Lesson xvnf.] SOUND. 103 



any sound is at a distance, there must be an inter, 

 val of time elapse before the sound itself can arrive 

 tt the ear j and, indeed, this may often have been 

 manifested to us by our own observations. Thus, 

 for instance, when a gun has been fired at a dis- 

 tance, we may have taken notice, that after we 

 have seen the flash, sometime has elapsed previous 

 to our hearing the report : or again, when a wood- 

 man, remote from us, has been felling trees, we 

 may have observed that an interval of time passes 

 away after we have seen the action performed 

 before we hear the stroke. The experiments which 

 have been made to determine the velocity of sound, 

 have neither been so numerous nor so accurate as 

 could be wished ; and on this account it is, that 

 opinions differ concerning the nature of its pro- 

 gressive motion. Because the intensity of sound 

 diminishes as the distance increases, some persons 

 suppose that the velocity does also : they therefore 

 imagine that the velocity is inversely as the dis- 

 tance, and consequently the time will be directly 

 as the distance : but an opinion which meets with 

 more advocates, and may now bt regarded as con- 

 firmed, is, that sound moves uniformly in common, 

 atmospheric air, at the rate of 1 142 feet in one se- 

 cond of lime, or about an English mile in 4f se- 

 conds ; and this rate of motion being allowed, it 

 will not be any way difficult to determine the time 

 for the passage of sound, the distance being given, 

 and vice versa, in a variety of cases. 



A curious phaenomenon with regard to sounds, 



is an echoy which is caused by the vibrating air 



r 5 being 



