412 • WRITINGS OF JOSEPH HENRY, [1856 



at which the apparatus is placed, the second sound will com- 

 bine with the first, and thus a loud and sustained vibra- 

 tion will be produced. It will be evident from this that 

 every room has a key-note, and that to an instrument of the 

 proper pitch 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 perception, will depend 

 on several conditions; — first, on the size of the apartment; 

 secondly, on the strength of the sound or the intensity of 

 the impulse; thirdly, on the position of the reflecting sur- 

 faces; and fourthly, 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 required for the impulse along the axis 

 to reach the wall; and if we suppose that at each collision a 

 portion of the original force is absorbed, it will 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 througli which the sound has to travel. 



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

 or the intensity of the impulse, must be evident, when we 

 consider that the cessation of the reflections is due to the 

 absorption by the walls, or to irregular reflection, and that 

 consequently the greater the amount of original disturb- 

 ance the longer will be the time required for its complete 

 extinction. This principle was abundantly shown by our 

 observations on diff'erent 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, that may tend to break 

 up the wave and interfere with the reflection through the axis 

 of the room will serve to lessen the resonance of the apart- 

 ment. Hence, though the panelling, the ceiling, and the 

 introduction of a variety of oblique surfaces, may not pre- 



