248 
Mechanical Action of Light. [April, 
have an astatic combination inside the bulb, so that a very 
faint light would be sufficient to start it, but in this case 
I am obliged to set it going by an eleCtric current. I have 
placed a candle near the magnetic radiometer. I now touch 
the key ; the instrument immediately responds ; the paper 
unwinds from the Morse instrument, and on it you will see 
dots in regular order. I put the candle 8 inches off, and the 
dots come wide apart. I place it 5J inches off, and two dots 
come where one did before. I bring the candle 4 inches from 
the instrument, and the dots become four times as numerous 
(Fig. 10), thus recording automatically the intensity of the 
light falling on the instrument, and proving that in this case 
also the radiometer obeys the law of inverse squares. 
This instrument, the principle of which I have illustrated 
to-night, is not a mere toy or scientific curiosity, but it is 
capable of giving much useful information in climatology. 
You are well aware that the temperature, the rainfall, the 
atmospheric pressure, the direction and force of the wind, 
FIG.IO. 
are now carefully studied in most countries, in order to elu- 
cidate their sanitary condition, their animal and vegetable 
productions, and their agricultural capabilities. But one 
most important element, the amount of light received at 
any given place, has been hitherto but very crudely and 
approximately estimated, or rather guessed at. Yet it can- 
not be denied that sunlight has its effeCt upon life and 
health, vegetable, animal, and human, and that its relative 
amount at any place is hence a point of no small moment. 
The difficulty is now overcome by such an instrument as 
this. The radiometer may be permanently placed on some 
tall building, or high mountain, and, by connecting it by 
