Chap. 10.] Experiments on the Velocity of Sound. 447 



progrefs. 4th. That the velocity of found is the 

 fame, whether a cannon 'is placed towards the pcr- 

 fon who hears its report, or turned a contrary ways 

 in other words, a great gun fired from the Tower of 

 London eaftward, would be heard at Weflminfter in 

 the fame interval of t]me as if it was difcharged to- 

 wards the latter place. And if the gun was dif- 

 charged in a direction perpendicular to the horizon, 

 it would be heard as foon as if difcharged in a right 

 line towards the hearer. By other experiments, how- 

 ever, the progrefs of found appears to be impeded bf 

 a (Irong wind, fo that it travels at the rate of about 

 One mile flower in a minute againft a ftrong wind thaa 

 with it. 



A knowledge of the progrefTion of found is not an 

 article cf mere fterile curiofity, but in feveral in- 

 fiances ufeful j for by this we are enabled to deter- 

 mine the diftance of mips or other moving bodies. 

 Suppofe, for example, a veiTel fires a gun, the found 

 of which 4s heard five feconds after the flafh is feenj 

 as found moves 1 142 Englifh feet in one fecond, this 

 number multiplied by 5 gives the diftance of 57 10 

 feet. The fame principle has been already mentioned 

 as applicable in ftorms of lightning and thunder. 



The waves or pulfes of found being reflexib!^ in 

 their courfe when they meet with an extended folid 

 body of a regular furface, an ear placed in the paf- 

 fage of thefe reflected waves will perceive a found 

 fimilar to the original found, but which will feem to 

 proceed from a body fituated in a fimilar pofitioa 

 and diftance behind the plane of reflection, as the real 

 (bunding body is before it. This reflected found is 

 commonly called an ECHO, which, however, cannot 

 take place at lefs than fifty-five feet; becaufe it is 

 2 nrcefTary 



