ACOUSTICS. 



59 



fasten with marine glue, or sealing-wax, a small bit of 



mirror, about 



the fourth of 

 an inch square. 

 The wire must 

 be held tightly 

 at some point, 

 in a vice upon 

 a table. The 

 Fig. 37. light from the 



J>orU lutniere falls upon the plane mirror m, and is 

 thence reflected upon the small mirror on the end of 

 the wire at /, whence it is reflected to the screen. If 

 the wire be now carefully plucked, it will give a line of 

 light upon the screen, but will probably soon change 

 into an ellipse or a circle. If the wire be struck with 

 a small billet of wood, like a hammer-handle, there will 

 be heard two sounds, the fundamental with some over- 

 tone that will give a beautiful compound figure upon the 

 screen, some circle or ellipse made up of small undu- 

 lations,, which will vary as the wire is struck in different 

 places. If the wire be made fast at its middle, and the 

 other end of it be plucked, the end with the glass will 

 take up the vibrations at once — a case of sympathetic 

 vibration. If it is not fastened in the middle there will 

 be little or no movement when the lower end is struck. 

 (See Tyndall on Sound, pp. 133, 135.) 



MELDE S EXPERIMENT. 



To one prong of a small pocket tuning-fork tie a 

 piece of silk thread, six or eight inches long, and to 

 the other end tie a pin-hook and hang upon it a small 

 weight, say a shirt-button. Project this with the large 



