194 LECTURES TO SCIENCE TEACHERS. 



solid bodies. If we take a lighter medium, such as air, a 

 more vigorous and prompt reinforcement of the sound is 

 produced. At the same time the decay of the sound is 

 more rapid on the removal of the source. Here I have a 

 tuning-fork. When I strike it and hold it over a par- 

 ticular column of air, the air within the jar responds very 

 vigorously. The reinforcement of a note of definite sound 

 in this manner is termed resonance. 



If I pour water in the jar, thereby obtaining a column 

 of air vibrating at a different rate, the response will not be 

 nearly so loud. Of course by decreasing the length of the 

 column of air we shall increase the difference between the 

 two vibrations. If now, in place of a simple tone, as in 

 this tuning-fork, I produce a compound tone, the jar will 

 pick out from the compound tone that one constituent 

 which corresponds to its own rate of vibration, and will 

 reinforce that tone to the exclusion of all the others. 

 This is the principle of Helmholtz's resonators which are 

 capable of responding to notes of a certain definite pitch ; 

 that is to say to simple pendular vibrations. If the 

 nozzle of one of these resonators be placed in the ear, and a 

 compound tone be sounded, the resonator will respond to 

 one particular tone only, and thus it can be ascertained 

 if that tone be among the constituents of the compound 

 tone. Helmholtz has thus made an analysis of vowel 

 sounds by means of these resonators. 



I have here a series of brass pipes, which are of different 

 lengths, and on pulling out one of these pipes you hear 

 a little explosion. That explosion generates an extremely 

 intricate wave-form, inasmuch as it is produced by the coal- 

 escence of a great many vibrations of different periods. 

 Nevertheless, this column of air within the tube can re- 

 spond to vibrations of only one period. Out of the multitude 

 of vibrations existing in the explosion which follows the 

 sudden withdrawal of this tube, the air within this tube 

 selects one tone and reinforces it, and thus you get, quite 

 audibly, the note of the tube by the sudden pulling out of 

 the stopper. Inasmuch as these tubes are tuned so as to 

 give notes of definite ratio, I think we shall find that from 

 the noise made by simply pulling out these tubes suc- 

 cessively we may get the notes of the common chord. 



This is an explanation of the sound given by organ pipes 



