gs should be made, by driving a
3/4-inch pointed iron rod to the rock, or to a depth of _five_ feet where
the rock falls away. By this means, measuring the distance from the
soundings to the ranges of the stakes, we can denote on the map the shape
and depth of sunken rocks. The shaded spot on the east side of the map,
(Fig. 8,) indicates a rock three feet from the surface, which will be
assumed to have been explored by sounding.

In most cases, it will be sufficient to have contour lines taken only at
intervals of two feet, and, owing to the smallness of the scale on which
these maps are engraved, and to avoid complication in the finished plan,
where so much else must be shown, each alternate line is omitted. Of
course, where drains are at once staked out on the land, by a practiced
engineer, no contour lines are taken, as by the aid of the level and rod
for the flatter portions, and by the eye alone for the steeper slopes, he
will be able at once to strike the proper locations and directions; but
for one of less experience, who desires to thoroughly mature his plan
before commencing, they are indispensable; and their introduction here
will enable the novice to understand, more clearly than would otherwise be
possible, the principles on which the plan should be made.

[Illustration: Fig. 9 - WELL'S CLINOMETER.]

Fig. 9 - WELL'S CLINOMETER.

For preliminary examinations, and for all purposes in which great accuracy
is not required, the little instrument shown in Fig. 9,--"Wells'
Clinometer,"--is exceedingly simple and convenient. Its essential parts are
a flat side, or base, on which it stands, and a hollow disk just half
filled with some heavy liquid. The glass face of the disk is surrounded by
a graduated scale that marks the angle at which the surface of the liquid
stands, with reference to the flat base. The line 0.----0. being parallel to
the base, when the liquid stands on that line, the flat side is
horizontal; the line 90.----90. being perpendicular to the base, when the
liquid stands on that line, the flat side is perpendicular or _plumb_. In
like manner, the intervening angles are marked, and, by the aid of the
following tables, the instrument indicates the rate of fall per hundred
feet of horizontal measurement, and per hundred feet measured upon the
sloping line.(6)

Table No. 1 shows the rise of the slope for 100 feet of the horizontal
measurement. Example: If the