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THE POPULAR SCIENCE MONTHLY. 



the whole bulb is hot, and I remove the lamp : see what happens. 

 The rotation quickly diminishes. Now it is at rest ; and now it is 

 spinning round just as fast the reverse way. I can produce this 

 reverse movement only with difficulty with a pith instrument. The 

 action is due to the metal being a good conductor of heat. As it 

 absorbs heat it moves one way ; as it radiates heat it moves the op- 

 posite way. 



At first I made these instruments of the very lightest material 

 possible, some of them not weighing more than half a grain ; and, 

 where extreme sensitiveness is required, lightness is essential. But 

 the force which carries them round is quite strong enough to move a 

 much greater weight. Thus the metallic instrument I have just ex- 

 perimented with weighs over thirteen grains, and here is one still 

 heavier, made of four pieces of looking-glass blacked on the silvered 

 side, which are quickly sent round by the impact of this imponderable 

 agent, and flash the rays of light all round the room when the electric 

 lamp is turned on the instrument. 



Before dismissing this instrument let me show one more experi- 

 ment. I place the looking-glass and the metal radiometer side by 

 side, and, screening the light from them, they come almost to rest. 

 Their temperature is the same as that of the room. What will hap- 



MAGNET 



MORSE 

 INSTRUMENT 



pen if I suddenly chill them ? I pour a few drops of ether on each of 

 the bulbs. Both instruments begin to revolve. But notice the differ- 

 ence. While the movement in the case of the metal radiometer is 

 direct, that of tire looking-glass instrument is reverse. And yet to a 

 candle they both rotate the same way, the black being repelled. 



