372 R. Spice— Method of tuning two Forks t 



Though the optical method of tuning, of Lissajous, gives good 

 results, I find that two forks thus tuned to unison, may be a 

 fraction of a vibration out^ without in any way disturbing the 

 steadiness of the figure. 



In the 2d edition (English) of Tjndall's " Sound," in lecture 

 VII, the author says, ''I divide this jar by a vertical diaphragm, 

 and bring OTie of the forks over one of its halves, and the other 

 fork over the other. The two semi-cylinders of air produce 

 beats by their interference. On removing the diaphragm, the 

 beats continue as lond as before, one half of the same column of 

 air interfering with the other." 



Dr. Tyndall does not, however, mention the fact, that precisely 

 the same result would have been obtained if no diaphragm had 

 been employed, yet this is so. When two unison forks are 

 struck on the knee, (or by a piece of lead covered with leather,) 

 and then held together over their proper resonant column, the 

 following phenomena will be observed. 



If there is a diflFerence between their rates, of several vibra- 

 tions, there will of course be rapid beats ; if the forks are very 

 nearly in tune, the beats will succeed each other at long inter- 

 vals; further, when they are almost perfectly in tune, there will 

 not be any beats properly so called, but after the sound of the 

 forks has nearly died away, it will rise or swell out again very 



gradual decrease of sound, down to silence, without any rein- 

 forcement at any time. 



t find that to carry out this tuning absolutely, both forks must 

 be at the same temperature; consequently, after using a file on 

 one of them, I place both forks in a vessel of water to equalize 

 their temperatures, wipe them dry, and test them. To show the 

 accuracy of this method I select the following example: 



A pair of Ut=» forks (256 vibrations) will sound over a column 

 for about 185 seconds ; suppose that the sound decreases up to 

 the 100th second, and then begins to rise ; obviously 100 sec- 

 onds is the time of half a heat, or 200 seconds the beating time ; 

 that is to say, it will have been demonstrated that one of the 

 forks gave jirr of a vibration per second more than its fellow. 



What has been said of the unison, applies to other intervals. 



I have recently executed by this method, Ut,, Ut,, Ut^ and 

 Ut^ forks for the physical cabinet of Columbia College. 



230 Bridge Street, Brooklyn, Jan., 1876. 



