c
qualities, attracts one end of all compass needles and causes them to
point towards it, and as it is near the True North Pole, this serves
to indicate the direction of north to a person using a compass.

Of course, the angle which the Magnetic needle makes with the True
Meridian (called the Magnetic Declination) varies at different points
on the earth. In some places it points east of the True Meridian and
in others it points west of it.

It is important to know this relation because maps usually show the
True Meridian and an observer is generally supplied with a magnetic
compass. Fig. 15 shows the usual type of Box Compass. It has 4
cardinal points, N, E, S and W marked, as well as a circle graduated
in degrees from zero to 360 deg., clockwise around the circle. To read the
magnetic angle (called magnetic azimuth) of any point from the
observer's position the north point of the compass circle is pointed
toward the object and the angle indicated by the north end of the
needle is read.

[Illustration: Fig. 15]

You now know from the meridians, for example, in going from York to
Oxford (see Elementary Map) that you travel north; from Boling to
Salem you must travel south; going from Salem to York requires you to
travel west; and from York to Salem you travel east. Suppose you are
in command of a patrol at York and are told to go to Salem by the most
direct line across country. You look at your map and see that Salem is
exactly east of York. Next you take out your field compass (Figure 15,
Par. 1870), raise the lid, hold the box level, allow the needle to
settle and see in what direction the north end of the needle points
(it would point toward Oxford). You then know the direction of north
from York, and you can turn your right and go due east towards Salem.

Having once discovered the direction of north on the ground, you can
go to any point shown on your map without other assistance. If you
stand at York, facing north and refer to your map, you need no guide
to tell you that Salem lies directly to your right; Oxford straight in
front of you; Boling in a direction about halfway between the
directions of Salem and Oxford, and so on.

=1871. Determination of positions of points on map.= If the distance,
height and direction of a point on a map are known with respect to any
other point, then the position of the first point is fully determined.

The scale of the map enables us to determine the distance; the
contour