MR. W. CROOKES ON REPULSION RESULTING FROM RADIATION. 
289 
pendent of the material of which the fly is made, and is only slightly increased or 
diminished according to the conducting power of the fly for heat. The lighter the 
weight of the fly to be driven round the easier it moves, and the heavier the fly the 
longer it keeps in motion after it is once started. 
Fig. 17. 
Fig. 18. 
303. When heat is applied to either pole of the bulb the action is of a different 
character, negative rotation taking place (295, 296, 298, 301). A moment’s considera- 
tion will show that the molecular pressure proceeding from a hot pole of the bulb will 
strike the inner surface of the sloping vanes, and driving them before it, will cause 
a rotation which appears negative to an observer, although it is really positive to the 
direction of pressure. It is not easy to represent the lines of force on a flat diagram, 
but fig. 18 will sufficiently illustrate the mode of action. The heat is supposed to be 
applied near the centre, and the molecular pressure radiating on all sides presses 
the vanes chiefly on the inner surfaces. 
This action, like the positive rotation when the bulbs are heated equatorially, is in 
great measure independent of the material of which the vanes are made and of its 
conducting power for heat. 
304. The anomalous results obtained when the radiometers were heated with hot 
glass shades or hot water (282 to 293), the vanes sometimes rotating one way and 
sometimes the other, but chiefly in the negative direction, are now completely ac- 
counted for. Polar heating gives negative, and equatorial positive rotation, and there- 
fore when both are heated together by immersion in hot water the direction of motion 
is governed by the stronger of these two forces. A much larger surface of the bulb is 
effective in producing negative than positive rotation, and therefore the tendency of 
hot water is to drive the vanes negatively. If, however, the lower part only of the 
bulb is dipped into hot water (297), strong negative rotation takes place. Owing 
to accidental conditions, such as the shape of the bulbs, the size of the vanes, and 
their position in respect to the centre, the two opposing forces may be more or less 
evenly balanced, and a very little will cause one or the other to preponderate. A 
hot shade, for instance, if it is very large, may heat the bulb uniformly all over and 
give negative rotation, whilst if it is tall and narrow, the equator of the bulb receives 
most heat, and positive rotation ensues. 
305. When explaining the action of light from a candle on the radiometers 
M DCCCLXX VIII. 2 P 
