

VELOCITY OF SOUND. 333 



the report or the whistling is heard, if the observer is at 

 a distance of several hundred, or still better several thou- 

 sand, feet. Even at a distance of 200 feet, the sound 

 produced by a person striking a hard object, such as a 

 stone or a log of wood, with an axe or a hammer, is 

 not heard at the instant when the object is seen to be 

 struck, but the sound is heard sensibly later. The 

 velocity with which sound is propagated has been de- 

 termined by observing the time which elapses between 

 seeing the flash when a very distant gun is fired and 

 hearing the report. The average velocity thus found 

 is 340 m per second ; it is somewhat smaller in cold air, 

 and somewhat greater when the air is warm. 



From the interval in time between lightning and 

 thunder, during a thunder-storm, the distance of the 

 thunder-cloud may be approximately estimated. Thus 

 an English mile is about 1609 m , and sound requires 



therefore - = 4' 73 seconds, or very nearly 4 se- 

 conds, to travel an English mile. To pass over 4 miles, 

 19 seconds are required, or nearly ^ of a minute ; 

 similarly, if between lightning and thunder there is an 

 nterval of say half a minute, we may conclude that 



OA 



lie thunder-cloud is at a distance of -^ miles, or a 



4f 

 little more than 6 miles. 



Air is the most usual conductor of sound, but not 

 he best. Many solid bodies transmit sound very well 

 'tfid much better than air. The transmission of sound 

 an be strikingly shown by means of a tightly-stretched 

 >iece of twine, or still better an iron wire. Each end 

 >f the cord or wire is fixed into the middle of a thin 



