1856] WRITINGS OF JOSEPH HENRY. 411 



On the same principle we may at, once assert that the 

 panelling of a room, or even the introduction of reflecting 

 surfaces at different distances will not prevent the echo, pro- 

 vided they are in parallel planes, and situated relatively to 

 each other within the limit of perceptibility. 



Important advantage may be taken of the principle of 

 reflection of sound by a proper arrangement of the reflecting 

 surfaces behind the speaker. We frequently see in churches, 

 as if to diminish the efi'ect of the voice of the preacher, a mass 

 of drapery placed directly in the rear of the pulpit. However 

 satisfactory this may be in an sesthetical point of view, it is 

 certainly at variance with correct acoustic arrangements, 

 the great object of which should be to husband every articu- 

 lation of the voice, and to transmit it unmingled with other 

 impulses and with as little loss as possible to the ears of the 

 audience. 



Another efiect of the transmission and reflection of sound 

 is that which is called reverberation, which consists of a pro- 

 longed musical sound, and is much more frequently the 

 cause of indistinctness of perception of the articulations of 

 the speaker than the simple echo. 



Reverberation is produced by the repeated reflection of a 

 sound from the walls of the apartment. If for example a 

 single detonation takes place in the middle of a long hall 

 with naked and perpendicular walls, an impulse will pass 

 in each direction, will be reflected from the walls, cross each 

 other again at the point of origin, be again reflected, and so 

 on until the original impulse is entirely absorbed by the 

 solid materials which confine it. The impression will be 

 retained upon the ear during the interval of the trans- 

 mission past it of two successive waves, and thus a contin- 

 ued sound will be kept up, particularly if the walls of any 

 part of the room are within 30 feet of the ear. If a series of 

 impulses, such as that produced by the rapid snaps of a quill 

 against the teeth of a wheel be made in unison with the 

 echoes, a continued musical sound will be the result. Sup- 

 pose the wheel to be turned with such velocity as to cause 

 a snap at the very instant the return echo passes the point 



