Sounding of two Notes. 521 



ordinarily powerful; while the difference- and summation-notes 

 15 and 17 are but faintly perceptible at 1 : 16, and the notes 

 31 and 33 at 1 : 32 can scarcely be perceived at all. 



In experiments with the last-named tuning-forks, which 

 are therefore the most favourable for the observation of the 

 intermitting notes, I move the disk directly in front of the 

 forks. When, however, I use deeper forks, I insert between 

 them and the disk suitable resonators of the same diameter 

 as the holes in the disk, so that the note always sounds loudly 

 when one of these holes is in front of the opening in the re- 

 sonator. It may be remarked, by the way, that with this 

 arrangement the variation-notes especially sound wonderfully 

 beautiful, and when the disk is moved alternately quicker and 

 slower they may be distinctly heard to retreat from and approach 

 one another. 



In the above, only a note of constantly equal intensity was 

 allowed to approach the ear intermittingly by mechanical 

 means; the transition of periodical vibration maxima to a note, 

 however, can also be observed in notes which themselves pos- 

 sess a periodically changing intensity. For this purpose I 

 have constructed siren-disks with circles in which the holes 

 are at equal distances, but get periodically larger and smaller, 

 so that a series of isochronous impulses of periodically changing 

 intensity is produced if they are sounded through reeds of the 

 same diameter as the largest holes. One of these disks con- 

 tained three circles, each of 96 equidistant holes, whose dia- 

 meter varied on the first circle 16 times from 1 to 6 millims., 

 on the second 12 times, and on the third 8 times. If these 

 circles were sounded with a tube of 6 millims. diameter, 

 while the disk was at first turned slowly, the single-hole 

 periods were heard in each circle like separate beats ; when it 

 was turned faster, first the 16 periods of the first, then the 12 

 of the second, and lastly the 8 of the third circle changed to 

 one note ; when, lastly, the high note of 96 holes with 8 turns 

 of the disk had reached the second g", the deep notes C, G, 

 and double C, corresponding to the number of periods, were 

 clearly and powerfully heard with this g" . 



On another still larger disk, 70 centims. in diameter, I 

 arranged seven circles of 192 equidistant holes, which periodi- 

 cally increased and decreased in size 96, 64, 48, 32, 24, 16, 

 and 12 times. In the first, therefore, a whole period was con- 

 tained in two different large openings, and the note of the 

 periods in it was therefore only an octave deeper than the 

 note of the 192 holes, while in the seventh circle each period 

 was formed of 16 openings, and the note of the periods was 

 consequently four octaves deeper than the note of the 192 



