the stream, M-N, the Saddle, M, the water shed
from F to H, and steep bluff at K, but they also give the slopes of
the ground at all points. From this we see that the slopes are
directly proportional to the nearness of the contours--that is, the
nearer the contours on a map are to one another, the steeper is the
slope, and the farther the contours on a map are from one another, the
gentler is the slope. A wide space between contours, therefore,
represents level ground.

[Illustration: Fig. 5]

[Illustration: Fig. 6]

The contours on maps are always numbered, the number of each showing
its height above some plane called a datum plane. Thus in Fig. 6 the
contours are numbered from 0 to 100 using the surface of the lake as
the datum plane.

The numbering shows at once the height of any point on a given contour
and in addition shows the contour interval--in this case 20 feet.

Generally only every fifth contour is numbered.

The datum plane generally used in maps is mean sea level, hence the
elevations indicated would be the heights above mean sea level.

The contours of a cone (Fig. 7) are circles of different sizes, one
within another, and the same distance apart, because the slope of a
cone is at all points the same.

[Illustration: Fig. 7]

The contours of a half sphere (Fig. 8), are a series of circles, far
apart near the center (top), and near together at the outside
(bottom), showing that the slope of a hemisphere varies at all points,
being nearly flat on top and increasing in steepness toward the
bottom.

[Illustration: Fig. 8]

The contours of a concave (hollowed out) cone (Fig. 9) are close
together at the center (top) and far apart at the outside (bottom).

[Illustration: Fig. 9]

The following additional points about contours should be remembered:

(a) A Water Shed or Spur, along with rain water divides, flowing away
from it on both sides, is indicated by the higher contours bulging out
toward the lower ones (F-H, Fig. 6).

(b) A Water Course or Valley, along which rain falling on both sides
of it joins in one stream, is indicated by the lower contours curving
in toward the higher ones (M-N, Fig. 6).

(c) The contours of different heights which unite and become a single
line, represent a vertical cliff (K, Fig. 6).

(d) Two contours which cross each other represent an overhanging
cliff.

(e) A closed contour without another contour in it, represents either
in elevation or a depression,