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the con federation of the Council of the Royal So- 
ciety, and offered to carry it into execution, at the 
expence of the Society, if they thought proper. The 
Council determined that fo ufeful and important a 
work fhould be completed, and accordingly fent out 
inftrudions to Mefheurs Mafon and Dixon for the 
regulation of their operations; particularly requiring 
them to meafure the lines carefully over-again with 
fir-rods, which they fent to them, together with a 
brafs ftandard, of 5 foot, with whicli the rods were 
to be compared frequently, and the difference noted, 
and alfo the height of the thermometer at the 
time; for the lines had been all meafured before with 
a ffandard chain, which, though lufficient for the 
common purpofes of furveying, was by no means to 
be depended upon in fo nice an operation as that of 
meafuring a degree of latitude. The Honourable 
Mr. Penn was pleafed, at the requeff of the Royal 
Society, to grant the further ufe of his fedtor, before 
mentioned, and other inftruments, to the obfervers, 
for compleating this meafure. 
The method purfucd in this work, is, that which 
the level difpofition of the country pointed out. But 
the refult may be expected to be more accurate on 
this account, as meafures taken in a ffraight line, and 
on a level furface, are known to be capable of great 
exadtnefs ; and no adventitious errors are here intro- 
duced from any poffible errors of a chain of triangles. 
Meffieurs Mafon and Dixon having alfo determined 
the angle which the oblique line made with the me- 
ridian, by proper aftronomical obfervations, and the 
amplitude of the arch of their meridian line by fe- 
veral obfervations of zenith diffances of fixed ffars, 
made 
