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 to 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° W plus a Deviation of 5° W equals a compass error of 15° W, or a Variation of 10° W plus a Deviation of 5° E leaves a net compass error of 5° W.