202 Sir G. S. Mackenzie, Bart., on the 



vacuum, more or less perfect and extensive, precedes ; and 

 its being filled, and the production of sound, are simultaneous. 

 We shall begin with loud sounds. 



When the electric fluid passes from one cloud to another, 

 or descends to the earth, the velocity is so great, that the air 

 is suddenly displaced along its whole passage, and a very 

 perfect vacuum is formed. It is as suddenly supplied, and a 

 loud crash is instantly heard. In this case, no hard bodies 

 are interposed ; it is merely the quick passage of the light- 

 ning that causes the vacuum. 



. When a cannon is fired, the sudden and forcible expansion 

 of the gases which are set loose, causes them to rush out of 

 the tube, driving the wadding, shot, and air which previously 

 occupied it before them, and thus a vacuum is produced in 

 the tube. The instant the expelling force ceases to act, the 

 air violently enters to supply the vacuum, and the sound pe- 

 culiar to the firing of cannon is produced. 



When a string or wire is stretched between two points, 

 and made to vibrate by its elasticity, each passage of the 

 string from one position to another (which is rapid in propor- 

 tion to the tension), displaces the air in the same manner as 

 the electric spark, and on the filling up of the vacuum sound 

 is produced. If there were only a single movement, there 

 would be but a single sound ; but when vibrations succeed 

 each other at intervals so short as to be inappreciable to our 

 sense, the formation of vacua, and their filling up, are so quick 

 in succession, as to render it impossible for the ear to per- 

 ceive any interruption, and the sound is accordingly heard, 

 while the vibrations proceed, as prolonged "or musical. 



The case is the same in regard to sound produced by fric- 

 tion on strings, an extremely rapid succession and filling of 

 vacua being the result of the very minute starts of the bow 

 in overcoming the friction. 



When a blow is struck with any substance on another, with 

 a hammer, for instance, on a resisting body, the force of the 

 blow, at the moment of contact, displaces the air at and 

 around the spot, so as to form a vacuum, and the air instantly 

 recovering its position, sound is evolved. 



