254 



LETTERS ON NATURAL MAGIC. 



ference. The method of damping or pinching plates is 

 shown in Fig. 41. In No. 1 a square plate of glass A B, 

 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 circumference, 

 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. 41. 



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

 Fig. 45, 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 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 A B, the sand will arrange itself in three 



