320 SOUND. 



time ; and that it had passed over , , and , of the 

 same space, in , 4-, and ^-, of the same time. As to intense 

 and languid sounds, Mr Derham found that they pass 

 through the same space in the same time ; as appears from 

 the following experiments. At Tilbury Fort were fired one 

 or two common guns, and also a great gun, into which 

 the powder was well rammed ; the report of all these 

 reached Mr Derham, who was about three miles off, at 

 the same time. After sun-set, some muskets, sakers,* 

 and mortars were discharged on Blackheath. Mr Der- 

 ham could not hear the muskets, either on account of the 

 great distance, or because the air was not clear enough ; 

 yet he heard the sakers and mortars in the same space of 

 time, though the report of the mortars was more languid 

 and weak than that of the sakers.' 



4. Various Phenomena produced by the Motion of 

 Sound. j 



In consequence of the fact that it requires some time 

 for sound to pass through the air, it is impossible for two 

 sounds at any distance from each other, to be heard at 

 the same moment of time, by persons who are at those 

 places. 



If A and B are standing at the distance of one mile 

 from each other, and each fires a gun at the same mo- 

 ment, A will riot hear B's gun until several seconds 

 after he hears his own, because the sound will require 

 that time to pass through the distance between them. 

 And the same will be the case with B. One might 

 at first suppose that if A should wait and fire at the mo- 

 ment he hears the report from B the two sounds would 

 then be heard together. A would hear them together, 

 but the time that must elapse after B had fired, before 

 the sound from A would come to him, would be greater 

 than if they fired at the same moment. For he must 

 wait till the sound of his own gun had gone to A, and 

 then until the sound of A's discharge should return to 

 him. It is thus evidently impossible for two persons, 

 standing at a distance from each other, to produce a 

 sound which shall be heard by both, at the same time. 



* A spicies of ordnance. 



