L ] 
E/, which is to a third proportional to the diameter of the earth and the line B R, as 
r % l t /• 1 • . 1 1 • 1 1 M r\ r 
the tangent of the latitude of the point 13 , to the radius. Whence B./ — 15,8 feet 
which i'ubtracted from At juft found = 
433094.6 leaves 
To which add 
as found before 
The fum is 
es 
!{ 
AR — 
433078 8 feet 
NP = 
78290,7 
CD = 
26608 0 
D *’= 
89,7 
— 
538067 feet 
~ an arch of meridian intercepted between the parallels of latitude palling through the 
points N and A, anfwering to the celelfial arch i° 28' 45". 
Then fay, as x° 28' 45'''; is to i° : : fo is 538067 feet, to 363763 Englifh feet, 
which is the length of a degree oflatitude in the provinces of Pennfyivania and Mary- 
land. The latitude of the* Northernmoft point N, was determined from the zenith 
diftances of feveral ftars, fee page 323 — 39° 56' 19" and the latitude of the Souther- 
moll point A — 3S 0 27' 34". Therefore the mean latitude exprefled in degrees and 
minutes is ==39° 126 
To reduce tin’s meafure of a degree to the meafure of the Paris toife, it mull be 
premifed, that the meafure of the French foot was found upon a very accurate 
cornparifon, made by Mr. Graham, of the toife of the Royal Academy of Sciences at 
Pa ris, with the Royal Society’s brafs ftandard, to be to the Englilh foot, as ii4to 107. 
See Philofophical Tranfadf, Vol. XLII. p. 185. Therefore fay as 1 14 : is to 107 : : fo 
is 363763 the meafure of the degree in EngJifh feet, to 34.1427 the meafure of the 
degree in French feet, which divided by 6, the number of feet in a toife, gives the 
length of the degree 3=56904! Paris toifes, in the latitude 39° 12' North. 
Such is the length of a degree in this latitude, fuppofing the five feet brafs ftandard 
made ufe of in this meafure to have been exactly adjufted to the length of the Royal 
Society’s brafs ftandard. It was really adjufted by Mr. Bird, by his accurate brafs fcale 
ol equal parts, which he makes fuch excellent ufe of in dividing aftronomical inftru- 
ments, and which is juft -roWth part of an inch fhorter than the Royal Society’s brafs 
ftandard upon a length of three feet. If one would take notice of fo fmall a difference, the 
length of a degree juft found muft be lefTened by ^-s-Wo-th part, or by ten feet, in order 
t.0 reduce it to the meafure of the Royal Society’s ftandard. Since I am treating of fuch 
niceties, may it be allowed me to add, that the five feet brafs ftandard having been 
again compared with Mr. Bird’s fcale, ftnee its return from North America, appeared 
both to myfelf and Mr. Bird to be juft — oVo-th part of an inch fhorter than the fcale, 
upon that fide on which the hundredths of an inch are placed at one end, and T-oVsths 
of an inch fhorter than the fcale upon the oppofite fide ? which diminution of its length 
is undoubtedly owing to the fmall wearing or battering which it has met with in the 
frequent ufe that was made of it. But the divided lide of the rod having been that 
which was made ufe of in meafuring the levels, is what is to be regarded in the prefent 
cafe. If one would allow for the wearing of the rod, one may fuppofe it to have fufter- 
ed a gradual diminution ; and then one muft take a mean between its firft length, which 
was the fame with Mr. Bird’s fcale, and its prefent length, which is t- 0 Vo th of an inch 
fhorter as one may fuppofe it a medium to have been rcVoth part of an inch {hotter 
than 
