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.
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.