SILENCE FEOM TWO SOUNDS. 195 



dredths 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 corresponding parts of 

 the other half. 



As these singular phenomena have not yet 

 been made available by the scientific conjuror, 

 we must be satisfied with this brief notice of 

 them ; but there is still one property of sound, 

 which has its analogy also in light, too remark- 

 able 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 lowl 

 sounds may be 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 vibrations in a second, they will pro- 

 duce each equal waves of sound, and these waves 

 will conspire in generating an uninterrupted sound, 

 double of either of the sounds, heard separately. 

 If the two strings or the two columns of air are 

 not in unison, but nearly so, as in the case where 

 the one vibrates 100 and the other 101 times in a 

 second, then at the first vibration the two sounds 

 will form one of double the strength of either : 

 o 2 



