450 PHILOSOPHICAL TRANSACTIONS. [aNNO 1747-8. 



sectJhd.* AccorcHng to this last measure, the velocity of sound, when the 

 wind -|- is still, is settled at the rate of a mile, or 5280 English feet in A-,W". 



To return to our purpose: the length of the conducting wire from the ma- 

 chine to the observers near the 7-mile stone was 6732 feet; the length of that 

 to the Q-mile stone 3868 feet. The first of these measures only was made use 

 of in the present operations concerning the velocity of electricity. In 12 dis- 

 charges of the coated phial, which were felt by the observers near the 7-mile 

 stone, and who, by a second watch of Mr. Graham's, measured the time be- 

 tween feeling the electrical commotion and hearing the report of the gun, with 

 the utmost attention and exactness, was at a medium 5J- seconds. And as the 

 gun was distant from these observers 6732 feet, it follows, from the experiments, 

 which have been made on the velocity of sound, that the real instant of the dis- 

 charge of the gun preceded that of the observers hearing its report, at this time, 

 when the strength of the wind was not so great as to enter into the computation, 

 6-nroV ; or preceded the instant when the electrical commotion was felt only 

 O-ToVo"- But this instant was, from the nature of the experiment, necessarily 

 prior to that of the electrical explosion, which was not made till the fire of the 

 gun was actvially seen; and therefore the time between the making of that explo- 

 sion and its being actually felt by the observer, which must have been less than 

 O-rtroV"* w^s really so small, as not to fall under any certain observation, when it 

 is to be distinguished from that which must of necessity be lost, between the 

 firing of the gun and the electrical explosion itself 



In all the experiments where the circuit was formed to any considerable length, 

 though the coated phial was very well charged, the snap at the gun-barrel, on 

 the explosion, was not near so loud as when the circuit is formed in a room ; so 

 that a by-stander, though versed in these operations, from seeing the flash and 

 hearing the report, would imagine the stroke at the ends of the conducting wire 

 to be very slight; the contrary of which, when the wire has been properly con- 

 ducted, has always happened. 



From a review of these experiments, the following observations may be 

 deduced. 



1 . That in all the preceding operations, when the wires have been properly 



• M. Cassini de Thury afterwards measured the velocity of sound at Aiguemortes in Languedoc, 

 and found the observations there from those made about Paris vary only half a toise in a second. See 

 Mem. de I'Acad. Royale des Sciences, pour I'annte 1739, p. 126.— Orig. 



+ Dr. Derham found, that when sound was carried against the wind, not only its distance but its 

 velocity was lessened; aud in M. Cassini's memoir, there is an experiment, where sound being carried 

 against ihe wind, which then blew very strong, was retarded near a twelfth part of the usual time in 

 its progress. — Orig, 



