Manner in which Sound is produced. 201 



If, in the experiment with the bell in vacuo, no sound be 

 produced, or if it be produced but not conducted by the ma- 

 terials of the aparatus, how is it made known that sound is 

 produced when the air is admitted into the receiver ; and 

 how is it then conducted to the ear by the materials of the 

 apparatus ? 



Motion was produced by the bell in vacuo, and no sound 

 was heard. The same motion being produced when the air 

 was admitted, sound was heard ; therefore, some effect must 

 have been caused in the air, and some motion produced, in- 

 dependent of that caused by the impulse given to the bell. 

 That this was the case, and how the sound was produced, I 

 will endeavour presently to shew. In this, that motion was 

 given to the air in the receiver ; that the air communicated 

 motion to the receiver ; that the receiver imparted it to the 

 air without ; the air without to the drum of the ear ; and the 

 drum to that portion of the air within it, and next to the 

 sensorium — all agree. 



Instead of saying that sound consists of motion given to the 

 air, I am induced by many observations to believe the cause 

 of sound being evolved to be, in all cases, the rushing of air 

 into a vacuum, the effect being a peculiar movement by which 

 sound is evolved and conveyed. This new result of observa- 

 tion I will now proceed to illustrate. 



That the motion of the air rushing into a vacuum, and which 

 it does with very great velocity, does produce sound, is illus- 

 trated by the extension of an experiment usually made to ex- 

 hibit the pressure of the atmosphere. A hollow vessel, hav- 

 ing its mouth covered by a piece of bladder stretched over it, 

 so as to have the fastening air-tight, is applied to an air- 

 pump or exhausting-syringe. As soon as a portion of air is 

 abstracted from the vessel, the bladder is seen to be pressed 

 into it. Let the exhaustion be carried on as far as may be, 

 without the bladder tearing. If an opening be now made 

 with the point of a knife, the air rushes in, and a loud report 

 is heard. The sound produced in this case is evidently the 

 result of the filling up of a vacuum. It remains to be shewn 

 that, in all cases when sound is evolved, the formation of a 



