2 7 o THE POPULAR SCIENCE MONTHLY. 



near and the speed increases. I now lift the radiometer up, and place 

 it full in the electric light, projecting its image direct on the screen, 

 and it goes so rapidly that if I had not cut out the four pieces of pith 

 of different shapes you would have been unable to follow the movement. 



The speed With which a sensitive radiometer will revolve in the 

 sun is'almost incredible; and the electric light, such as I have it in 

 this lantern, cannot be far short of full sunshine. Here is the most 

 sensitive instrument I have yet made, and I project its image on the 

 screen, letting the full blaze of the electric light shine upon it. Noth- 

 ino- is seen but an undefined nebulous ring, which becomes at times 

 almost invisible. The number of revolutions per second cannot be 

 counted, but they must be several hundreds, for one candle has made 

 it spin round forty times a second. 



I have called the instrument the radiometer because it will enable 

 me to measure the intensity of radiation falling on it by counting the 

 revolutions in a given time ; the law being that the rapidity of revo- 

 lution is inversely as the square of the distance between the light and 

 the instrument. 



When exposed to different numbers of candles at the same distance 

 off, the speed of revolution in a given time is in proportion to the 

 number of candles ; two candles giving twice the rapidity of one can- 

 dle, and three, three times, etc. 



The position of the light in the horizontal plane of the instrument 

 is of no consequence, provided the distance is not altered ; thus two 

 candles, one foot off, give the same number of revolutions per second, 

 whether they are side by side or opposite to each other. From this 

 it follows that if the radiometer is brought into a uniformly lighted 

 space it will continue to revolve. 



It is easy to get rotation in a radiometer without having the sur- 

 faces of the disks differently colored. Here is one having the pith- 

 disks blacked on both sides. I project its image on the screen, and 

 there is no movement. I bring a candle near it, and shade the lisrht 

 from one side, when rapid rotation is produced, which is at once altered 

 in direction by moving the shade to the other side. 



I have arranged here a radiometer so that it can be made to move 

 by a very faint light, and at the same time its rotation is easily fol- 

 lowed by all present. In this bulb is a large six-armed radiometer 

 carrying a mirror in its centre. The mirror is almost horizontal, but 

 not quite so, and therefore, when I throw a beam of electric light ver- 

 tically downward on to the central mirror, the light is reflected off at 

 a slight angle, and, as the instrument rotates, its moA T ement is shown 

 by the spot of light traveling round the ceiling in a circle. Here 

 again the fog helps us, for it gives us an imponderable beam of light 

 moving round the room like a solid body, and saving you the trouble 

 of looking up at the ceiling. I now set the radiometer moving round 

 by the light of a candle, and I want to show you that colored light 



