268 Dr. A. M. Mayer's Researches in Acoustics. 



In all the experiments the driving-wheel of the rotator was 

 revolved either once in a second or twice in a second. This 

 is accomplished, after some practice, in the following way : — 

 The rotator, on which is mounted the disk and fly-wheel, is 

 placed near a clock giving loud beats of seconds, and the 

 driving is revolved by the guidance of the hand and ear. 

 The results of the experiments showed that the velocities thus 

 given to the disk were sufficiently uniform, and the measures 

 of the durations of sonorous sensations sufficiently concordant 

 and precise to obtain the data of the physiological law. 



I adopted this method of rotation in preference to mechan- 

 ical means for controlling and measuring the revolutions of 

 the disks. To determine when the interrupted sounds have 

 blended requires, so to say, a flexible apparatus whose velocity 

 is under the immediate control of the hand and ear. This is 

 important in making the final judgement between sounds, one 

 of which appears to have too few interruptions, the other a 

 few more interruptions than are necessary to give a continuous 

 uniform sensation. It is evident that when we can at once 

 slightly increase or diminish the velocity of rotation of the 

 disk, we have the means of making comparisons rapidly suc- 

 ceeding one another. A rotator, driven as described, forms 

 more a part of the observer than one driven and regulated by 

 mechanism. 



As there were three grooves in the driving-wheel and three 

 in the pulley on the axle, and as the driving-wheel was re- 

 volved either once or twice in a second, eighteen different 

 velocities could be given to the rotating disk. 



Disks were made having numbers of holes from 5 to 19, so 

 that, with 18 velocities and the various numbers of holes in 

 the disks, it was easy to select a disk, driven with a known 

 velocity, which gave the exact number of interrupted sounds 

 per second to blend. 



The 18 ratios of velocities of the driving-wheel and of the 

 pulley on the axle of the rotator were obtained as follows : — 

 A circle of cardboard, divided into one hundred parts, was 

 clamped on the rotator in front of a disk. The driving- 

 wheel was rotated either once or twice in a second, so that the 

 conditions were the same as in the experiments. From 10 to 

 100 revolutions of the driving-wheel were made before the 

 ratio was determined. The division to which a fixed index 

 pointed on the divided circle gave the fraction of a revolution. 

 The whole number of revolutions was given by a simple 

 counter which moved with very little friction. 



The rotating disks were made of mahogany, 5 millim. thick, 

 with disks of cardboard about 2 millim thick screwed to the 

 wooden disks. The circumferences of the holes in the wooden 



