MR. W. CROOKES OK REPULSION RESULTING FROM RADIATION. 
101 
413. The exhaustion was now carried to T8 
were taken :— 
Fig. 13 c. 
M, and the following observations 
Exhaustion 18 M. 
In position A (Fig. 13 c ), the speed was 53 revolutions a minute positive. 
In position E, the screen being 13 millims. from the convex side of the vanes, there 
was no rotation. 
In position F, the screen and vanes being 5 millims. apart, there was negative 
rotation of 4 revolutions a minute. 
In position B, the vanes and screens touching on the convex side, there was negati ve 
rotation at the rate 12 5 turns a minute. 
414. From these observations, which are in conformity with several hundreds of 
experiments tried at different distances and pressures, the following laws can be traced. 
a. When the screen touches the concave surface of the vanes, the rotation is always 
positive. It commences at a low exhaustion, increases in speed till the rarefaction is 
so high that an ordinary radiometer would begin to lose sensitiveness, and after¬ 
wards remains at about the same speed up to the highest rarefaction yet obtained. 
b. At any rarefaction after 87 M, there is a neutral position for the screen. When 
it is on the concave side of this neutral position the direction of rotation is positive, 
and when on the convex side of the neutral position the direction of rotation is 
negative. The speed of rotation is greater as the vanes are further removed from this 
neutral position on either side. 
c. The position of this neutral point varies with the degree of exhaustion. Thus, 
at 12 M the screens must be 3 millims. from the convex side ; at '18 M they must be 
13 millims. from the convex side. The higher the exhaustion the greater the distance 
which must separate the convex side of the hemi-cylinders and the screens. 
415. The various behaviours of these screened flies can be explained in the following 
manner. Fig. 14, A and B, represents the instrument in full sized plan, when the 
screen touches the convex surface of the hemi-cylinder. The radial black and dotted 
lines are supposed to represent the direction and extent of lines of pressure in a 
manner sufficient to enable the mind to follow the train of reasoning, but without 
implying that they indicate more than a very limited part of the whole action. It 
will be remembered that in this position, when the exhaustion is 117 M, the fly 
rotates positively (409), and that at an exhaustion of - 18 M, the fly rotates negatively 
(414). 
