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stationary, I placed my finger nearest to a black disc ; it was at 
once repelled ; the white approached, passed the warm spot, 
the next black was then repelled — thus continuous rotation was 
produced. I then removed my finger, and allowed the globe 
to cool. Friction soon overcame momentum, the vanes stopped, 
and u reverse motion ” immediately commenced . With a finger 
on opposite sides of the globe, the same effects are produced, 
but with greater intensity. When perfectly still I turned the 
flame of the lamp down so low that I could not distinguish 
anything in the room, and then surrounded the globe with my 
warm hands. In about half a minute, suddenly turning up the 
flame. I found the vanes rotating rapidly (making 40 or 
50 complete revolutions per minute), the black discs being 
repelled. Eemoving my hands, reverse motion ensued ; and as 
soon as the vanes had become stationary, the temperature 
being equalised, I turned down the flame, so that there was only 
just enough light to enable me to see the vanes, and again 
placed my hands round the globe. The black discs were 
repelled, rotation quickened, reached its maximum, and then 
gradually decreased in velocity until the vanes were perfectly 
stationary. The little air inside having been raised to the 
same temperature as that of my hands, when they were re- 
moved rapid reverse action at once ensued. When the vanes 
again became stationary, I allowed two or three drops of 
sulphuric ether to fall upon the globe. The rapid evaporation, 
producing cold, immediately caused reverse motion, which 
continued for two or three minutes, when the vanes again be- 
came stationary, and the motion was reversed, the black discs 
being now repelled as the glohe was receiving heat from the 
outside air. This experiment can be repeated for any number 
of times ; the hotter the surrounding air, the greater the effect. 
It can be produced as well in the dark as in the light, and if a 
single drop of ether fall on the globe every four or five minutes, 
rotation — first in one direction, and then in the other — must 
continue until the temperature of the whole of the surrounding 
air be reduced to that degree of cold obtained by the rapid 
evaporation of the ether. Thus, with a perfect radiometer, 
unlimited supply of ether, and means of dropping it when 
required, motion may be produced which must practically be 
perpetual. 
The next experiments were tried in daylight. I placed B 
in front of the fire for a few seconds. The black discs were 
repelled with such rapidity that I could not distinguish the 
vanes. On putting it quickly outside my window, where the 
thermometer stood three or four degrees below the freezing 
point, the rotation soon ceased, and reverse motion set in, at 
first with a velocity equal to thirty-three complete revolutions 
