the line of
sight passes through this point.

1760 (A--B) : 500 (B--C) : : 50 (elev. A) : X
solving, X = 14.2

Now, since the ground at point C is 25 feet above the base and the
line of sight passes within 14.2 feet of the base at this place, an
observer at A is unable to see B.

The matter of profiling is very simple. Merely mark where the contours
cut the edge of a piece of co-ordinate paper and extend the proper
elevations, then pass a line through these points, remembering that
the surface of the ground has a natural curve.

LESSON 6. (FIELD WORK--FIVE HOURS.)

PROBLEM.--By use of the slope scale on the sketch board and the
contour interval scale on the alidade, each man will secure vertical
data on the flat sketch made in the fourth lesson. Certain critical
elevations will be determined and marked with red flags before hand.
The elevations of two points on the ground will be furnished, one as
the datum and the other as a check. Draw in contours of this sketch
with the help of drainage lines and elevations already secured.

The chief points to be considered are to take slopes from points
established on the sketch; to take several sights and average the
angle of slope; to properly lay off the elevation by using the slope
scale on the alidade; and finally to put in the contours along these
lines of sight _on the spot_ thus allowing for difference in
topography between the point of sight and the station from which the
elevation is taken. Careful note must be made of the drainage systems
as these are the keynotes to the sketch and finally the contours are
connected together, keeping in mind always that no contour stops
unless it makes a closed curve or goes off the map. Remember also
that contours make fingers pointing up stream and are blunt around
hill sides. Contours cross streams to opposite points and break at
roads, continuing on the other side. Uniform slopes have
equally-spaced contours. Do not try to measure every slope, two
intersecting elevation sights on a hill will check the height. Put the
intervening contours in by eye.

LESSON 7. (CLASSROOM--FOUR HOURS.)

PROBLEM.--Completing the map sketch previously made and making a
landscape sketch.

It is important to complete a map and no matter how good it is, if
certain points are omitted, the value of the work is very much
decreased. The sketcher must clear the sketch of all unnecessary lines
and notes and make his lettering cl