ADVANCEMENT OE SCIENCE. 135 



made in unison with, the echoes, a continued musical sound will be the result. 

 Suppose the wheel to be turned with such velocity as to cause a snap at the very- 

 instant the return echo passes the point at which the apparatus is placed, the se- 

 cond sound will combine with the first, and thus a loud and sustained vibration 

 will be produced. It will be evident from this that every room has a key note, 

 and that if an instrument be sounded on this, it will resound with great force. It 

 must be apparent also that the continuance of a single sound and the tendency 

 to confusion in distinct articulation will depend on several conditions — first, on the 

 size of the apartment ; second, on the strength of the sound, or the intensity of 

 the impulse ; third, on the position of the reflecting surface ; and, fourth, on the 

 nature of the material of the reflecting surfaces. 



In regard to the first of these, the larger the room, the longer time will be re- 

 quired for the impulse to reach the wall along the axis ; and if we suppose that 

 at each collision a portion of the original force is absorbed, it Avill require double 

 the time to totally extinguish it in a room of double the size, because the velocity 

 of sound being the same, the number of collisions in a given time will be inversely 

 as the distance through which the sound has to travel- 



Again, that it must depend upon the loudness of the sound, or the insecurity of 

 the impulse must be evident, when we consider that the cessation of the reflections 

 is due to the absorption of the walls, or irregular reflection, and that consequently 

 the greater the amount of original disturbance the longer will be the time required 

 for its complete extinction. This principle was abundantly shown by our observa- 

 tions on different rooms. 



Thirdly, the continuance of the resonance will depend upon the position of the 

 reflecting surfaces. If these are not parallel to each other, but oblique, so as to 

 reflect the sound, not to the opposite but to the adjacent wall, without passing 

 through the longer axis of the room, it will evidently be sooner absorbed. Any 

 obstacle also which may tend to break up the wave and interfere with the reflec- 

 tion through the axis of the room, will serve to lessen the resonance of the apart- 

 ment. Hence, though pannelling the ceiling and introducing a variety of oblique 

 surfaces may not prevent an isolated echo, provided the distance be sufficiently 

 great and the sound sufficiently loud, yet that they do have an important 

 effect in stopping the resonance is evident from theory and experiment. In a 

 room fifty feet square, in which the resonance of a single intense sound continued 

 six seconds, when eases and other objects were placed around the wall, its con- 

 tinuance was reduced to two seconds. 



Fourthly, the duration of the resonance will depend on the nature of the material 

 of the wall. A reflection always takes place at the surface of a new medium, and 

 the amount of this will depend on the elastic force or power to resist compression 

 and the density of the new medium. For example, a wall of nitrogen, if such could 

 be found, would transmit nearly the whole of a wave of sound in air, and reflect, 

 but a very small portion. A partition of tissue paper would produce nearly the 

 same effect. A polished wall of steel, however, of sufficient thickness to prevent 

 yielding, would reflect, for practical purposes, all the impulses through the air 

 which might fall upon it. The rebound of the wave is caused, not by the oscillation 

 of the wall, but the elasticity and mobility of the air. A single ray of sound 

 striking against a yielding board would probably increase the loudness of the 

 reverberation, but not its continuance. On this point a series of experiments were 

 made by the use of the tuning fork. In this instrument the motion of the foot 



