ACOUSTIC FIGUEES. 



191 



figures assumed by sand or fine lycopodium 

 powder, strewed over the surface of the glass 

 plate. In order to produce these figures, we 

 must pinch or damp the plate at one or more 

 places, and when the sand is strewed upon its 

 surface, it is thrown into vibrations by drawing 

 a fiddle-bow over different parts of its circum- 

 ference. The method of damping or pinching 

 plates is shown in Fig. 45. In No. 1, a square 

 plate of glass AB, ground smooth at its edges, 

 is pinched by the finger and thumb. In No. 2, 

 a circular plate is held by the thumb against the 

 top c of a perpendicular rod, and damped by the 

 fingers at two different points of its circumference. 

 In No. 3 it is damped at three points of its cir- 

 cumference ; c and d by the thumb and finger, 

 and at e by pressing it against a fixed obstacle 

 a b. By means of a clamp like that at No. 4, it 

 may be damped at a greater number of points. 



Fig. 45. 



N'2, 



