NOISE AND MUSICAL SOUNDS. 171 



nished with carpets, curtains, etc., and their reflection from 

 smooth, hard surfaces. By carefully-arranged convex sur- 

 faces, the waves of sound may be readily collected to a focus. 

 These laws of the reflection of sonorous waves explain echoes 

 and the conduction of sound by confined strata of air, as in 

 tubes. We thus explain the mechanism of speaking-trum- 

 pets, the collection of the waves by the pavilion of the ear, 

 and their transmission to the tympanum by the external 

 auditory meatus. To make the parallel between sonorous 

 and luminous transmission more complete, it has been ascer- 

 tained that the waves of sound may be refracted to a focus 

 by being made to pass through an acoustic lens, as a balloon 

 filled with carbonic-acid gas. The waves of sound may also 

 be deflected around solid bodies, when they produce what 

 have been called by Tyndall, shadows of sound. 1 



Any one observing the sound produced by the blow of an 

 axe can note the important fact that sound is transmitted with 

 much less rapidity than light. At a short distance, our view 

 of the body is practically instantaneous ; but there is a con- 

 siderable interval between the blow and the sound. This in- 

 terval represents the velocity of the sonorous conduction. 

 This fact is also illustrated by the interval between a flash of 

 lightning and the sound of thunder. The velocity of sound 

 depends upon the density and elasticity of the conducting 

 medium. Without discussing the formulae of Newton and 

 their correction by Laplace, it is sufficient to state that the 

 rate of conduction of sound by atmospheric air at the freez- 

 ing-point of water is about 1,090 feet per second. This rate 

 presents comparatively slight variations for the different 

 gases, but is very much more rapid in liquids and in solids. 

 In ordinary water, it is 4,708 feet per second ; in iron or steel 

 wire, about 16,000 feet, and in most woods, in the direction 

 of the fibre, about the same. 8 



Noise and Musical Sounds. There is a well-defined 



1 TYXDALL, Sound, London, 1867, p. 22. 2 TYNDALL, op. cit., p. 26, et seq. 



