dge of a lake, but horizontally they wander back and
forth to just as great a degree.

The line marked 880, at the penitentiary, passes through on that
particular piece of ground every point that is 880 feet above
sea level. Should the Missouri River rise in flood to 880 feet,
the penitentiary would be on an island, the edge of which is
marked by the 880 contour.

Contours show several things; among them the height of the ground
they cross. Usually the contour has labeled on it in figures the
height above some starting point, called the DATUM PLANE--generally
sea level. If, with a surveying instrument, you put in on a piece
of ground a lot of stakes, each one of which is exactly the same
height above sea level--that is, run a line of levels--then make
a map showing the locution of the stakes, a line drawn on the
map through all the stake positions is a contour and shows the
position of all points of that particular height.

On any given map all contours are equally spaced in a vertical
direction, and the map shows the location of a great number of
points at certain fixed levels. If you know the vertical interval
between any two adjacent contours, you know the vertical interval
for all the contours on that map, for these intervals on a given
map are all the same.

With reference to a point through which no contour passes, we
can only say that the point in question is not higher than the
next contour up the hill, nor lower than the next one down the
hill. For the purposes of any problem, it is usual to assume
that the ground slopes evenly between the two adjacent contours
and that the vertical height of the point above the lower contour
is proportional to its horizontal distance from the contour, as
compared to the whole distance between the two contours. For
instance, on the map, find the height of point A. The horizontal
measurements are as shown on the map. The vertical distance between
the contours is 20 feet. A is about one-quarter of the distance
between the 800 and the 820 contours, and we assume its height
to be one-quarter of 20 feet (5 feet) higher than 800 feet. So
the height of A is 805 feet.

The vertical interval is usually indicated in the corner of the
map by the letters "V. I." For instance: V. I.=20 feet.

On maps of very small pieces of ground, the V. I. is usually
small--perhaps as small as 1 foot; on maps of large areas on a
small scale it may be very great--even 1,000 feet.

Contours also