58 . CALCULATIONS, &C, .TRACT 26.

angle of elevation and the angle of depression, and take themean between the two computed differences of altitude; forthis excludes the necessity of making the proper allowancesfor refraction, and for the curvature of the' earth ; since theeffect of each of these is balanced and corrected by that ofthe counter observation. But as to those points in the sec-tions which are far distant from the observer, and where greataccuracy is required, it may be proper to make the allowancefor refraction and curvature, as there is generally no back ob-servation, by which their effects may be balanced. Theseare the chief hints which at present occur, besides the generalinformation to be derived by the computer from the perusalof the modes of computation that have been described in thistract. As to the surveyor, he will strike out other conve-nient ways of measurement, adapted to the circumstanceswith which the nature of the survey may happen to be at-tended.

A map of the country about Shichallin is hereunto annexed,(pi. 4), to convey a general idea of the nature of the ground,and for the better illustration of the description given in theformer parts of this tract.