348 The Mechanical Action of Light. [July, 
exhausted to the highest attainable point and hermetically 
sealed. 
The arms of this instrument rotate with more or less velo- 
city under the action of radiation, the rapidity of revolution 
being directly proportional to the intensity of the incident rays. 
Placed in the sun or exposed to the light of burning magne- 
sium, the rapidity is so great that the separate discs are lost 
in a circle of light. Exposed to a candle 20 inches off another 
instrument gave one revolution in 182 seconds; with the 
same candle placed at a distance of 10 inches off the result 
is one revolution in 45 seconds; and at 5 inches off one re- 
volution was given in 1m seconds. Thus it is seen that the 
mechanical action of radiation is inversely proportional to the 
square of the distance. At the same distance 2 candles give 
exactly double, and 3 candles give three times, the velo- 
city given by 1 candle, and so on up to 24 candles. A 
small Radiometer was found to revolve at the velocities 
shown in the following table, when exposed to the radiation 
of a standard candle 5 inches off. 
Time required for One. Revolution. 
Source of Radiation. Time in Seconds. 
1 candle, 5 inches off, behind green glass . . 40 
bP) 5 33 99 blue 99 ~ < 38 
a 5 mn ss purple’, ) 2) aeaeee 
- 5 as 4» Orange §,,. A eee 
as 5 es 25 yellow ,; 22 Seaman 
., 5 up 1) . light red 5; /e.emeeee 
In diffused daylight the velocity was one revolution in 
from 1°7 seconds to 2°3 seconds, according to the intensity 
of the incident rays. In full sunshine, at Io A.M., it revolved 
once in 0°3 second, and at 2 P.M. once in 0°25 second. 
When heat is cut off by allowing the radiation to pass 
through a thick plate of alum, the velocity of rotation is 
somewhat slower, and when only dark heat is allowed to fall 
on the arms (as from a vessel of boiling water) no rotation 
whatever is produced. 
In all respects, therefore, it is seen that the Radiometer 
gives indications in strict accordance with theory. 
Several radiometers, of various constructions as regards 
details, but all depending on the above-named discovery, 
have been exhibited at the Royal Society, where their 
novelty and unexpected indications excited a considerable 
amount of interest. 
