THE MECHANICAL ACTION OF LIGHT. 273 



Now, having found that this force would carry round a compara- 

 tively heavy weight, another useful application suggested itself. If 

 I can carry round heavy mirrors or plates of copper, I can carry 

 round a magnet. Here, then (Fig. 9), is an instrument carrying a 

 magnet, and outside is a smaller magnet, delicately balanced in a ver- 

 tical position, having the south pole at the top and the north pole at 

 the bottom. As the inside magnet comes round, the outside magnet, 

 being delicately suspended on its centre, bows backward and forward, 

 and, making dontact at the bottom, carries an electric current from a 

 battery to a Morse instrument. A ribbon of paper is drawn through 

 the " Morse " by clock-w T ork, and at each contact at each revolution 

 of the radiometer a record is printed on the strip of paper by dots ; 

 close together if the radiometer revolves quickly, farther apart if it 

 goes slower. 



Here the inner magnet is too strong to allow the radiometer to 

 start with a faint light without some initial impetus. Imagine the 

 instrument to be on the top of a mountain, away from everybody, 

 and I wish to start it in the morning. Outside the bulb are a few 

 coils of insulated copper wire, and by depressing the key for an in- 

 stant I pass an electric current from the battery through them. The 

 interior magnet is immediately deflected from its north-south position, 

 and the impetus thus gained enables the light to keep up the rotation. 

 In a proper meteorological instrument I should have an astatic com- 

 bination 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 



Fig. 10. 



unwinds from the Morse instrument, and on it you will see dots in 

 regular order. I put the candle eight inches off, and the dots come 

 wide apart. I jjlace it five and three-quarters, inches off, and two dots 

 come where one did before. I bring the candle four 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 



VOL. IX. 18 



