234 Professor Silvanus P. Thompson [June 13, 



immediate front of the apparatus will hear the resultant sounds 

 properly. 



Upon the edge of the first of the series there has been cut a curve 

 graphically compounded of 24 waves as a fundamental, together with 

 a set of four perturbed harmonics of equal intensity. The first har- 

 monic consists of 49 waves (2 X 24 -j- 1) ; the second of 75 waves 

 (3 X 24 + 8) ; the third of 101 (4 X 24 + 5) ; the fourth of 127 

 (5 X 24 -}- 7). The resulting curve possesses 24 waves, no two of 

 them alike in form, and some highly irregular in contour. The 

 efi'ect of blowing air through a slit against this disk is to produce a 

 disagreeable sound, quite lacking in unitary character, and indeed 

 suggesting intermittence. 



The second wave-disk is constructed with the same perturbed 

 harmonics, but with their amplitudes diminishing in order. Tliis 

 disk produces similar etfects, but with more approach to a unitary 

 character. 



In the third disk there are also 24 fundamental waves, but there 

 are no harmonics of the lower terms, the superposed ripples being 

 perturbed harmonics of the hfth, sixth, and seventh orders. Their 

 numbers are 6 x 24 -j- 6 ; 7 X 24 + 7 ; and 8 X 24 -f- 8 ; being, 

 in fact, three harmonics of a fundamental 25. This disk gives a 

 distinctly dual sort of sound ; for the ear hears the fundamental 

 quite separate from the higher tones, which set themselves to blend 

 to a unitary effect. There is also an intermittence corresponding to 

 each revolution of the disk, like a beat. 



The fourth disk resembles the preceding ; but the gap between 

 the fundamental and the three perturbed harmonics has been filled 

 by the addition of three true harmonics. This disk is the first in 

 this research which gives a real timbre, though it is a peculiar one : 

 it preserves, however, a unitary character, even when the slit is tilted 

 in either direction. The 24 waves in this disk all rake forward like 

 the teeth of a circular saw, but with multiform ripples upon them. 

 The quality of tone becomes more crisp when the slit is tilted so as 

 to slope across the teeth, and more smooth when in the reverse 

 direction. 



The fifth disk, which is larger, has 40 waves at its edge ; these 

 are cut with curves of all sorts, taken haphazard from various com- 

 binations of pure harmonics in all sorts of proportions and varieties, 

 no two being alike, the maxima and minima of the separate waves 

 being neither isochronous nor of equal amplitude. This disk gives 

 an entirely unmusical effect, amid which a fundamental tone is heard, 

 accompanied by a sort of rattling sound made up of intermittent and 

 barely recogTiisable tones. 



The sixth disk is derived from the preceding by selecting eight 

 only of the waves, and repeating them five times around the periphery. 

 In this case each set of eight acts as a single long curve, giving beats, 

 with a slow rotation, and a low tone (accompanied always by the 

 rattling mixture of higher tones) when the speed is increased. 



