120 
MR, W. CROOKES ON REPULSION RESULTING FROM RADIATION. 
black mica screen, the cups being unexposed, the fly is driven rapidly round in one 
direction or the other, according to which black side is illuminated, the cups going with 
almost equal readiness whether the convex retreats, as in an ordinary cup radiometer, 
or the concave retreats, as in Robinson’s anemometer. "When the light shines on both 
screen and cups the rotation of the fly is always positive ; but the speed is much 
greater when the molecular pressure from the black screen conspires with, than when 
it opposes the positive motion of the fly. 
449. It was suggested to me by Professor Stokes that it might be possible to get 
rotation in a radiometer with a perfectly flat fly, alike on both sides, by throwing the 
obliquity from off the fly on to the case. Following out the suggestion, three vertical 
partitions of thin clear mica were fixed in the bulb of a radiometer, with their planes 
not passing through the axis of rotation, but inclined, as shown in fig. 24, and slightly 
Fig. 24. 
cut away to let the fly pass closely. The fly had four aluminium vanes, polished alike 
on both sides. Candles at a, b, c make the fly revolve rapidly in the direction of the 
arrow. Indeed, one candle is sufficient to produce rotation. Breathing gently on the 
bulb causes negative rotation. A hot glass shade inverted over the instrument causes 
strong negative rotation, changing to positive on cooling. When the fly is furnished 
with clear mica, or with silver-flake mica vanes, the same results are obtained as when 
aluminium vanes are employed. The strongest action is produced by warming the 
bulb. 
THE OTHEOSCOPE. 
450. The experiments on the rotation of the envelope (442 to 447) show that the 
surface generating the molecular pressure need not be the one which rotates. The 
apparatus shown in fig. 23, as well as the one described in a former paper (360), prove 
that it is an advantage to have the driving surface stationary. In the radiometer the 
surface which produces the molecular disturbance is mounted on a fly, and is itself 
driven backwards by the excess of pressure between it and the sides of the containing 
vessel. Regarding the radiometer as a heat-engine, it is seen to be imperfect in many 
respects. The black or driving surface, corresponding to the heater of the engine, being 
also part of the moving fly, is restricted as to weight, material, and area of surface. It 
must be of the lightest possible construction, or friction will greatly interfere with its 
movement; it must not expose much surface, or it will be too heavy ; and it must be a 
very bad conductor of heat, so as to retain the excess of pressure on one side. Again, 
