THE VELOCITY OF SOUND 189 



the storm-cloud that we see from here, there first 

 came a flash of lightning; then, after waiting some 

 time, the sound of the thunder was heard. Why do 

 not the sound and the lightning come together !" 



"Two things tell us of the thunderbolt: light and 

 noise. The light is the flash of the lightning, the 

 noise is thunder. Likewise in the discharge of fire- 

 arms there is the light produced by the ignition of 

 the powder and the noise resulting therefrom. At 

 the scene of the explosion light and noise are coin- 

 cident ; but for persons at a distance the light, which 

 travels at an incomparably greater velocity, arrives 

 before the sound, which moves more slowly. If you 

 note the discharge of a gun a considerable distance 

 away, you see first the flash and smoke of the ex- 

 plosion, and do not hear the report until some time 

 after; the more distant the explosion, the longer 

 the time. Light travels an immense distance in an 

 exceedingly short time. The flash of the explosion, 

 therefore, reaches the eye at the very instant of its 

 occurrence. If the sound does not arrive until after, 

 it is because it travels much less rapidly and, in order 

 to cover a considerable distance, requires consider- 

 able time, which is easily measured. 



" Suppose ten seconds pass between the flash of a 

 cannon 's discharge and the arrival of the sound. 

 The distance is measured between the place wlim- 

 the explosion occurred and that where it was heard. 

 It is found to be 3400 meters. Sound, therefore, 

 moves through the air, in a single second, a distance 

 of 340 meters. That is a good rate of speed, com- 

 parable with that of the cannon-ball, but nothing, 



