Mr Watt an the Principles of Attraction^ &c. 18$ 



small quills ; having 25 grains weight of magnetic steel-filings 

 rubbed over its surface. Two or four of the illuminated tops 

 of the smaller caudal feathers of the peacock (Pavo cristatus), 

 with their dingy sides (which are little attracted by light) ap- 

 plied to each other, formed another kind of disc. Their planes 

 were placed perpendicularly, and they were stuck into the end 

 of the revolving quill. They were formed into a convenient 

 size, by cutting off the straggling filaments of the feathers. 

 One disc was made of gold, and another of silver leaf. They 

 were formed by bending a piece of very fine silver-wire, of about 

 the thickness of a hair, into a circle of three or four inches diame- 

 ter. The wire, after being attached by its edge to the end of 

 the quill, was wetted by a little water, in which a small portion 

 of gum arabic was dissolved. The circle was placed upon a 

 leaf of the gold or silver which adhered to the wire, and the 

 corners of the leaf were then cut off. 



The other substances were gold-beaters' leaf; very thin pa- 

 per, coated with lamp-black ; and thin laminae of mica. 



All these were successively put under a hemispherical glass- 

 cover, placed upon a marble slab, and secured from any current 

 of air, by being surrounded at the edge by a layer of wax or 

 putty. The effect of light was also tried upon them under the 

 exhausted received of an air-pump. 



Effects qf the Light of' a Candle. — The first experiments 

 made upon these bodies, to ascertain in some measure how far 

 they were affected by the attracting or repelling influences of 

 light, were by the flame of a candle ; all other sources of par- 

 tial light or heat being excluded. 



The velvet disc, with the steel-filings, rendered magnetic, 

 moved to the light of a candle at the distance of 1 foot from 

 the edge of the cover. It turns its edge to the source of light, 

 and consequently its plane nearly parallel to the rays. 



The discs made of the feathers were moved by the candle at 

 the distance of 3 and 4 feet, measuring from the flame to the 

 point of suspension. A broad caudal feather of any of the gal- 

 linaceous tribe, if suspended by a fine filament of silk from the 

 top of the cover, and balanced horizontally, with its flat sides 

 opposite to the sides of the cover, will indicate the attractive 

 power of the light at the distance of from 4 to 6 feet. They 



