314 
MR. W. CROOKES ON REPULSION RESULTING FROM RADIATION. 
368. Pressure 388 millionths of an atmosphere. — No movement of the vanes on 
making contact. Negative rotation of the disk, at a speed of 1'25 revolutions a 
minute. Viscosity = 0‘103. Candle repulsion = 9‘3. 
369. Pressure 212 millionths of an atmosphere. — On first making contact the vanes 
and disk move in the same direction, the vanes going ^ revolution negatively, and the 
disk ^ revolution positively, thus . The vanes then remain still, but the disk 
rotates negatively, at a uniform speed of 1 revolution a minute. Viscosity 
= ’099. Candle repulsion = 127. 
370. Pressure 118 millionths of an atmosphere.— The vanes now rotate continuously 
in the positive direction at the rate of 1 9 revolutions a minute. The disk first 
rotates positively — i. e. , in the opposite direction to the vanes — then stops and rotates 
very slowly the same way as the vanes. Viscosity = '092. Candle repulsion = 24. 
371. Pressure 94 millionths of an atmosphere. — On igniting the wire, the general 
action is as last described, but the disk continues the preliminary positive rotation 
( TUP for a longer time before it takes up its negative rotation (SUP. The vanes rotate 
positively at a rate of 30 revolutions a minute. Viscosity= ‘089. Candle repulsion= 29. 
372. Pressure 59 millionths of an atmosphere. — The positive rotation of the disk 
now continues, at a uniform speed of 2‘5 revolutions a minute, without changing 
to negative, as in the last two cases. The vanes move positively, at a rate of 68 
revolutions a minute < §* ) . Viscosity = ‘080. Candle repulsion = 35 ‘5. 
When the disk and vanes are moving positively at a uniform speed, if the battery 
current is turned off so as to let the platinum ring cool, the positive motion of the 
disk soon stops, the vanes continuing to rotate. The disk now changes its direction 
and rotates negatively the same way as the vanes ; its velocity soon becomes equal to 
that of the vanes, and they then both rotate together as if fixed to the same axis. 
This change of direction in the disk and its subsequent following the vanes are due 
to the viscosity of the air causing the rapidly moving vanes to drag the disk round 
with it. 
373. Pressure 14 millionths of an atmosphere. — The positive rotations of both disk 
and vanes continue as in the last experiment, the speed increasing. The vanes rotate 
150 times a minute, and the disk 10 times a minute t S p. Viscosity = ‘044. Candle 
repulsion = 27. 
374. Pressure 11 millionths of an atmosphere. — The positive rotations are the same 
as before. The velocity of the vanes is about 600 revolutions a minute, but it is 
difficult to register accurately these high speeds. The disk rotates 12 times a minute. 
Viscosity = ‘039. Candle repulsion = 23‘5. 
At this pressure, when the battery is turned off, and the wire allowed to cool, the 
disk continues its positive movement for a considerable time. The viscosity of the 
rarefied air is now only ‘039, and the drag of the vanes on the disk takes longer to 
exert its influence than when the viscosity was ‘08. 
