ELECTRICITY. |7I 



hard blow, the powder will be divided into small heaps, 

 describing a line, the length of the wire ; those nearest 

 the point struck will be the largest, the others will gra- 

 dually diminish in size as they lie further off. This ex- 

 periment may be made in a still more simple manner : take 

 a square of glass or metal, whose edges are quite even, 



cover thesurface with powder of lycopodium, and hold it ^^^ ^^^^ 



with the fingers by opposite sides, leaving the other sides with a square 



quite free. If one of the free sides be struck with apiece P^^^^* n 



of wood, the powder m ill be immediately thrown into 



lines parallel to the direction of the blow, in which may 



be observed many elevations and depressions. But if the 



blow be given with a rough board, oh the whole extent of 



the edge, the powder will dispose itself in lines parallel to 



the side struck. The lines will be liiore or less wavy, in 



precisely the same degree as the side struck was more or 



less even. If one of the surfaces be struck, a number of 



little heaps will be formed. This is doubtless the result 



of an oscillatory movement, and most decidedly of a pro- 



gressive and an undulatory motion. 



But if the tablet be held between the finger and thumb 

 upon the two surfaces, without touching the edges, and a 

 blow be giren on the upper surfaces, not only heaps are 

 formed by the powder, but a sound is emitted. The heaps 

 receive a motion which obliges them to reunite at the ex- 

 tremity and they nearly assume the figure described by 

 Chladni. Prepare the whole as if to obtain the figure of 

 Chladni, using lycopodium instead of sand, and the figure 

 will gradually appear. At the first blow the heaps will Figures formed 

 be formed like small knots, ranged about the points where by lycopodium 

 the largest were formed. Let ABC D (Fig. 3, PI. Ill,) °" Slass, &c. 

 represent the tablet struck on the point E ; the heaps 

 described by e ^, //', g g\ &c. will be formed : e<?will 

 be formed sooner than //', and //' sooner than gg\ 

 The first on the point E, and on all the line E £', will 

 be driven towards E' ; but that on e and e will be deter- 

 mined by two powers in the direction EE or ED and 

 E E'y it will then describe the curved line e h'y and all 

 the other points will traverse similar lines. 



In this manner the curve C E' D will be described ; and 

 jis all the other squares of the tables ^£'(7, AE' B, 



A 



