Manner in which Sound is produced, 203 



In writing, a scratching noise is heard. This is caused by 

 the pen jerking over the rugosities of the paper, causing 

 minute vacua, which are instantaneously filled. 



With respect to soft or low sounds, it must be premised, 

 that a body in motion, when surrounded by a medium, neces- 

 sarily displaces that medium in its progress. A vacuum is 

 the effect of the removal of the body from its position ; and 

 in proportion to the velocity of removal, is the extent of the 

 vacuum left at any one instant of time. Sound is modified 

 in its amount by the bulk of air displaced, and in its intensity 

 by the velocity of displacement. The latter is regulated by 

 elasticity ; for example, the sound produced by the collision 

 of two elastic bodies is far more intense than when two in- 

 elastic bodies come into contact, the velocity being the same 

 in both cases. The faster we write, the louder is the sound 

 of scratching ; and we may write so slow that no sound is 

 produced that is sensible to the ear. Writing with the com- 

 paratively inelastic pencil, the sound is far less intense than 

 when the elastic pen is used. 



When a body is moved slowly through the air, or whe 

 the air slowly passes a body at rest, which is the same thing 

 in effect, the vacuum at any one instant is small, and is every 

 instant supplied, and no sensible sound follows. When a 

 body is moved rapidly, a more extensive vacuum is every in- 

 stant formed, the collapse is proportional, and sound is heard. 



A farther illustration may be found in the case of the 

 cannon. When a full charge is put into a cannon, the spaces 

 for the gases set free by the firing of the powder is confined, 

 and the resistance to their expansion causes a high degree 

 of velocity in their escape ; and the rush of air to supply 

 their place in the tube, causes the sound precisely in the 

 same manner as it does in the case of the exhausted vessel 

 covered by bladder, already mentioned. When a charge 

 sufficient only for a musket is put into a cannon, there is 

 more room for expansion, in proportion to the quantity of 

 gas evolved ; the velocity of expulsion is less, and the sound 

 produced is feeble. But if the small charge be put into a 

 musket, there is a proportional resistance to the expansive 

 force, and a proportional intensity of sound. The amount, 



