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



or mass, of sound evolved in the case of circumscribed por- 

 tions of air, depends, cwterls paribus, on the extent ol the 

 vacuum produced. This is illustrated by the pipes in an or- 

 gan. The velocity of the air passing into the pipes, is regu- 

 lated by the weight on the bellows, so that the pressure is 

 uniform for all the pipes, from the smallest to the largest. 

 The size of the pipe, and width of the languid, regulates the 

 amount of sound, on which gravity or acuteness depend, the 

 pressure being the same. The intensity depends on the force 

 or velocity of the air passing into the pipe. The pressure 

 being uniform, when it is exerted on a large pipe, brings out 

 a grave sound ; and when exerted on a small pipe — on a 

 small quantity of air — the sound is more intense or acute. 

 In the former case, the mass of sound being greater, though 

 not so intense, is heard at a greater distance than that pro- 

 duced by the small pipes. Accordingly, we hear the sound 

 produced by the pedal pipes of an organ, on the outside of a 

 church, when we can distinguish no other. Supposing that 

 all the pipes were of the same size, and each to have its own 

 bellows, if the force applied to expel the air from all the bel- 

 lows were as 1, the same sound would be produced from all 

 the pipes. But suppose the force to be augmented as 2, 3, 

 and so on, in different bellows, the sound would be propor- 

 tionally intense or acute. This is illustrated also by the 

 flute, in which a note may be raised an octave by blowing 

 harder on the same hole. The greater the force exerted on 

 the same amount of air, the more perfect is the vacuum, and 

 more intense or acute the sound. The force being dimi- 

 nished, the sound becomes more grave. The sound from or- 

 gan pipes is brought out by forcing air over a serrated sur- 

 face (the languid), which, by causing a number of rapid move- 

 ments, produces the necessary condition. In reed pipes, the 

 vibration of a piece of metal produce the vacua. The sound 

 of wooden pipes in the organ is modified by what is called 

 stopping them — closing them with a plug. By this means, 

 a short pipe is made to give the same note as a long one, but 

 softer. The cause of this is, that the air forced into the pipe 

 is resisted and sent back, thus diminishing the rate of mo- 

 tion in the air, and the amount and intensity of the sound. 



