SILENCE FROM TWO SOUNDS. 257 



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 remarkable. If a glass 

 plate about twenty-seven inches long, six-tenths of an inch 

 broad, and six-hundredths of an inch thick, is held by the 

 edges between the finger and thumb, and has its lower 

 surface, near either end, rubbed with a piece of wet cloth, 

 sand laid upon its upper surface will arrange itself in 

 parallel lines at right angles to the length of the plate. 

 If the place of these lines is marked with a dot of ink, 

 and the other side of the glass ruler is turned upwards, 

 and the ruler made to vibrate as before, the sand will 

 now accumulate in lines intermediate between the former 

 lines, so that the motions of one-half the thickness of the 

 glass ruler are precisely the reverse of those of the cor- 

 responding parts of the other half. 



As these singular phenomena have not yet been made 

 available by the scientific conjurer, we must be satisfied 

 with this brief notice of them ; but there is still one pro- 

 perty of sound, which has its analogy also in light, too 

 remarkable to be passed without notice. This property 

 has more of the marvellous in it than any result within 

 the wide range of the sciences. Two loud sounds may 6 

 made to produce silence, and two strong lights may be made 

 to produce darkness ! 



If two equal and similar strings, or the columns of air 

 in two equal and similar pipes, perform exactly 100 





