280 Action of the Radiometer with both sides bright. 



ence, that a given displacement in the normal direction will 

 cause the atoms in a given superficial area to be spread over a 

 larger portion of convex surface than of concave surface, 

 the motion in both cases being from within to without, and 

 the lines of motion being divergent for the convex surface and 

 convergent for the concave surface. On this account there 

 will be, cmteris paribus, greater atomic density at the con- 

 cave surface than at the convex surface, and the latter will 

 be in the condition of the blackened surface of the ordi- 

 nary radiometer. Hence the cup-shaped radiometer will 

 move as if it were pushed on the convex side ; as is found ex- 

 perimentally to be the case. 



The experiments referred to showed that the motion of the 

 radiometer was in the same direction and nearly the same 

 in amount, whether the light fell only on the convex side, 

 or only on the concave side, and that in either case the rate of 

 revolution was about half what it is when the light falls on 

 both sides. These results may be considered to be consistent 

 with the theory, inasmuch as we may draw the inference from 

 the supposed thermo-electrical character of the disturbance, that 

 the state of displacement of the atoms induced by a disturb- 

 ance on one surface is spread equally over both by superficial 

 conduction, and consequently that the action is of the same 

 kind whether one surface or both be illumined. 



When this radiometer was heated by a hot shade or plunged 

 in hot water, rotation was caused in the opposite direction to 

 that caused by the light. Under either of these circum- 

 stances the superficial atomic density of a vane must be the 

 same throughout in order that its molecular forces may coun- 

 teract the effect upon it of the uniform surrounding tempera- 

 ture. But to satisfy this condition at the convex surface, not 

 only must the tendency of the induced heat to diminish the 

 superficial atomic density by expansion be counteracted, but 

 that also which is due to the divergence of the lines of motion 

 normal to the surface ; whereas at the concave surface the 

 convergence of the lines of motion helps to counteract the ten- 

 dency of the temperature to diminish the superficial atomic 

 density. Hence the molecular force from without to within 

 acts in greater degree at the convex surface than at the con- 

 cave surface. Hence also the interior atomic density dimi- 

 nishes in the direction from the convex to the concave surface, 

 because where the molecular force is greater, the atomic den- 

 sity must, cceteris paribus, be greater. Consequently, as the 

 greater atomic density is on the convex side, the greater velo- 

 city of current and less density of the aether is also on that 

 side, so that the vane is urged in the direction from the con- 



