, F.R.S. &c. 



I have recently been engaged in experiments which are likely 

 to throw much light on some obscure points in the theory of the 

 repulsion resulting from radiation. In these I have been mate- 

 rially assisted by Professor Stokes, both in original suggestions 

 ind in the mathematical formulae necessary for the reduction of 

 che results. Being prevented by other work from completing the 

 experiments sufficiently to bring them before the Royal Society 

 prior to the close of the session, I have thought that it might be 

 of interest were I to publish a short abstract of the princ pal 

 results I have obtained, reserving the details until they are 

 ready to be brought forward in a more complete form. 



In the early days of this research, when it was found that no 

 movement took place until the vacuum was so good as to be 

 almost beyond the powers of an ordinary air-pump to produce, 

 and that as the vacuum got more and more nearly absolute, so the 

 force increased in power, it was justifiable to assume that the action 

 would still take place when the minute trace of residual gas 

 which theoretical reasoning proved to be present was removed. 

 The first and most obvious explanation therefore was that the 

 repulsive force was directly due to radiation. Further con- 

 sideration, however, showed that the very best vacuum which I 

 had succeeded in producing might contain enough matter to offer 

 considerable resistance to motion. I have already pointed out 

 that in some experiments, where the rarefaction was pushed to a 

 very high point, the torsion-beam appeared to be swinging in a 



