W, etc. Some lines are marked "No
Variation." In such cases no allowance need be made. On harbor charts or
other small charts, the Variation is shown by the compass-card printed
on the chart. The North point of this card will be found slewed around
from the point marking True North and in the compass card will be some
such inscription as this: "Variation 9 deg. West in 1914. Increasing 6' per
year."

Now let us see how we apply this Variation so that although our compass
needle does not point to true North, we can make a correction which will
give us our true course in spite of the compass reading. Note these
diagrams:

[Illustration]

The outer circle represents the sea horizon with the long arrow pointing
to true North. The inner circle represents the compass card. In the
diagram to the left, the compass needle is pointing three whole points
to the left or West of True North. In other words, if your compass said
you were heading NE x N, you would not actually be heading NE x N. You
would be heading true North.

[Illustration]

In other words, standing in the center of the compass and looking toward
the circumference, you would find that every true course you sailed
would be three points to the _left_ of the compass course. That is
called Westerly Variation.

Now look at the diagram to the right. The compass needle is pointing
three whole points to the right or East of True North. In other words,
standing in the center of the compass and looking toward the
circumference, you would find that every true course you sailed would be
three points to the _right_ of the compass course. That is called
Easterly Variation.

Hence we have these rules, which put in your Note-Book:

_To convert a compass course into a true course_

When the Variation is westerly, the true course will be as many points
to the left of the compass course as there are points or degrees of
Variation. When the Variation is easterly, the true course will be as
many points or degrees to the right of the compass course.

_To convert a true course into a compass course_

The converse of the above rule is true. In other words, Variation
westerly, compass to the right of true course; variation easterly,
compass course to the left.

DEVIATION

As stated before, Deviation causes an error in the Compass due to the
magnetism of the iron in the ship. When a ship turns, the compass card
does not turn, but the relation of the iron's magnetism t