by Rotating Tuning-forks. 535 



evident that the rotation does not prevent the vibration of the 

 tuning-fork, but simply the communication of the motion to the 

 air. As yet we are unable to give any explanation of this re- 

 markable phenomenon." 



I repeated this experiment a long time ago, but arrived at a 

 quite different result, an account of which I laid before the Phy- 

 sical Society of Berlin*. I in fact never heard the sound of the 

 tuning-fork cease, but only become weaker; but I distinctly 

 heard at the same time a higher tone, as well as a series of beats 

 which coincided in number with the number of half-revolutions 

 of the tuning-fork. MM. W. and E. H.Weber, to whom I 

 communicated my discordant result, unfortunately could not 

 a^ain find the tuning-fork which they had used. Professor W. 

 Weber, however, wrote me that it was a common aa-iovk, and 

 suggested that the reason why the higher tone had not been 

 perceptible in their experiments was that they had employed a 

 slower and more noisy lathe. But a satisfactory explanation was 

 still wanting. 



My attention was lately recalled to this almost forgotten ob- 

 servation by Konig's beautiful experiment, in which the tone of 

 a tuning-fork sounds higher when it is approaching the ear, so 

 that the number of beats which it gave with another fork while 

 at rest is altered by its motion f. The idea easily suggested 

 itself that tones of different pitch must reach the ear from the 

 two prongs of the rotating tuning-fork, since one is moving 

 towards the ear and the other away from it. I consequently 

 took up my experiments again ; but this time they have led me 

 to an entirely different view of the phenomenon. 



The excellent tuning-forks which are now available allow of the 

 experiment being made in a very decisive manner. I used prin- 

 cipally a "middle c"-fork (512 vibrations), by Lange of Berlin, 

 and a cc-fork (1024 vibrations j), by Konig of Paris ; the prongs 

 of the former were 155 millims. long, and those of the latter 

 100 millims. The section of the prongs of both forks was rect- 

 angular, as is usually the case in tuning-forks ; at the point, the 

 prongs of the "middle c"-fork measured 11 millims. by 6 mil- 

 lims., and those of the cc-fork 14 millims. by 6 millims., the 

 smaller dimension being in both cases in the direction in which 

 the fork vibrates when sounded. When these forks were fixed 

 in the lathe and set in rotation at the rate of about twelve turns 

 in a second, after being caused to vibrate, the tone c rose about 

 three-quarters of a tone, and cc a little more than half a tone. 



* Fortschritte der Physik, 1850-51, vols, viii. & ix. p. viii. 



t Konig, Catalogue des appareils d'Acoustique, p. 16, No. 75. 



% [The above numbers correspond respectively to 256 and 512 complete 

 vibrations, according to the mode of counting usually adopted in England. 

 — Transl.] 



