WATER-WAVES AND SOUND-WAVES. 



171 



ing with phenomena about which Science says she does know some- 

 thing : from a consideration of these known facts we shall be able, 

 slowly but surely, to grasp some of the much less familiar phenomena 

 with which spectrum analysis is especially concerned. 



Fig. 8. 



We all know that some sounds are what is termed high and others 

 low, a difference which in scientific language is expressed by saying 

 that sounds have a difference in pitch. We know that the difference 

 between a sound which is pitched high and a sound which is pitched 

 low is simply this that the pulses or waves, as we may call them for 

 simplicity's sake, which go from the sender-forth of the sound (which 

 may be a cannon, a piano, or anything else) to the receiver, which is 

 generally the human ear, are of different lengths. What in physics is 

 called a sound-wave is constructed as follows : We have a line A JC, 

 which represents the normal condition of the air through which the 

 sound is traveling, and curves which represent to the eye first, the 

 relative amounts of compression ( + ) and rarefaction ( ) brought about 

 by the sound in the case of each pulse, and secondly the relationship 

 of this to the actual length of the wave, or, what is the same thing, the 

 time taken for the pulse to travel. Thus we may have long waves and 

 short waves independently of the amount of compression or rarefaction, 

 and much or little compression or rarefaction independently of the 

 length of the wave. We know that the difference between a high note 

 and a low note, whether of the voice or of a musical instrument, is, that 

 the high note we can prove to be produced by a succession of short 



