124) Mr Watt on the Prificiples of Attraction, 



also traversed 5° either way to the influence of a powerful horse- 

 shoe magnet, when placed so as to rest against the glass, the 

 hand being quickly withdrawn. 



The feathers generally begin to move slowly ; in a few se- 

 conds they uniformly turn the points of their filaments toward 

 the source of light, and their sides being parallel to the direction 

 of the rays ; and whenever they assume this relative position 

 they rest. If the flame is placed opposite to the tips of the fea- 

 thers at once, they move little, or not at all. If the rays of 

 light are made to fall upon their planes, at angles of 40°, 90°, or 

 150°, they will traverse only to the extent of these degrees, and 

 then remain stationary. 



The gold-leaf, for the first hour or two after it is formed into 

 a disc, and put under the cover, shews extraordinary sensibility 

 to the influence of light. It indicates the effects of the light of 

 a candle at the distance of from 15 to 20 feet from the flame. 

 If not kept in the dark, and in vacuo, it soon loses this suscep- 

 tibility ; and, in six or eight hours, will not move at a greater 

 distance from the flame than two feet. 



The gold leaf always turns the edge of its disc to the light, in 

 whatever position the candle may be placed. 



The silver leaf is equally sensitive to the impulses of light, 

 and never loses this property to the same extent as the gold. If 

 thoroughly dry, and placed in vacuo, it indicates the influence 

 of light, when 20 and 25 feet distant from the flame of a candle. 

 Several of the leaves tried, whether kept m vacuo or not (if 

 preserved from the light), when exposed to the attractive and 

 repulsive properties of the rays issuing from the flame of a can- 

 die, always moved toward the light, at a distance of eight and 

 ten feet. The silver leaf has a movement peculiar to itself. It 

 first turns the front of its disc, and then its edge ; and this move- 

 ment is often so constant that it will oscillate for hours in an arc 

 of 90°. When it has lost part of its susceptibility to the im- 

 pressions of light, it is so attracted as to move till its disc con- 

 fronts the source of the light. In this state, it loses its vibratory 

 motion, and takes a minute or two to traverse 45°. 



The gold-beaters'* leaf moves at the distance of six feet from 

 the flame. It turns its edge to the point from which the light 

 emanates, and then rests. 



