[Vide Plate 2, [Fig. 4.]]
From any point A, taken in the line C D, erect the perpendicular A E, in which set off from A to E 40 yards, set off from E to G, in the prolongation of A E, 10 yards, at G raise the perpendicular G F, and produce it towards I, plant pickets at E, and G, then move with another picket on G F, till F is in a line with E, and D; and on the prolongation of the perpendicular F G place another picket at I in the line with E, and C: measure F I (54 yards), then—
as G E : A E :: F I : C D;
Or, as 10 : 40 :: 54 : 216 yards.
8.—To find the inaccessible length, A, B, of the front of a fortification.
[Plate 2, [Fig. 5.]]
Plant a picket at C, from whence both points may be seen; find the lengths C A, C B (by the method in No. 5); make C E one-fourth, or any part of C B, and make C D bear the same proportion to C A: measure D E; then
as C D : D E :: C A : A B.
Nearly in the same manner the distance from B to A may be ascertained, when the point B is accessible; for having measured the line C B, and made the angle C E D equal to C B A, the proportion will be as C E : D E :: C B : B A.
9.—BY MEANS OF THE TANGENT SCALE OF A GUN,