the point (b), Fig. 1 X. Other points, such as
C, D, etc., would be located in the same way.

=1882. The determination of the heights of hills, shapes of the
ground, etc., by contours.= To draw in contours on a sketch, the
following steps are necessary:

[Illustration: Fig. 2]

(a) From the known or assumed elevation of a located station as A,
Fig. 1 Y, (elevation 890), the elevations of all hill tops, stream
junctures, stream sources, etc, are determined.

(b) Having found the elevations of these critical points the contours
are put in by spacing them so as to show the slope of the ground along
each line such as (a)-(b), (a)-(c), etc., Fig. 1 Y, as these slopes
actually are on the ground.

[Illustration: (Tally Register)--Fig. 3]

[Illustration: (Clinometer)--Fig. 4]

To find the elevation of any point, say C (shown on sketch as c),
proceed as follows:

Read the vertical angle with slope board, Fig. 2, or with a
clinometer, Fig. 4. Suppose this is found to be 2 degrees; lay the
scale of M. D.[22] (ruler, Fig. 2) along (a)-(c), Fig. 1 Y, and note
the number of divisions of -2 degrees (minus 2 deg.) between (a) and (c).
Suppose there are found to be 5-1/2 divisions; then, since each
division is 10 feet, the total height of A above C is 55 feet (5-1/2 x
10). C is therefore 835 ft. elev. which is written at (c), Fig. 1 Y.
Now looking at the ground along A-C, suppose you find it to be a very
decided concave (hollowed out) slope, nearly flat at the bottom and
steep at the top. There are to be placed in this space (a)-(c), Fig. 1
Y, contours 890, 880, 870, 860 and 850, and they would be spaced close
at the top and far apart near (c), Fig. 1 Y, to give a true idea of
the slope.

The above is the entire principle of contouring in making sketches and
if thoroughly learned by careful repetition under different
conditions, will enable the student to soon be able to carry the
contours with the horizontal locations.

=1883.= In all maps that are to be contoured some plane, called the
_datum plane_, must be used to which all contours are referred. This
plane is usually mean sea level and the contours are numbered from
this plane upward, all heights being elevations above mean sea level.

In a particular locality that is to be sketched there is generally
some point the elevation of which is known. These points may be bench
marks of a survey, elevation of a railroad station above sea level,
etc. By using such points as t