k 232 Royal Society. 



As regards the possibility of the motion being iivany way the 

 direct result of radiation. This supposition the author had pre- 

 viously 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 sur- 

 rounding 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 experienced by the enve- 

 lope ; but when the vanes had acquired full speed, the envelope 

 was subjected to no force whatever ; when, however, the light w r as 

 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 produced showed that, when making 240 

 revolutions per minute, the torsional force on the vanes does not 

 exceed one forty millionth part of a pound acting on a lever a 

 foot long ; that the pressure of the gas on the vanes to produce 

 this was not more than one two million five hundred thousandth part 

 of a pound on the square inch, or one thousandth part of the pres- 

 sure in a Torricellian vacuum, thus "placing the extreme minute- 

 ness of the forces in a clear light, a light from which the extreme 

 delicacy of Mr. Crookes's instrument had altogether withdrawn 

 them. 



It is then shown, on theoretical grounds, that the difference of 

 temperature on the two sides of the vanes necessary to cause heat- 

 reactions of this magnitude could not be less than 1°*7 F., while 

 the probability is that it is considerably more. 



In order to apply this test and see how far the actual difference 

 of temperature in Dr. Schuster's experiments corresponded with that 

 deduced from the theory, a new photometer was devised by the 

 author with an immediate view of measuring the difference of tem- 

 perature caused by light on a black and a white surface. 



Of two thin glass globes, 2| inches in diameter, connected 

 by a siphon-tube § inch internal diameter, one was blackened 

 with lampblack on the inside over one hemisphere and the other 

 was whitened with chalk in a similar manner, the two clean faces 

 of the globes being turned in the same direction. Oil was put 



