ACOUSTIC FIGURES. 



193 



and if the bow is applied at a point 45 from the 

 last point, the figure of the sand will be as in Fig. 

 47, No. 1. If with the same plate, similarly 

 pinched, the bow is drawn over a part 30 from 

 the pinched point of the circumference, the sand 

 will form six radii as in No. 2. When the centre 

 of the plate is left free, a different set of figures 

 is produced, as shown in No. 3 and No. 4. When 

 the plate is pinched near its edge, and the bow 

 applied 45 from the point pinched, a circle of 

 sand will pass through that point, and two dia- 

 meters of sand, at right angles to each other, will 

 be formed as in No. 3. When a point of the cir- 

 cumference is pressed against a fixed obstacle, 

 and the bow applied 30 from that point, the 

 figure in No. 4 is produced. 

 Fig. 47. 

 No. 1. No. 2. 



No. 3. 



No. 4. 



If, in place of a solid plate, we strew the sand 

 o 



