o the magnets
in the compass is altered. Hence, every change in course causes a new
amount of Deviation which must be allowed for in correcting the compass
reading. It is customary in merchant vessels to have the compasses
adjusted while the ship is in port. The adjuster tries to counteract the
Deviation all he can by magnets, and then gives the master of the ship a
table of the Deviation errors remaining. These tables are not to be
depended upon, as they are only accurate for a short time. Ways will be
taught you to find the Deviation yourself, and those ways are the only
ones you can depend upon.

Put in your Note-Book:

Westerly Deviation is applied exactly as westerly Variation. Easterly
Deviation is applied exactly as easterly Variation.

The amount of Variation plus the amount of Deviation is called the
Compass Error. For instance, a Variation of 10 deg. W plus a Deviation of 5 deg.
W equals a compass error of 15 deg. W, or a Variation of 10 deg. W plus a
Deviation of 5 deg. E leaves a net compass error of 5 deg. W.

LEEWAY

Leeway is not an error of the compass, but it has to be compensated for
in steaming any distance. Hence it is mentioned here. A ship steaming
with a strong wind or current abeam, will slide off to the leeward more
or less. Hence, her course will have to be corrected for Leeway as well
as for Variation and Deviation.

Put in your Note-Book:

Leeway on the starboard tack is the same as westerly Variation. Leeway
on the port tack is the same as easterly Variation. This is apparent
from the following diagram:

[Illustration]

As the wind, blowing from the North, hits the left hand ship, for
instance, on her starboard side, it shoves the ship to the left of her
true course by the number of points or degrees of leeway.

Leave a space and put the following heading in your Note-Book:

I. Complete rule for converting a compass course into a true course:
1. Change the compass course into a new compass reading.
2. Apply Easterly Variation and Deviation +.
3. Apply Westerly Variation and Deviation -.
4. Apply port tack Leeway +.
5. Apply starboard tack Leeway -.
II. Complete rule for converting a true course into a compass course:
1. Reverse the above signs in applying each correction.

I will now correct a few courses, and these are to be put into your
Note-Book:

---------+------+-------------+--------+--------+-------+---------
C Cos | Wind | Leeway