and Repulsion in the Lunar Rays^ &c. 125 



Very thin paper, coated with lamp-black, or gilded with gold 

 or silver-leaf, and varnished with spirit of turpentine, when the 

 disc is about five inches diameter, move, by the influence of the 

 light of a candle, at the distance of three and four feet. 



As the light passed through the glass of the cover, which 

 would intercept any degree of heat, whilst it admitted the light, 

 and as the movements begin generally in a few seconds, there is 

 no reason to believe that any increment of heat can have any 

 share in producing the motions. 



All these bodies, however, move to the influence of heat, when 

 it proceeds from a given point, at various distances. Yet the ef- 

 fect of heat is evidently very inferior, in point of power, to the 

 influence of light. 



A piece of coal, for example, two inches square, ignited to red 

 heat, when presented to the velvet disc with the filings, only ex- 

 cites it to move towards it, though held close to the cover ; but 

 if it is exposed to the clear rays of the sun, during summer, as 

 soon as it has absorbed a certain quantity of the rays, it is strong- 

 ly repelled, and will continue, when first made, to revolve for 

 hours without intermission, performing each revolution in about 

 5". They all turn their edges to the point from which the heat 

 proceeds. 



Effects of the Lunar Beam. — As the candle used in trying the 

 effects of light on these bodies was of a moderate size, and as 

 there appeared to be little difference between its illuminating 

 power, at 16 or 20 feet from the flame, and the light afforded 

 by the moon, when nearly full, it did not appear to the author 

 unreasonable to expect, or surprising to find, that the discs were 

 affected by the influence of the lunar rays, in nearly a similar 

 manner. They were made the subjects of experiment both in 

 the open air, under the cover, and in a room with the windows 

 shut. When tried in an apartment, the window was darkened, 

 and they were made to rest (by moving the stand a little), in 

 such a position that the rays of the moon, when admitted, fell 

 upon the discs nearly at right angles to their planes. They all 

 turned their edges toward the luminary, and their planes nearly 

 parallel to the incidental beams ; and tliey frequently maintained 

 this relative position for hours, moving slowly and regularly, by 



