718 
DE. AETHUE SCHUSTEE ON THE FOECE PEODUCING- THE 
stancy of the position of rest, and from the deflection caused by the internal forces, that 
the existence of an external force equal to 5 per cent, of the internal force could not 
have escaped observation ; and had this force been even less than half that amount, it 
would most likely have been detected. We therefore have at present no evidence 
whatever of any force directly referable to radiation. 
The motion in the light-mill is wholly due to the forces acting between the revolving 
mill and its enclosure. It has not been the object of this investigation to find out what 
these forces are ; and I must leave it therefore to Professor Reynolds to show in how far 
the experiments agree with his theory of the phenomenon. I ought, however, to mention 
that on suggesting the experiments described above to him, he at once told me how the 
reaction would make itself apparent according to his theory. His predictions have been 
fulfilled throughout. It was also through his courtesy that I was enabled to work with 
the neat instrument made by Mr. Geissler, and I have had his valuable aid throughout 
this investigation. 
It is evident that the experiments agree in detail with the fact that two internal 
forces exist: — one, X 1? independent of the velocity; the other, —x x f{u), increasing as 
the velocity increases and acting in a direction opposite to that of X x . The force acting 
on the vessel is the reaction of the force which moves the mill, and is therefore 
expressed by 
-Xx+^/M. 
As long as which vanishes with u, increases, it has not yet arrived at the value 
given by the equation 
-X, +*,/>)=(). 
In that case X! is greater than *1 f{u) and the vessel will be impelled in the negative 
direction, that is, in the direction opposite to that in which the mill revolves. When 
the velocity of the mill has become constant, the force acting on the vessel is zero, and 
the vessel will therefore return to its position of rest. When the light is suddenly 
removed, X x is suddenly removed, and the only force acting on the vessel is 
*1 /(«*)• 
This force drives the vessel round in the opposite direction, that is, in the direction 
in which the mill is moving. As the velocity diminishes this force diminishes until it 
vanishes with the velocity. 
The fact that, on suddenly removing the light, the deflection of the vessel was never 
found as large as when the light was turned on, is easily explained. The mill acquires 
the maximum speed in a shorter time than that required to reduce the velocity to zero 
when the light is removed. The force acting up to the time of the first elongation is 
smaller when the light is removed than when it is turned on, and hence the smaller 
elongation. 
