258. =Loudness of Sound.=--The loudness of sound depends upon the width of the vibrations producing it. The harder you strike the end of the tuning-fork, Fig. 182, the farther will it vibrate the one way and the other, and the louder will be the sound. The same thing is true of the strings of a piano. A round bell, when it is struck, tends in its vibration to take an oval form, and the extent of its vibration back and forth as it does this governs the loudness of the sound. As sound passes from the sounding body the vibration gradually lessens, and at length dies away. It is like the successive vibrations or waves of water produced by dropping a stone in it. The louder the sound is the larger are the first vibrations, and the farther will the vibrations extend, as in water a large stone dropped into it will produce larger waves than a small one, and the waves will extend over a greater space. 259. =Diffusion of Sound.=--When there is no hindrance sound spreads equally in all directions. It is in this respect with the vibrations or waves of air as it is with the waves of water when a stone is dropped into it. Light is also diffused in the same manner, as you will see in another chapter. [Illustration: Fig. 186.] 260. =Reflection of Sound.=--As waves of water striking against any object bound off, so it is with the vibrations or waves of sound. And the same is true of this as of all motion, as stated in § 206, that the angle of incidence is equal to the angle of reflection. The reflection of sound is the cause of _echoes_. In order that an echo be perfect the sound must be reflected back to the ear from some plane surface of some size. Sometimes when there are successive plane surfaces of rocks along a river there are successive echoes. Thus in Fig. 186 (p. 200) is represented a locality on the Rhine where a sound is reflected at successive places, 1, 2, 3, 4. The rolling of thunder, though sometimes caused by the different distances of parts of the same flash of lightning, is commonly owing to reflections of the sound among the clouds. From this cause the report of a cannon is more apt to be a rolling sound when there are clouds above than when the sky is clear. Sound is continually reflected in every variety of direction from obstacles with which it meets. Thus in a room it is reflected from the walls and from all the objects in the room; and the more varied are the surfaces the more varied and confused are the reflections. You know that a voice has a very different sound in a room when it is empty from what it has when the room is filled with an audience. Indeed, a blind speaker can estimate very nearly the size of his audience by the sound of his own voice. The explanation is, that with a full audience the surfaces for reflection are vastly multiplied, and so deprive the sound of the sharp and ringing character which is given to it by reflection from comparatively few surfaces which are plane and firm. The effect