414 POPULAR LECTURES AND ADDRESSES. 



mony C E, G, arc of half the frequency of 

 the former, and of one-third of the frequency of 

 the latter, and to verify absolutely that they 

 arc of twice the frequency of the error of E,. 



If when the approximate harmony C E / is 

 being sounded, and the beats on it heard, the 

 faintest sound of G is produced by a very 

 gentle excitation of the fork by the bow, 

 instantly a loud beat at half speed is heard. The 

 phenomenon is rendered very striking by alter- 

 nately touching the top of the G fork by the bow 

 so as to stop its vibrations, and then drawing the 

 bow very gently for a fraction of a second l along 

 one side to re-excite them. It is marvellous how 

 small an intensity of the sound G is required to 

 give a smooth unbroken loud beat in the double 

 period. I have found it difficult to excite the G 

 gently enough to give the gradual transition from, 



1 In every case, lo obtain regular beats, each tuning-fork, after 

 being set in vibration by the bow, must be left to itself. The 

 sound is sensibly graver as long as the bow is applied to augment 

 or sustain the vibration than when the fork is left free. Thus, if 

 two tuning-forks nearly, but not quite, in unison, are alternately 

 acted on by the bow and left free, the beats arc less rapid during 

 the time the bow is applied to the higher fork, and more rapid 

 when to the lower, than when both forks arc vibrating freely. 



