intervening point C is 475 feet above sea level and 500 yards from B.
As B is the lowest we will call its elevation zero or at datum, then
elevation of A is 50 feet and C 25 feet.

[Illustration: Plate #10]

Another method of deciding visibility is by proportion. Measure the
distance between the three points A, B, and C, and obtain their
elevations above the datum (lowest of the 3) and using similar
triangles. Take the same case as above, letting X represent the point
above which the view is clear at 1,260 yards from point A, the line of
sight passes through this point.

1760 (A--B) : 500 (B--C) : : 50 (elev. A) : X
solving, X = 14.2

Now, since the ground at point C is 25 feet above the base and the
line of sight passes within 14.2 feet of the base at this place, an
observer at A is unable to see B.

The matter of profiling is very simple. Merely mark where the contours
cut the edge of a piece of co-ordinate paper and extend the proper
elevations, then pass a line through these points, remembering that
the surface of the ground has a natural curve.


LESSON 6. (FIELD WORK--FIVE HOURS.)

PROBLEM.--By use of the slope scale on the sketch board and the
contour interval scale on the alidade, each man will secure vertical