the mean Denftty of the Earth. 717 
Thefe feftions are very numerous, made in all direc- 
tions from the primitive points before mentioned, and 
many of them extended to great defiances far beyond the 
bounds of the plan hereunto annexed, fo as to include 
the nearefl hills and valleys of the furrounding country. 
They are rnoftly made in vertical planes in. the manner 
deferibed in the article of the Phil. Tranf. before refer- 
red to, excepting fome few of them which are level fee- - 
tions in planes parallel to the horizon,, and fome indeed^ 
irregular as being neither vertical nor horizontal. To 
compute the relative altitude of each point in thefe fee-- 
tions, it is evident, requires the refolution of two different 
triangles, viz. a horizontal triangle by which its place in 
the plan is afeertained, and a vertical triangle of which 
one fide is the elevation or deprefiion of the point. Of 
thefe fedlions there are above 70, containing near 1000 
points, whole places in the plan and relative altitudes 
have been computed: fo that the number of triangles, 
whofe numerical refolutions have been performed in the 
courfe of this bufinefs, amounts to feveral thoufands. 
Before the abftraft of the computation of the feci ions, 
I {hall here put down at large the calculation of one of. 
them, to fhew the manner in which they have been com- 
puted in the readied: and eafieft way that occurred to me, 
preferving at the fame time the proper degree of accu- 
racy,. 
