692 



THE POPULAR SCIENCE MONTHLY. 



which is then exhausted to the highest attainable point and hermeti- 

 cally sealed. 



The arms of this instrument rotate with more or less velocity un- 

 der the action of radiation, the rapidity of revolution being directly 

 proportional to the intensity of the incident rays. Placed in the sun, 



Pigs. 7 and 8. cr, a very fine needle-point; b, two pieces of straw ; c, jewel-cup ; rf, d, d, d, four 

 pith-disks, blackened on one side. The arms between the straw in the centre and the disks 

 are bent glass-fibres ; e, glass support holding cup ; /, cement to keep the support c in its 

 place. 



or exposed to the light of burning magnesium, the rapidity is so great 

 that the separate disks are lost in a circle of light. Exposed to a can- 

 dle twenty inches off another instrument gave one revolution in 182 

 seconds ; with the same candle placed at a distance of ten inches off 

 the result is one revolution in 45 seconds ; and at five inches off one 

 revolution was given in 11 seconds. Thus it is seen that the mechani- 

 cal action of radiation is inversely proportional to the square of the 

 distance. At the same distance two candles give exactly double, and 

 three candles give three times the velocity given by one candle, and 



