218 



Professor Silvanus P. Thompson 

 TABLE III. 



[June 13, 



These forks are excited by striking tliem with a steel hammer. 

 Some of the resulting beat-tones will be heard all over the theatre ; 

 but, in the case of the very low tones of 40 and 32 vibrations, only 

 those who are close at hand will hear them. The case in which 

 there are 26 beats is curious. Most hearers are doubtful whether 

 they perceive a tone or not. There is a curious fluttering eflect, as 

 though a tone were there, but not continuously. 



We have seen, then, that the beat-tones correspond in pitch to 

 the number of the beats ; that they can themselves interfere, and 

 give secondary beats ; and that the same number of beats will always 

 give the same beat-tone irrespectively of the interval between the 

 two primary tones. What better proofs could one desire to support 

 the view that the beat-tones are caused, as Dr. Young supposed, by 

 the same cause as the beats, and not, as von Helmholtz maintains, by 

 some other cause ? Yet there are some further points in evidence 

 which are of significance, and lend additional weight to the proofs 

 already adduced. 



Beats behave like primary impulses in the following respect, that 

 when they come with a frequency between 32 and 128 per second, 

 they may be heard, according to circumstances, either discontinuously 

 or blending into a continuous sensation. 



It has been objected that, whereas beats imply interference 

 between two separate modes of vibration arising in two separate 

 organs, combination-tones, whether summational, or differential, or 

 any other, must take their origin from some one organ or portion of 

 vibratile matter vibrating in a single but more complex mode. To 

 this objection an experimental answer has been returned by Dr. 

 Koenig in the following way. He takes a prismatic bar of steel, 

 about 9 inches in length, and files it to a rectangular section, so as 

 to give, when it is struck at the middle of a face to evoke transversal 

 vibrations, a sound of some well-defined pitch. By carefully adjusting 

 the sides of the rectangular section in proper proportions, the same 



