19-2 



LETTERS ON NATUBAL MAGIC. 



If we take a square plate of glass, such as that 

 shown in Fig. 46, No. 1, and, pinching it at its 

 centre, draw the fiddle-bow near one of its angles, 

 the sand will accumulate in the form of a cross, 

 as shown in the figure, being thrown off the parts 

 of the plate that are in a state of vibration, and 

 Fig. 46. 



settling in the nodes or parts which are at rest. 

 If the bow is drawn across the middle of one of 

 the edges, the sand will accumulate as in No. 2. 

 If the plate is pinched at N, No. 3, and the bow 

 applied at F and perpendicular to AB, the sand 

 will arrange itself in three parallel lines, per- 

 pendicular to a fourth passing through F and N. 

 But if the point N, where it is pinched, is a little 

 farther from the edge than in. No. 3. the parallel 

 lines will change into curves as in No. 4. 



If the plate of glass is. circular, and pinched at 

 its centre, and also at a point of its circumference, 



