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PEOF. 0. REYNOLDS ON THE FORCES CAUSED BY THE 
The limit which this resistance would impose on the speed would, at low speeds, be 
very simple ; the velocity would be proportional to the force causing it. 
If the light from each of two candles would cause the mill to turn with a certain 
velocity, then the two caudles acting together should cause the mill to turn with double 
velocity ; and this is exactly what happens, as the following Table shows : — 
Distance from the candles 
in feet. 
Number of revolutions per minute. 
1 candle. 
2 candles. 
2 
1-2 
3 
1 
5| 
11 
1 
23 
36 
I 
65 
120 
It will be seen that at very small velocities the effect of 2 candles is rather more than 
double that of 1 ; this is owing to the friction of the pivot, which is constant. 
Also at the higher velocities there is a falling off in the speed, exactly as might be 
expected from the air. Hence we see that the force which limits the speed of the mill, 
follows the same complicated law as that of the resistance which would result from the 
friction of the air ; and hence there cannot be a doubt but that they are the same. 
The Force which turns the Mill is not directly referable to Radiation. 
With reference to the assumption that the force is radiant or in any way directly refer- 
able to radiation, I pointed out at Bristol, before Section A (Brit. Assoc.), that in any such 
supposition the results of the experiments are directly opposed to one of the fundamental 
laws of motion, viz. that action and reaction are equal. In these experiments a hot 
body causes a cold body to recede, while a cold body causes the hot body to approach ; 
so that if both the bodies were free to move, we should have the cold body running 
away and the hot body running after it. This fact is, I take it, a conclusive proof that 
the force does not act from body to body, but between each body and the medium in 
which it is placed ; that each body, as it were, propels itself through the surrounding 
medium in a direction opposite to its hottest side. 
The truth of this reasoning has been set beyond all doubt by a very beautiful experiment 
made by Hr. Schuster. The results of this he is about to communicate to the Royal 
Society ; and as his paper will contain a full account of the experiment, it is only neces- 
sary here for me to refer to the results and the way in which they bear on the subject 
in hand. Hr. Schuster suspended my light-mill by a double fibre, so that if undisturbed 
by any torsional force it would hang with the vessel always turned in one direction, but 
in such delicate equilibrium that the smallest torsional force would cause it to take 
a fresh position. In this way he was enabled to ascertain whether the action of light 
on the vanes of the mill was attended with any effect to turn the envelope. 
Some such effect must have been caused whatever had been the nature of the force, 
either in the commencement or in the maintenance of the motion. 
