Sometimes it pointed east of
north, sometimes west of north, and on occasion it even turned tail
on north and pointed south. The variation at the particular spot on
the globe occupied by the Snark was 9 degrees 40 minutes easterly.
Well, that had to be taken in to account before I gave the steering
course to the man at the wheel. I read:
"The Correct Magnetic Course is derived from the True Course by
applying to it the variation."
Therefore, I reasoned, if the compass points 9 degrees 40 minutes
eastward of north, and I wanted to sail due north, I should have to
steer 9 degrees 40 minutes westward of the north indicated by the
compass and which was not north at all. So I added 9 degrees 40
minutes to the left of my west-by-south course, thus getting my
correct Magnetic Course, and was ready once more to run to open sea.
Again alas and alack! The Correct Magnetic Course was not the
Compass Course. There was another sly little devil lying in wait to
trip me up and land me smashing on the reefs of Vatu Leile. This
little devil went by the name of Deviation. I read:
"The Compass Course is the course to steer, and is derived from the
Correct Magnetic Course by applying to it the Deviation."
Now Deviation is the variation in the needle caused by the
distribution of iron on board of ship.
