1-18 MR. W, CROOKES ON REPULSION RESULTING FROM RADIATION. 
444. The radiometer was floated in water contained in a large beaker, and a cover 
was placed over it to prevent interference from air currents. The screens were brought 
close to one side of the vanes, and the whole was exposed to sunlight. The force of 
repulsion at once overcame the power of the magnet, and the compound fly rotated 
rapidJy. A strong magnet was then brought near the bulb, the motion of the fly was 
arrested, and immediately the bulb rotated in the opposite direction, making about 
6 revolutions a minute. So strong was the force of rotation that when the cover 
was removed, and a paper arrow 3 feet long was cemented to the top of the bulb, as 
long as the sunshine lasted this paper index was swung round the room 3 times 
a minute. On tapping the screens to the other side of the vanes, rotation took place 
equally well, but in the opposite direction. 
445. The radiometer was brought into a dark room, and four candles placed near, 
the outside control magnet being in position. The bulb rotated about 3 times a 
minute. When its speed was uniform the control magnet was removed, liberating the 
fly, which then revolved under the influence of the internal pressure, the case and the 
vanes going opposite ways. After the bulb had made one-third of a revolution it 
stopped, and went back for about the same distance. Then it again stopped and made 
one-third of a revolution in the original direction. These oscillations kept on for a con¬ 
siderable time ; they seemed to get gradually less and less, but I did not observe that 
they stopped altogether. The motion of the bulb in the opposite direction to the fly 
was always more rapid than when it went with the fly. The movement of the fly 
kept up at a uniform speed as long as the experiment lasted. 
446. It would seem that this oscillatory movement was due in whole or in part to 
internal friction, either of the steel point on the glass socket or of the vanes against 
the residual gas, or to both these causes combined. To ascertain what power this 
friction possessed the candles were removed, and as soon as the whole instrument had 
come to rest, a bar magnet was moved alternately from one side of the radiometer to 
the other, so as to cause the fly to rotate as if it had been exposed to light. The fly 
rotated rapidly, and the internal friction carried the glass envelope round in the same 
direction at the rate of about I revolution in 3 minutes, in opposition to the friction of 
the water against its sides. 
447. The rotation of the envelope, the fly being fixed by a magnet, was also effected 
in another manner. Oblique mica vanes were fixed round the inner horizontal circum¬ 
ference of the bulb, and the movable fly, which carried a small magnet, was furnished 
with metallic vanes favourably presented (273), so that molecular pressure acting 
between them and the envelope should cause the fly to rotate. Inside the fly, coiled 
round the supporting glass stem, but not in contact with the fly, was a platinum wire 
spiral, the extremities of which were connected with thicker wires passing through 
and sealed into the glass bulb. By connecting these terminals with a battery the 
spiral was ignited, and the heat warming the metallic fly caused pressure to be 
exerted between the fly and the outer envelope, and produced rapid rotation. The 
