100 SPECIAL SENSES. 



pose, as we wish only to ascertain the laws of the vibrations 

 of sounds in harmony and the mechanism of discords. 



Discords. A knowledge of the mechanism of simple ac- 

 cords enables us to understand more easily the rationale of 

 discords, and vice versa. As in the case of harmony, the 

 fact that certain combinations of musical tones produce a 

 disagreeable impression was ascertained empirically, with no 

 knowledge of the exact cause of the palpable dissonance. 

 Thanks to the labors of modern physicists, however, the 

 mechanism of discords is now pretty well settled. We shall, 

 in our explanation, begin with a combination of tones slightly 

 removed from perfect unison. 



Suppose, for example, that we have two tuning-forks giv- 

 ing precisely the same numbers of vibrations in a second ; 

 the tones are then in perfect unison. We load one of the 

 forks with a bit of wax, so that its vibrations are slightly re- 

 duced, and start them both in vibration at the same instant. 

 Taking the illustration given by Tyndall, we assume that one 

 fork has 256, and the other, 255 vibrations in a second. 

 While these two forks are vibrating, we have one gradually 

 gaining upon the other ; but, at the end of half a second, one 

 will have made 128 vibrations, while the other will have 

 made 127J. At this point the two waves are in direct oppo- 

 sition to each other ; they are moving in exactly opposite di- 

 rections ; and, as a consequence, the sounds neutralize each 

 other, and we have an instant of silence. 1 The perfect sounds, 



1 It is almost essential to this point in our argument to ascertain if two 

 sounds in unison, so arranged that the maximum of the waves of one will coin- 

 cide with the minimum of the waves of the other, will, when thus combined, 

 produce silence. This fact has been illustrated by an example, which we quote 

 from Helmholtz : 



"Suppose that we have two organ-pipes, precisely similar, tuned in unison, 

 placed near each other upon the same sound-board. Each one, sounded simply 

 by a current of air, gives an intense tone ; but if the current of air strike them 

 both simultaneously, the movement of the air is modified in such a manner, that 

 the current enters one of the pipes when it emerges from the other ; no sound 

 strikes the ear of an observer at a little distance, who then can only hear the 



