Cook was assiduous in obtaining observations to ascertain the variation of the compass -- i.e., the difference between the direction shown by the magnetic needle and the true north. He is constantly puzzled by the discrepancies in these observations made at short intervals. These arose from the different positions of the ship's head, whereby the iron within a certain distance of the compass is placed in different positions as regards the needle working the compass card, the result being that the needle is attracted from its correct direction in varying degree. This is known as the Deviation of the compass. The cause of this, and of the laws which govern it, were only discovered by Captain Flinders in 1805.

Happily for the navigators of those days, little iron entered into the construction of ships, and the amount of the Deviation was not large, though enough to cause continual disquiet and wonderment.

Cook's longitudes in this voyage are all given as West of Greenwich, not divided into east and west, as is usual at this day. The latter system again has only been adopted universally since his time. (Wharton 1893)

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