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VARIATION, the angle contained between the true meridian and the magnetic meridian;

After the discovery of that most useful property of the magnet, or load-ftone, namely, the giving hardened iron and steel a polarity, the compass was for many years used without knowing that its direction in any ways deviated from the poles of the world: and about the middle of the 16th century, so certain were some of its inflexibly pointing to the north, that they treated with contempt the notion of the variation, which about that time began to be suspected.

Mr. Robertson, librarian of the Royal Society, favoured the author with an inspection of several curious remarks concerning the history of modern navigation; in which it appears, that the most early discoveries with regard to the magnetical variation were made about the year 1570. Mr. Robert Norman, from a variety of observations made by him nearly at that time, ascertains it to have been 11 15' easterly, or one point of the compass.

However, careful observations soon discovered, that in England, and its neighbourhood, the needle pointed to the eastward of the true north: but the quantity of this deviation being known, mariners became as well satisfied as if the compass had none; because they imagined that the true course could be obtained by making allowance for the true variation.

From successive observations made afterwards, it was found, that the deviation of the needle from the north was not a constant quantity; but that it gradually diminished, and at last, and about the year 1660, it was found at London that the needle pointed due north, and has ever since been getting to the westward, and now the variation is more than 20 degrees to the westward of the north: so that in any one place it may be suspected the variation has a kind of libratory motion, traversing through the north to unknown limits eastward and westward.. But the settling of this point must be left to time.

"During the time of the said observations it was also discovered, that the variation of the needle was different in different parts of the world, it being west in some places when it was east in others; and in places where the variation was of the same name, yet the quantity of it greatly differed. It was therefore found necessary, that mariners should every day, or as often as they had opportunity, make, during their voyage, proper observations for an amplitude or azimuth; whereby they might be enabled to find the variation of the compass in their present place, and thence correct their courses." Robertson's Elements of Navigation.

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© Derived from Thomas Cadell's new corrected edition, London: 1780, page 303, 2004
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