MR. W. CROOKES ON REPULSION RESULTING FROM RADIATION. 
presented radiometers, when tested with the hot metal rings applied equatorially and 
at the poles (298 to 305), showed that the action of radiation was in some cases 
entirely different, according as it fell on the vanes horizontally or nearly vertically. 
The experiments just described with the sloping adjustable cup (432) show still more 
forcibly the great difference in speed of rotation caused by a little alteration in the 
angle presented by the vanes to the direction of pressure. These experiments were, 
however, tried with somewhat complicated apparatus. A simpler form of instrument, 
in which full advantage is taken of the peculiarities of sloping vanes, was exhibited 
before the Royal Society on April 5, 1876, under the name of the turbine radiometer. 
Fig. 20. 
In the earlier radiometers of this kind the vanes were of mica, blacked on both sides, 
and inclined at an angle like the sails of a windmill, instead of being in a vertical 
plane. These are not sensitive to horizontal radiation, but move readily, in one or 
other direction, to a candle held above or below. If one side only of each vane is 
blacked, the fly becomes more sensitive to radiation falling on the black side, but 
is not sensitive when radiation strikes it on the other side. In the ordinary form 
of radiometer, the number of disks constituting the fly is limited to six or eight, a 
greater number causing interference one with the other, and obstruction to the inci¬ 
dent light. In the turbine form of fly there is no such difficulty ; the number of vanes 
may be increased to a considerable amount without overcrowding, and with corres¬ 
ponding advantage. The action of the vanes is evident. Fig. 20 shows the turbine 
vanes ; light falling from above generates molecular pressure between the surface of 
the vanes and the top of the exhausted bulb, and this pressure, acting as at A, drives 
the fly round in the direction of the arrow. A vertical light gives the strongest 
action, but rotation takes place whatever be the incident angle, provided the light is 
caught by one surface more than by the other. If the finger, or any warm substance, 
touches the top of the bulb, so as to generate pressure from the inner surface of the 
exhausted bulb, the fly is strongly driven round in the direction shown at fig. 20, A 
