MR. W. CROOKES ON REPULSION RESULTING FROM RADIATION. 
287 
Fie. 15. 
radiometers in succession. They all moved in the positive direction, except the bright 
aluminium radiometer, which remained stationary. When the funnel was removed, 
the two aluminium and the thick mica radiometers rotated positively till they were cold. 
294. The funnel was allowed to cool. • It was then inverted over a radiometer, and 
steam was passed through for a second or two. The same experiment was repeated 
with each radiometer. The results were now equally uniform with those of the last 
experiment, but the rotation was negative ; the bright aluminium fly moving the best 
of all, and the pith fly the least. 
295. Order now began to evolve itself from the mass of contradictions I had 
hitherto encountered. Let fig. 15 represent the funnel 
covering one of the radiometers ; and first let it be sup- 
posed that the funnel has been heated in hot water, and 
then inverted over the bulb. The funnel touches the 
bulb in a ring at a a, in a nearly equatorial position, and 
therefore this ring of the bulb gets hotter than any other 
part, as radiant heat from hot glass has much inferior 
warming power to actual contact with hot glass. Next, 
imagine the funnel to be cold, and steam passed in for a 
second or two. The upper portion of the funnel, a a b, 
together with the north pole portion (if I may use the 
expression) of the bulb gets warm, whilst the equator is 
cold. 
296. The rule appears to be that when the equatorial 
part of the bulb of the radiometer is warmed, positive 
rotation takes place, and when the polar portion is 
warmed negative rotation takes place, 
297. Further experiments soon showed that this law 
was likely to be the true one. The radiometers were 
dipped into hot water one-third of the way up the bulb. 
Strong negative rotation took place in each case. 
298. A thick brass ring was selected with an internal 
diameter about half that of the bulb, fig. 16, a. It was 
heated to about 400° C., and then put on the five radio- 
meters in succession at a. All moved in the negative 
direction at a tolerably good speed. 
On removing the hot ring from the two aluminium 
radiometers a reversal of movement took place and faint 
positive rotation was observed. With the thin mica 
and the pith radiometers no change of direction could 
be detected on cooling, the negative rotation keeping up 
for a considerable time. 
Fie. 10. 
