322 On the Audibility of single Sound Waves, $c. [Jan. 21,. 



holes could be left open or closed at will. The wheel was turned by 

 hand, about twenty-five rotations of the disk being the maximum 

 attainable with the required regularity. 



By leaving only one hole open, only one puff was made at each 

 revolution. By varying the rate of rotation, waves of different dura- 

 tion, i.e., corresponding to vibrations of different pitch, could be 

 produced. No matter how the rate of revolution was increased, up 

 to the maximum, twenty-five per second, only the single puffs could be 

 heard ; at first quite separate, then as a soft purr, and at the quickest 

 rate like a kind of rapid patter. 



With this siren, the sound caused by the single impulse is most 

 distinct and clear, and its invariability in character with varying rate 

 and varying blast of air particularly noticeable. The single puffs of 

 a duration not exceeding -J^Q-Q second were thus perfectly audible. 



As the rates just referred to were estimated from the number of 

 revolutions made by the driving wheel, we thought it advisable to 

 control this method of measuring the length of the waves by ascer- 

 taining the note produced by the siren when all the holes were open 

 while the disk revolved at the same rates. It was thus found that 

 clear tones could be heard varying with the rate up to the maximal 

 attainable limit, somewhat above the note C'", i.e., 1056 per second. 



II. 



No doubt adventitious secondary oscillations followed the single 

 puffs for a short period, and probably gave them character as well as 

 making them more distinctly audible. Hence we wished to hear 

 what character the tones would have if caused by a short series of 

 these vibrations. We commenced with a number of holes open 

 which, at our maximal rate, would have a duration of y^ second. 

 This gave a tone similar to, and quite as distinct as, that produced 

 when all the holes were open. We then gradually reduced the 

 number of holes used, and we found a fact which, as far as we know, 

 has not been previously observed, and for which we were not prepared 

 by the importance given by our physical friends to the necessity of a 

 series of waves in the production of sounds. When only two holes 

 remained open, the variation in tone following changes in rate was 

 perfectly distinct, and a note even higher than C'", corresponding to 

 1056 V.D. could be heard perfectly. That is to say, a series made up 

 of two puffs which lasted T ^s^th of a second each, or less than g-^th 

 of a second for the series, was capable of stimulating the terminals of 

 the auditory nerve in such a way as to make the tone C'" readily 

 recognisable. 



When the single hole and the double hole were tried alternately, 

 the result was striking. The pitch of the note caused by the short 

 series of two puffs rose and fell with the increase or decrease of the 



