ACOUSTICS. 6 1 



upon a mirror a, and is received upon the bit of mirror 

 upon the end of the tube. The open end of this tube 

 is to be held at the mouth and various sounds produced, 

 varying in pitch and intensity. The vibrations of the 

 membrane will move the mirror, and the beam of light 



Fig. 39. 



reflected from it upon the screen will describe various 

 beautiful and regular curves, depending upon the man- 

 agement of the voice. It will be easy to find some 

 pitch and intensity which will give a straight line : then, 

 while the sound is being made, if the outer end be 

 swung sidewise at right angles to the line, an undulat- 

 ing line will appear, in every way like those produced 

 by the vibrating tuning-fork described on another page. 

 If there are prominent over-tones in the sound they will 

 be made apparent by their interference, giving a trace 

 just like the traces upon a smoked glass by Scott's 

 Phonautograph. The forms are regular enough for a 

 tone of a given pitch and intensity, to enable one to 

 write his music with them for notes ; and if a tune like 

 "Auld Lang Syne " be tooted in the instrument, the ef- 

 fect is quite amusing. The size of these figures, at the 

 distance of fifteen or twenty feet, may be six or eight 

 feet or more. 



