M

CALCULATIONS TO ASCERTAIN THE TRACT 26.

not horizontal.

Ft. In.

via = 70 11'mt = 68mp = 63 41

pt — 21 2,

The other measures that were taken, for determining thesections, will be delivered afterwards, when the results orcomputed altitudes have been obtained, in order to be placedopposite their correspondent angles.

Having now obtained, to a great degree of accuracy, themeasured lengths of two lines, which were to serve as basesfor all the future calculations; the next consideration washow to make the properest use of them. Every other line ordistance, drawn or conceived to be drawn, must be calculatedfrom them, by the help of the angles observed, either at theirextremities, or at all the other points and stations in the sur-vey and plan. As these two bases are situated iirthe low partsof the country, from which but a very few of the other prin-cipal stations are visible, one method evidently is, to computeimmediately from these bases, such of the great lines in thesurvey whose extremities are visible from them; and then,from these calculated lines, to compute others next to then),and so on quite around and within the whole figure. In thismanner, several values of each line will arise, both from thedouble computations by the two measured bases, and fromthe various sets of triangles, which can be formed from thevery numerous horizontal angles, which were observed at theseveral stations. But in this mode of computation, after greatlabour arid pains, I had frequently the mortification to find,that the several values of the same lines would differ so greatlyfrom one another, that it was often very doubtful whether Icould rely on any of them, or even on the. mean among themall. These differences arose from the small errors in the ob-served angles, which in some degree are unavoidable; andindeed they were so small, that the sum of the angles of theseveral triangles, which were used in the calculation, seldomdiffered by more than a minute or two from ISO'’. But in a