Mr Watt oil the Magnetic Influence of the Solar Rays. 17S 



body, was to observe what influence the solar rays might exerk 

 on it. 



I exposed pieces of tin, zinc, copper, and sealing-wax, to the 

 rays of the sun for two hours, and, being considerably heated 

 by this means, they sensibly attracted the magnetic needle, pro- 

 ducing a variation of a few degrees. When heated by the fire, 

 they had no effect on it. The copper and sealing-wax appear- 

 ed to possess the greatest power of attraction. The rays of the 

 sun, as far as I could judge, when passed through a lens, caused 

 a variation of two or three degrees. And it also appeared to 

 me, that, when the focal rays were passed through different co- 

 loured glasses, and made to impinge on the side of the wax sur- 

 rounding the needles, that they had different effects on the op- 

 posite poles. The blue rays formed in this manner, seemed to 

 attract the south pole, and repel the north. The blue and vio- 

 let ray produced a variation of several degrees, when directed to 

 the south pole. 



The rays of the sun, whether undivided or separated by the 

 prism, do not appear to exert their influence long on the mag- 

 net, in producing a variation not above a minute ; and this 

 arises, I suppose, from their coming into close contact with it ; 

 and from their being so suddenly generally diff*used over the 

 whole needle. It requires favourable circumstances to observe 

 the effect of these delii^ate influences ; and I should feel indebt- 

 ed to any one who would try these experiments with powerful 

 lenses, to attempt a farther illustration of them. 



When the magnetic needle is surrounded with wax, or glass, 

 and made to swim on the surface of water, it moves to much 

 gentler impressions than when placed on a pivot. The wax al- 

 so prevents the rays of light from coming into such full contact 

 as when the needle is uncovered. A shade should be thrown 

 over one pole, when the other is made the subject of experiment. 

 And the vessel ought to be large in which the magnet swims, 

 else it will not rest in the centre, being attracted to the edges; 

 and it should be marked at the bottom, to enable the observer 

 to detect the motions of the needle. 



