HARMONY EN MUSIC. 71 



heavy bell. A powerful man can scarcely move it sensibly by 

 a single impulse. A boy, by pulling the rope at regular intervals 

 corresponding to the time of its oscillations, can gradually bring 

 it into violent motion. 



This peculiar reinforcement of vibration depends entirely 

 on the rhythmical application of the impulse. When the bell 

 has been once made to vibrate as a pendulum in a very small 

 arc, and the boy always pulls the rope as it falls, and at a time 

 that his pull augments the existing velocity of the bell, this 

 velocity, increasing slightly at each pull, will gradually become 

 considerable. But if the boy apply his power at irregular in- 

 tervals, sometimes increasing and sometimes diminishing the 

 motion of the bell, he will produce no sensible effect. 



In the same way that a mere boy is thus enabled to swing 

 a heavy bell, the tremors of light and mobile air suffice to set 

 in motion the heavy and solid mass of steel contained in a 

 tuning-fork, provided that the tone which is excited in the air 

 is exactly in unison with that of the fork, because in this case 

 also every impact of a wave of air against the fork increases 

 the motions excited by the like previous blows. 



This experiment is most conveniently performed on a fork, 

 Fig. 7, which is fastened to a sounding-board, the air being 

 excited by a similar fork of precisely the same pitch. If one is 

 struck, the other will be found after a fe\v seconds to be sound- 

 ing also. Then damp the first fork, by touching it for a moment 

 with a finger, and the second will continue the tone. The 

 second will then bring the first into vibration, and so on. 



But if a very small piece of wax be attached to the ends of 

 one of the forks, whereby its pitch will be rendered scarcely 

 percept'bly lower than the other, the sympathetic vibration of 

 the second fork ceases, because the times of oscillation are no 

 longer the same in each. The blows which the waves of air 

 excited by the first inflict upon the sounding-board of the second 

 fork, are indeed for a time in the same direction as the motions 

 of the second fork, and consequently increase the latter, but 

 after a very short time they cease to be so, and consequently 

 destroy the slight motion which they had previously excited. 



