HARMONY IN MUSIC. 81 



Each separate wave-crest (or condensation) of air which 

 passes by the string is, of course, too weak to produce a sen- 

 sible motion in it. But when a long series of wave-crests 

 (or condensations) strike the string in such a manner that 

 each succeeding one increases the slight tremour which 

 resulted from the action of its predecessors, the effect 

 finally becomes sensible. It is a process of exactly the 

 same nature as the swinging of a 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 intervals, 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 tremours 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 

 few seconds to be sounding also. Then damp the first 



