Sound. 375 



point of origin. l The sound by way of the rail will reach the listener 

 in ^ part of a second in 8. 8 vibrations, and will be about ^ as loud as 

 when it started on the rail, or 225 times as loud as the same sound 

 brought by the air. It is well known that sound may be diverted from 

 a straight line, as often observed in the echo, where having struck a 

 solid body it is reflected back. 



It may be reflected at different angles according to the position of the 

 reflecting body with reference to the source of the sound ; just as a ball is 

 reflected from a wall in different directions according to whether it is 

 thrown square against the wall or at an angle. Hence, if the spokes of 

 sound be hurled against a concave wall they will be reflected in a manner 

 to make them converge, and so at the point of convergence or focus the 

 loudness will be greatly increased. Sound is said to move with greater 

 speed in proportion to the elasticity of the conductor, and to be retarded 

 in a certain ratio to its density. In passing through more than one con- 

 ductor, a sound will consequent!}^ travel at different rates, that is, will 

 have longer spokes in the more elastic medium and shorter ones in the 

 more dense, although if it pass from one medium to another that is 

 both more elastic and more dense, the elasticity might offset the density 

 and the rate remain the same. The shape of the medium through which 

 the sound passes must of course influence the direction taken by it. If 

 the sound passes into a denser medium and it is thicker on the left than 

 on the right, that is, wedged shaped, the spokes of sound will be short- 

 ened more on the left than on the right, and the spherical surf ace formed 

 by their front ends will wheel toward the left, thus taking a new direc- 

 tion. If the denser medium be the shape of a convex lens or a globe, 

 the spokes in the middle will be shortened more than those at the edges, 

 and consequently the outside ones will all be turned toward the middle, 

 the spherical front becoming flattened, and under proper conditions even 

 made concave and consequently converging. 



FIG. IBS. Convergence of Sound by a lens. 



Fig. 158 shows how a sound pulse made at A is first flattened and 

 finally made concave and converged into greater loudness at B by pass- 

 ing through a bladder of carbonic acid gas(C) which is heavier than air. 2 

 Every body upon being struck is caused to vibrate more or less accord- 

 1 The speed in cast steel is 16,354 feet per second, at 68 F. 2 See Gagre's Physics. 



