Intelligence and Miscellaneous Articles. 395 



ticular form assumed by the worn stone. Where veins of harder 

 material occur in the stones, these are left projecting from the 

 surface, and are sometimes even undercut. 



2. " The Bone-caves of Creswell Crags."— Third Paper. By the 

 Rev. J. Magena Hello, M.A., F.G.S. 



3. " On the Mammal-fauna of the Caves of Creswell Crags." By 

 Prof. W. Boyd Dawkins, M.A., F.R.S., F.G.S. 



LIV. Intelligence and Miscellaneous Articles. 



SUPPLEMENT TO A THEORY OF THE CUP-SHAPED RADIOMETER, 

 BY PROFESSOR CHALLIS, M.A., F.R.S. 



To the Editors of the Philosophical Magazine and Journal. 

 Gentlemen, 

 T BEG- to be allowed to make a few supplementary remarks re- 

 -*- lative to the Theory of the action of the cup-shaped radiometer 

 which I proposed in the April Number (p. 278). On further con- 

 sideration of the problem, it seems to me that this form of the 

 experiment is specially adapted to reveal the character of the forces 

 concerned in producing the observed movements. In that theory 

 it is assumed that the radiant light or heat incident on the vanes 

 is converted into heat of temperature, whereby the atoms in a very 

 thin superficial stratum are displaced from their neutral positions. 

 The only particular relative to this view that I have occasion to 

 advert to is the assertion (in p. 280) that " the state of displace- 

 ment of the atoms induced by a disturbance on one surface is 

 spread equally over both by superficial conduction." It now appears 

 to me that since the light is equally incident on all parts both of 

 the convex and the concave bright surfaces, there is no reason to 

 attribute to the accession of temperature any other effect than an 

 expansive force acting in directions normal to the surface, and 

 causing the superficial atoms of the concave surface to be more 

 crowded, and those of the convex surface to be less crowded, than 

 in their ordinary neutral state. As it may well be admitted that 

 when two substances are rubbed together, their superficial atoms 

 are displaced in like manner, and as it is known from experience 

 that friction induces in substances the power of attracting light 

 bodies, it might reasonably be inferred that the vanes of the 

 radiometer in motion are capable of similarly attracting. That 

 this is actually the case is proved by an experiment which showed 

 that a pith ball suspended in the neighbourhood of the revolving 

 vanes is attracted (see No. 175 of the ' Proceedings of the Royal 

 Society,' p. 313, and the theory of this attraction which I have 

 given in the Number of the Philosophical Magazine for November 

 1876). 



The compression of the superficial atoms on the concave surface 

 of the cup-shaped radiometer, and the dilatation of those on the 

 convex surface, would, according to what is said above, be pro- 



