Radiometer with both sides bright. 279 



ding communications contained in the Numbers of the ( Phi- 

 losophical Magazine ' for May and November 1876, and at 

 the same time departs in no respect from the principles of the 

 general hydrodynamical theory of the physical forces which I 

 have now for a long time upheld. 



Considering, first, the case of the radiometer with the vanes 

 blackened on one side, I assume that the radiant light or heat 

 which is incident on the vanes, being thereby converted into 

 heat of temperature, causes the atoms in a thin superficial 

 stratum to be displaced from their neutral positions, and in 

 greater degree on the blackened side than on the other, on 

 account of the greater accession of temperature on that side. 

 It is true that the intrinsic molecular forces of the vanes will 

 tend continually to make the atoms return to their neutral 

 positions ; but since at the same time the disturbing force is 

 continually in action, the result of the antagonistic forces, so 

 long as the disturbance is operative, will be a persistent ab- 

 normal condition of the superficial stratum. This is a real 

 change of condition of the vanes, by whatever name it be 

 called. It seems to me that it would be best described by 

 the term thermo-electric. But it is of chief importance to 

 remark, that every such superficial disturbance, however 

 caused, gives rise to a steady circulating cetherial current, in 

 which the pressure varies so as to be always less the greater 

 the velocity, and that consequently atoms immersed in such a 

 current will be dynamically acted upon by reason of the vari- 

 ation of pressure. In the present instance the course of the 

 current is from the blackened to the opposite surface, because, 

 on account of the greater expansion of the vane on the warmer 

 side, the channel for the current diminishes and the velocity 

 increases towards the bright side, and accordingly the dyna- 

 mical action is the same as if the vane were pushed on the 

 blackened side. [I have so fully discussed the above-cited 

 hydrodynamical proposition in previous communications, that 

 I do not think I am called upon to produce the demonstration 

 here.] 



Taking now the case of the cup-shaped radiometer (No. 1035), 

 we may assume that when the disturbing effect of the incident 

 radiant light or heat, converted into heat of temperature, is just 

 .co mterbalanced by the tendency of the superficial atoms to take 

 the positions in which they are ordinarily held by the molecular 

 forces, they will, as in the former case, be permanently dis- 

 placed from their neutral positions. But because both the 

 radiating surfaces are bright, the action will be the same on 

 both, so far as regards the amount of expansion caused by the 

 accession of temperature. There will, however, be the differ- 



