220 



Professor Silvanus P. Thompson 



[June 13, 



4 octaves lower in pitch, the latter being the beat-tone. On moving 

 the pipe so that wind is blown successively through each ring of 

 apertures, there is heard a shrill note, which is the same in each 

 case, and a second note (corresponding to the successive beat-tones) 

 which rises by intervals of fourths and fifths from circle to circle. 



Fig. 1. 



• • 



••••:.v^ 



.•• .,••-. •» . 



,•'•• ••*.. *•••. *•. ••. 



.♦• .• ,. ♦.. ••. •. 



. ,. .•• •., •. •. 



Siren Disk, with apertures varying periodically in size. 



These attempts to produce artificially the mechanism of beats 

 were, however, open to criticism ; for in them the phase of the 

 individual vibrations during one maximum is the same as that of 

 the individual vibrations in the next succeeding maximum ; whereas 

 in the actual beats produced by the interference of two tones the 

 phases of the individual vibrations in two successive maxima differ 

 by half a vibration; as may be seen by simple inspection of the 



Fig. 2. 



Siren Disk, pierced to imitate mechanism of beats. 



curves corresponding to a series of beats. When this difierence 

 was pointed out to Dr. Koenig, he constructed a new siren-disk (Fig. 

 2), having a similar series of holes of varying size, but spaced out so 

 as to correspond to a difference of half a wave between the sets. 

 With this disk, beats are distinctly produced with slow rotation, and 

 a beat-tone when the rotation is more rapid. 



