194 LETTERS ON NATURAL MAGIC. 



over a stretched membrane, the sand will form 

 itself into figures, even when the vibrations are 

 communicated to the membrane through the air. 

 In order to make these experiments, we must 

 stretch a thin sheet of wet paper, such as vege- 

 table paper, over the mouth of a tumbler-glass 

 with a footstalk, and fix it to the edges with glue. 

 When the paper is dry, a thin layer of dry sand 

 is strewed upon its surface. If we place this 

 membrane upon a table, and hold immediately 

 above it, and parallel to the membrane, a plate 

 of glass vibrating so as to give any of the figures 

 shown in Fig. 47, the sand upon the membrane 

 will imitate exactly the figure upon the glass. 

 If the glass plate, in place of vibrating horizon- 

 tally, is made to vibrate in an inclined position, 

 the figures on the membrane will change with 

 the inclination, and the sand will assume the 

 most curious arrangements. The figures thus 

 produced vary with the size of the membrane, 

 with its material, its tension, and its shape. 

 When the same figure occurs several times in 

 succession, a breath upon the paper will change 

 its degree of tension, and produce an entirely 

 new figure, which, as the temporary moisture 

 evaporates, will return to the original figure, 

 through a number of intermediate ones. The 

 pipe of an organ at the distance of a few feet, or 

 the notes of a flute at the distance of half a foot, 

 will arrange the sand on the membrane into 

 figures which perpetually change with the sound 

 that is produced. 



The manner in which flat rulers and cylinders 

 of glass perform their vibrations is very remark- 

 able. If a glass plate about twenty-seven inches 

 long, six-tenths of an inch broad, and six hun- 



