ACOUSTIC FIGURES. 



255 



parallel lines, perpendicular 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, and if the 

 bow is applied at a point 45 from the last point, the 

 figure of the sand will be as in Fig. 46. 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 diameters of sand at right angles to each 

 other will be formed, as in No. 3. When a point of the 

 circumference is pressed against a fixed obstacle, and the 

 bow applied 30 from that point, the figure in No. 4 is 

 produced, 



