THE REFRACTION OF SOUND. 



Bv William B. Taylor. 



As ordinarily received by the ear, sound may be considered as an 

 aerial impulse or succession of impulses radiating in all directions 

 from tbe origin of disturbance, and consisting in the main of a small 

 to-and-fro movement generating an expanding wave of comi^ression of 

 determinate velocity, necessarily followed by a corresponding wave of 

 attenuation. This vibration is a mass-movement of the air, and not a 

 molecular movement ; and the surface or surfaces of similar phase of 

 movement are equidistant from the origin ; or, in other words, the 

 wave-fronts are essentially spherical. 



The transmission of sound through liquid and solid mediums, though 

 similar in charactei", and subject to similar perturbations, will not hero 

 be considered. 



Sound, while difl'ering widely from light in the character of its waves 

 and their order of magnitude, yet thus moves like light in radial lines, 

 and like light is diverted from its rectilinear course whenever its waves 

 undergo an unequal retardation or acceleration ; that is, whenever any 

 segment of a series of advancing wave-fronts (regarded as an acoustic 

 beam) receives from any cause an unequal velocity on its opposite sides, 

 such beam is bent toward the side of least velocity, and from the side 

 of greatest velocity; the line of impulse or of acoustic effect being 

 always perpendicular to the surface of the wave front. 



By sound-beams, or sound-rays, the longitudinal direction of sound is 

 to be understood ; by sound-waves, the transverse surfaces of simul- 

 taneous movement are to be understood. The amplitude of the wave- 

 motion is very minute, being ordinarily a barely visible magnitude. It 

 lies in the direction of the wave-length, or of the sound-ray. In the 

 case of light, the ami)litude of vibration is transverse to the wave- 

 length, or to the direction of the ray. 



If we imagine a symmetrical boat on a p(H-fectly still sea or lake, 

 mechanically [>ropelled by oars of precisely similar character and move- 

 ment, it is obvious that such a boat must advance in a perfectly straight 

 course. If placed in a uiiilbrm current, the boat, though drifting with 

 the current, would still maintain a rectilinear path. If, however, such 

 moving boat were to enter a current, or to encounter a difference of cur- 

 rent on its oi)posite sides, or were it to encounter water of different 

 density, as by passing obliquely from salt-water into a margin of IVesh 

 water, then, at the moment of transition, the oars meeting with unequal 



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