172 



THE POPULAR SCIENCE MONTHLY. 



waves such pulses as have been described and the low note by a 

 succession of long waves. 



Now, the loudness or softness of a note does not alter its pitch, that 

 is, it does not alter the length of its waves or the rate at which they 

 travel. I can send a wave along the rope either violently or gently, 



Fig. 9. 



but with the same tension of the rope we shall find that the length of 

 the waves is the same, provided the period of vibration is the same. 

 Hence, then, the other idea added to the idea of pitch. 



There is another point which is worth noting, although it is not 

 needful to refer to it in any great detail, and that is, that we know that 

 sound travels with a certain velocity, and that this rate is subject to 

 certain small variations owing to different causes. 



We not only have to deal with amplitude that is, the departure of 

 the + and parts of the curve from the line AX- and velocity, but 

 we have this most important and very beautiful fact (for fact it is), 



Fig. 10. Sound-Waves op Different Lengths and Amplitudes. 



which some will have observed for themselves : If a person in a room 

 in which there is a piano sings a note, the string of a piano tuned to 

 that particular note will respond ; and, if he sing another note, then 

 another string will reply, the first string being silent. And if the ex- 

 perimenter were skilled enough to sing one by one all the notes to 

 which the strings of the piano are tuned, all the strings would be set 

 into vibration one by one, note for note. This fact may be explained 

 in this way : a piano-wire, or similar sonorous body, which is con- 

 structed to do a certain thing in this case to sound a particular note 

 always sounds that note when it is called upon in a proper way to do 



