THE RADIOMETER. 



77 



twisted nor broke. At one period it was piaced on the mantelpiece, 

 over an open fireplace, and whenever a pufF of wind came down the 

 chimney, driving the heat of the fire up toward it, the needle would then 

 spin with amazing rapidit}', reminding us of the whirlwinds that spin 

 up from heated plains of sand or dust. Our visitors were astonished to 

 see that needle whirling there, without any visible cause, and with no 

 apparent attachment, for the thread by which it was suspended was so 

 slight as almost to escape observation. 



The application of the hand to the side of the jar would always 

 cause the needle to move from a state of rest. It was very interesting 

 to watch its motion at night, whenever the light of a candle entered 

 the jar. It furnished a beautiful illustration of the effect which is pro- 

 duced upon the still night-air of summer when penetrated by the first 

 rays of the rising sun. We may conceive that the whole atmosphere at 

 that moment responds with infinite currents, breezes, and motions, awak- 

 ened into new life from a night's rest by the heat of the sun's rays. Of 

 course, light, as a substance, has nothing to do with the motion. It 

 results entirely from the expansion of the air by the force of heat. Our 

 needle moved on the same principle precisely — and on no other — that 

 the windmill moves in a current of air. 



If the glass jar were to be exhausted of its air, and the needle were 

 then to move when struck by a beam of light, the motion might be 

 supposed entirely due to that light; but it may well be doubted 

 whether it is possible to produce a vacuum so perfect that it would be 

 entirely void of gaseous substance of some kind. The planetary and 

 starry spaces themselves are probably not entirely free from matter 

 that would respond to the action of heat, if indeed such matter is not 

 necessary for the transmission of light and heat. An infinitesimal 

 portion of air, or even of vapor of mercury, spread through a large jar 

 or receiver, would doubtless obey the same law of expansion under 

 heat that is observed by the atmosphere in its densest conditions, and, 

 when set in motion, would prove sufficient to move a freely-suspended 

 needle. 



As our instrument, devised for proving the rotation of the earth, 

 did not prove that fact, but showed something else, so the radiometer, 

 which was supposed to prove the material character of light, did not 

 prove that fact, yet it may serve to show something else. But we do 

 not perceive why it should be called a radiometer, any more than the 

 windmill should be called by that name. 



