EMPIRICAL KNOWLEDGE, XXPLAXATIOX, <L-c. 177 



tion behindhand, the two portions would exactly neutralise 

 each other. This experiment has recently been performed 

 with success by Quincke and Konig*. The interference 

 arising between the waves from the two prongs of a 

 tuning-fork was also predicted by theory, and proved to 

 exist by Weber ; indeed it may be observed by merely 

 turning round a vibrating fork close to the ear u . 



It is a plain result of the theory of sound that, if we 

 move rapidly towards a sounding body, or if it move rapidly 

 towards us, the pitch of the sound will be a little more 

 acute ; and, vice versa, when the relative motion is in the 

 opposite direction, the pitch will be more grave. It arises 

 from the less or greater intervals of time between the 

 successive strokes of waves upon the auditory nerve, 

 according as the ear moves towards or from the source 

 of sound relatively speaking. This effect was predicted 

 by theory, and afterwards verified by the experiments of 

 M. Buys Ballot, on Dutch railways, and of Mr. Scott 

 Russell, in England x . Whenever, indeed, one railway 

 train passes another, on the locomotive of which the 

 whistle is being sounded, the drop in the acuteness of the 

 sound may be noticed at the moment of passing. This 

 change gives the sound a peculiar howling character, which 

 many persons must have noticed. I have calculated that, 

 with two trains travelling thirty miles an hour, the effect 

 would amount to rather more than half a tone, and it 

 would often amount to a tone. A corresponding effect is 

 produced in the case of light undulations, when the eye 

 and the luminous body rapidly approach or recede from 

 each other. It is shown by a slight change in the refrangi- 

 bility of the rays of light, and a consequent change in the 

 place of the lines of the spectrum, which has been made 

 to give most important and unexpected information con- 



' Tyndall's 'Sound,' p. 261. 

 u Ibid. p. 273. * Ibid. p. 78. 



VOL. II. N 



