1876.] Heat between a Surface and a Gas. 389 



Having obtained one of the beautiful little " light-mills " constructed 

 by Dr. Geissler, of Bonn, the author was in a position to make quantita- 

 tive measurements of the effects produced and of the force producing 

 them. 



In the first place, with regard to the sufficiency of the residual air to 

 cause the motion. It was found that this air is, with the exception of 

 the friction of the pivot, which is found to be so small as to be inappre- 

 ciable, the sole cause of the resistance which the mill experiences, of 

 the limit which is imposed on its speed for each intensity of light, and of 

 the rapidity with which it comes to rest when the light is removed. The 

 law of resistance, as determined by careful measurements, is found to 

 agree perfectly with the resistance which highly rarefied air would offer 

 to its motion ; and this law is distinctly special in its character, being 

 proportional to the velocity at low speeds, and gradually tending towards 

 the square of the velocity as the speed increases. 



Having established the fact that there is sufficient air in the mill (and 

 Mr. Crookes's behaves in the same manner as this mill) to balance, by its 

 resistance, the force which moves the mill, it is argued that all question 

 as to the sufficiency of the air to cause the forces is removed. What the 

 air can prevent it can cause. 



As regards the possibility of ihe motion being in any way the direct 

 result of radiation. This supposition the author had previously shown 

 to be directly contradicted by the fundamental law of motion that action 

 and reaction are equal. A cold body runs away from a hot body, while, 

 if free to move, the hot body will run after the cold body, showing that 

 the force does not act from body to body, but that each body propels 

 itself through the surrounding medium in a direction opposite to its 

 hottest side, the effect of one body on the other being due solely to the 

 disturbance which it causes in the equilibrium of temperature. 



The truth of this view was entirely confirmed by an experiment made 

 by Dr. Schuster, to be communicated to the Royal Society. Dr. Schuster, 

 by suspending the entire mill, was able to see whether the force which 

 causes the vanes of the mill to revolve caused any twisting force on the 

 envelope ; and he found that such twisting force, so far as it existed, 

 was exactly what must result from a force arising entirely within the 

 mill, i. e. between the vanes and the medium immediately surrounding 

 them. While the vanes were acquiring momentum a reaction was ex- 

 perienced by the envelope ; but when the vanes had acquired full speed, 

 the envelope was subjected to no force whatever ; when, however, the 

 light was turned off, the vanes, by virtue of the friction they experienced, 

 tended to drag the envelope with them. 



Besides proving that the force acts between the vanes of the mill and 

 the medium immediately surrounding them, Dr. Schuster's experiments 

 furnish a quantitative measure of the actual force. Taking the manner of 

 suspension and the weight of the mill into consideration, the effect pro- 



