respecting Sound and Light. 117 



When the walls of a passage, or of an unfurnished room, are 

 smooth and perfectly parallel, any explosion, or a stamping 

 with the foot, communicates an impression to the air, which is 

 reflected from one wall to the other, and from the second again 

 towards the ear, nearly in the same direction with the primitive 

 impulse : this takes place as frequently in a second, as double 

 the breadth of the passage is contained in 1130 feet; and the 

 ear receives a perception of a musical sound, thus determined in 

 its pitch by the breadth of the passage. On making the expe- 

 riment, the result will be found accurately to agree with this 

 explanation. If the sound is predetermined, and the frequency 

 of vibrations such as that each pulse, when doubly reflected, 

 may coincide with the subsequent pulse proceeding directly 

 from the sounding body, the intensity of the sound will be 

 much increased by the reflection ; and also, in a less degree, if 

 the reflected pulse coincides with the next but one, the next but 

 two, or more, of the direct pulses. The appropriate notes of a 

 room may readily be discovered by singing the scale in it ; and 

 they will be found to depend on the proportion of its length or 

 breadth to 1 130 feet. The sound of the stopped diapason pipes 

 of an organ is produced in a manner somewhat similar to the 

 note from an explosion in a passage ; and that of its reed pipes 

 to the resonance of the voice in a room : the length of the pipe 

 in one case determining the sound, in the other, increasing its 

 strength. The frequency of the vibrations does not at all imme- 

 diately depend on the diameter of the pipe. It must be con- 

 fessed, that much remains to be done in explaining the precise 

 manner in which the vibration of the air in an organ pipe is 

 generated. M. Daniel Bernoulli has solved several difficult 



