74 Correspondence of Governor Sharpe.

Letter Bk. IV

Line & it is much more difficult they say to run Southward
than Northward. In a hot Day the Plumb Lines tho there is
not a Breath of Wind have or seem to have a Vermicular
Motion which may deceive the Sight, & as the smallest
Thread being at the Distance of a few Feet from the Object
Glass of the Telescope will cover a considerable Space or a
small Rope at the Distance of a quarter of a Mile, how can
the Surveyors be they ever so careful be sure that they pre-
serve their exact & true Course. Could they take longer
Sights there would indeed be less Danger of Error & there-
fore we wished to have the Instrument D^r Bevis recommended
with which according to his Account a small Point on the
Surface of the Earth may be seen five Miles, however I am
afraid that on Tryal the Surveyors will not proceed at so great
a Rate, for a small Eminence between the Observer & an
Object on the Ground at so great a Distance will interrupt
the View & in the Country thro which the Line is to be run
many considerable Hills will intervene in the Space of two
or three Miles. I have for some time had the Sector fixed up
in my Garden & several others of the Commissioners besides
myself have frequently taken Observations with it agreeable
to D^r Bevis's Directions but have not for two Nights together
found the Star's (a in Lyra) Zenith Distance to be precisely
the same, the Variation being frequently twenty or thirty
Seconds & indeed the Silver Wire which is intended to plumb
or Rectify the Sector (extremely small as the Wire appears) is
thick enough to cover more than ten Seconds. Since then
there is little probability of All the Commissioners agreeing if
they are by Observations to determine & ascertain the Point
(to be found) fifteen Miles South of Philadelphia would it not
be most advisable to measure that Distance upon the Surface
of the Earth notwithstanding the Difficulties I mentioned to
you some time ago as the Surveyors will doubtless in such
Mensuration come nearer the Truth than sixty five Perches,
which considering the Length of that North Boundary is no
inconsiderable Thing, the Area contained in a Paralelogram
of 200 Miles by 65 Perches being no less than 26000 Acres.
Whether the Surveyors can or cannot run the Line they are
now directed to proceed on, we must I presume proceed on
some other part of the Work when we meet again in the
Spring: As to laying out a Part of the Circle & running a
North Line from the Tangent Point I suppose we shall agree
on a Mode of doing it, but what may happen among us when
the North Boundary & the Way of describing it shall come
under Consideration I cannot tell, much depends thereupon
& as all the Commissioners on both Sides will probably be
then present it is not likely they will be unanimous in their