THE SIX GATEWA YS OF KNOWLEDGE. 271 



you do not hear it, it is not to you a sound. 

 That is all I can say to define sound. To explain 

 what it is, I can say, it is change of pressure, 

 and it differs from a gradual change of pressure 

 as seen on the barometer only in being more 

 rapid, so rapid that we perceive it as a sound. 

 If you could perceive by the ear that the 

 barometer has fallen two-tenths of an inch to-day 

 that would be sound. But nobody perceives by 

 his ear that the barometer has fallen, and so 

 he does not hear the fall as a sound. But the 

 same difference of pressure coming on us sud- 

 denly a fall of the barometer, if by any means 

 it could happen, amounting to a tenth of an inch, 

 and taking place in a thousandth of a second, 

 would affect us quite like sound. A sudden 

 rise of the barometer would produce a sound 

 analogous to what happened when I clapped my 

 inds. What is the difference between a noise and 

 a musical sound ? Musical sound is a regular and 

 periodic change of pressure. It is an alternate 

 augmentation and diminution of air pressure, 

 occurring rapidly enough to be perceived as a 



