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 …swing, will point approximately N and S. (Notice that approximately) it may be news to you that there are only a very few places on the globe where the compass points directly to the North Pole. In the lat. of the U.S. the needle points considerably to the W. of the true N., and this "variation”, as it is called; changes materially with material changes at this ship's position, and even at the same spot, it is not the same every year. This variation must be allowed for when the "skipper" takes his course.

A compass, anyway, is an uncertain and delicate contrivance. Theoretically, all the compass on earth point to the same spot. Practically, no two compasses point alike on ship board, for each has certain local influences that tend to make it "deviate" as it is called, from the common point. Masses of [?], chains, wire rope, even an awning post, near the compass, affects it’s pointing, and affect it differently, in every position of the ship's head. In iron ships, which are themselves magnets, huge but weak, the compass is a marvel – lousily erratic. All these whims and faults must be studied out, corrected as far as possible, and when uncorrected, allowed for. But with a compass fully understood, the ship's head can be kept very near a straight line in any desired direction.

Now suppose that a ship sailed from New York Harbor, and at 2 o’clock in the afternoon passed close to Sandy Hook lightship. (Page 146) The course is say, S. E. and the ship it makes 10 knots per hour. At 5 o'clock, therefore, she has gone 30 knots, and if she has kept to a rigidly straight line, the captain, can mark a line on his chart from the lightship to this S. E., measure off 30 miles on it and be reasonably certain that he is at point. This is "dead reckoning "– that is, finding one's position by measuring the distance, and direction, sailed from a previous position.