726 
PROF. O. REYNOLDS ON THE FORCES CAUSED BY THE 
square, fastened so as to stand vertically with a corner at the top. The distance of 
the centres of these plates from the axis is about f- of an inch. The 
plates are very thin, and are covered on one of their sides (which sides 
are all turned the same way) with lampblack. 
Descending from the top of the vessel is a small tube, the function 
of which is to keep the wheel from falling from its pivot when the 
instrument is turned over. The air within the mill has been greatly 
rarefied ; electricity will not pass ; but more than this I cannot say. 
The Action of the Light-Mill. 
When placed in the light the mill quickly arrives at its maximum 
speed, and rotates continuously with a velocity depending on the in- 
tensity of the light. It will rotate steadily at speeds varying from 
1 revolution in 6 minutes (in the light of the full moon) to 240 revolutions in a minute 
(in the strongest light I have been able to obtain). 
When the mill is revolving, and the light is suddenly extinguished, it rapidly comes 
to rest. 
These two facts, namely (1) that the mill rapidly arrives at its maximum velocity 
when the light is turned on, and (2) that it as rapidly comes to rest when the light is 
turned off, are those to which I wish first to direct attention, for they appear to me 
to prove conclusively that the air within the envelope does exercise influence on the mill. 
(1) If it were true, as has been supposed, that the best results are obtained in a 
vacuum so perfect that there is not sufficient air to exercise any influence on the vanes 
of the mill, then it follows that the mill would move without experiencing any resistance 
from the air, and the only known resistance would be the friction of the pivot. Now 
whether or not this is the case is easily ascertained. The resistance of the pivot, what- 
ever may be its magnitude, does not increase with the speed of the mill, and hence does 
not oppose a greater resistance to its motion when it is turning fast than when it is 
turning slowly. The friction of the air, oh the other hand, increases rapidly with the 
velocity. There is therefore a difference in the manner in which these two resistances 
will affect the motion of the mill. If the mill were only subject to the resistance of 
the pivot, any force which would start it would continue to turn it with increasing 
velocity as long as it acted ; whereas, when subject to the resistance of the air, the 
resistance increasing with the speed, the mill would soon arrive at such a speed that the 
resistance balanced the turning force ; after which the motion would be steady. This 
difference in the action of the friction of a pivot and that of the air is well known in 
mechanics and utilized, as, for instance, in the striking part of a clock. If prevented by 
nothing but the friction of the spindles when the clock is striking 12 say, each stroke 
would follow after a less interval than the previous one. Now the invariable means by 
which this is prevented is by a fan like the wheel in the light-mill, which, by the 
resistance it experiences in moving through the air, prevents the clock striking at more 
than a certain rate. 
