[82] 
and where the Meafures of the different Places had 
been fettled with the ntmoft Exaftnefs, Sound was 
propagated at a Medium at the Rat-c only of 1038 
French Feet in a Second. The French Foot exceeds 
the Englifh by feven Lines and a half, or is as 107 
to 1 14: And confequently 1038 French Feet 
are equal' to 1106 Englifh Feet. The Differ- 
ence therefore of the Mcafurcs of Dr. < Derham 
and M. CaJJlni Feet in a Second. •f- Ac- 
cording to this laft Mealure, the Velocity of Sound, 
when the * * Wind is fill, is fettled at the Rate of 
a Mile, or 5280 Englifi Feet in 4" ~^f 0 . 
To return to our Purpoft 5 the Length of the con- 
ducing Wire from the Machine to the Obfervcrs 
near the feven Mile* Stone was (as has been before- 
mention'd) a Mile, a Quarter, and 8 Poles, 01*6732 
Feet: The Length of that to the nine Mile-Stone, 
3868 Feet. The firft of thefe Meafures only was 
made ule of in the prefent Operations concerning 
the Velocity of Ele&ricity. In twelve Difcharges 
of the coated Phial, which were felt by Mr. George 
Graham , Mr. Short , and Charles Stanhope Efq* 
the Obfervers near the feven Mile-Stone, and who, 
by a fecond Watch of Mr. Grahams , meafured the 
Time 
f M. CaJJini de Thury afterwards meafared the Velocity of Sound at 
Aiguemortes in Languedoc , and found the Obfervations there from thofe 
made about Paris vary only half a Toife iu a Second-. See Mem-, de 
P Acad. Roy ale des Sciences, pour i'annee 1739, P' I2 ^- 
* Dr. Derham found, that when Sound was carried againft the Wind, 
not only its Diftance but its Velocity was leflen’d ; and in M. CaJJini' & 
Memoir, there is an Experiment, where Sound being carried againit the 
Wind, which then blew very ftrong, was retarded near a twelfth Part 
of the ufual Time in its Progrefs. . 
