2/2 SOUND 



and the sound is heard at practically the instant it is made. 

 Ordinarily we are not conscious that it requires time for 

 sound to travel from its source to our ears, because the dis- 

 tance involved is too short. At other times we recognize 

 that there is a delay ; for example, thunder reaches our ears 

 after the lightning which caused the thunder has completely 

 disappeared. If the storm is near, the interval of time 

 between the lightning and the thunder is brief, because the 

 sound does not have far to travel ; if the storm is distant, 

 the interval is much longer, corresponding to the greater dis- 

 tance through which the sound travels. Sound does not 

 move instantaneously, but requires time for its transmission. 

 The report of a distant cannon is heard after the flash and 

 smoke are seen ; the report of a near cannon is heard the in- 

 stant the flash is seen. 



The speed with which sounds travels through the air, or its 

 velocity, was first measured by noting the interval (54.6 

 seconds) -which elapsed between the flash of a cannon and 

 the sound of the report. The distance of the cannon from 

 the observer was measured and found to be 61,045 f eet > and 

 by dividing this distance by the number of seconds, we find 

 that the distance traveled by sound in one second is ap- 

 proximately 1 1 1 8 feet. 



High notes and low notes, soft notes and shrill notes, all 

 travel at the same rate. If bass notes traveled faster or 

 slower than soprano notes, or if the delicate tones of the 

 violin traveled faster or slower than the tones of a drum, 

 music would be practically impossible, because at a distance 

 from the source of sound the various tones which should 

 be in unison would be out of time some arriving late, 

 some early. 



255. Sound Waves. Practically every one knows that a 

 hammock hung with long ropes swings or vibrates more slowly 



