32 EXCURSIONS ROUND LONDON. 



The harmonious combination of these 



* 



vibrations produces symphony. Thus, two 

 chords of the same substance, thickness, 

 and tenison, the one of which is a half 

 shorter than the other when struck at the 

 same moment, are in symphony, because 

 the shorter has two vibrations, while the 

 longer has but one. These two chords 

 produce an octave, the one with the other. 



A chord, which is one- third of the first, 

 will make three vibrations for one. 



When all the vibrations correspond with 

 ench other, they are in unison. 



But it is not sufficient that sonorous bo- 

 dies undergo external vibrations to produce 

 sounds } it is also necessary that those vi- 

 brations be extended to their internal mole- 

 cules. 



The concurrence of air is likewise neces- 

 sary to the production of sound. Thus 

 sounds gradually become weaker, in pro- 

 portion as the air is exhausted in the re- 

 ceiyei of an air-pump, and cease altoge- 

 ther 



