History of the inductive sciences, from the earliest to the present time

by William Whewell

Puis se torne la pointe toute
Contre l’estoile sans doute;

that is, it turns towards the pole-star. This account would make the knowledge of this property in Europe anterior to 1200. It was afterwards found[3] that the needle does not point exactly towards the north. Gilbert was aware of this deviation, which he calls the variation, and also, that it is different in different places.[4] He maintained on theoretical principles also,[5] that at the same place the variation is constant; probably in his time there were not any recorded observations by which the truth of this assertion could be tested; it was afterwards found to be false. The alteration of the variation in proceeding from one place to another was, it will be recollected, one of the circumstances which most alarmed the companions of Columbus in 1492. Gilbert says,[6] “Other learned men have, in long navigations, observed the differences of magnetic variations, as Thomas Hariot, Robert Hues, Edward Wright, Abraham Kendall, all Englishmen: others have invented magnetic instruments and convenient modes of observation, such as are requisite for those who take long voyages, as William Borough in his Book concerning the variation of the compass, William Barlo in his supplement, William Norman in his New Attractive. This is that Robert Norman (a good seaman and an ingenious artificer,) who first discovered the dip of magnetic iron.” This important discovery was made[7] in 1576. From the time when the difference of the variation of the compass in different places became known, it was important to mariners to register the variation in all parts of the world. Halley was appointed to the command of a ship in the Royal Navy by the Government of William and Mary, with orders “to seek by observation the discovery of the rule for the variation of the compass.” He published Magnetic Charts, which [219] have been since corrected and improved by various persons. The most recent are those of Mr. Yates in 1817, and of M. Hansteen. The dip, as well as the variation, was found to be different in different places. M. Humboldt, in the course of his travels, collected many such observations. And both the observations of variation and of dip seemed to indicate that the earth, as to its effect on the magnetic needle, may, approximately at least, be considered as a magnet, the poles of which are not far removed from the earth’s poles of rotation. Thus we have a magnetic equator, in which the needle has no dip, and which does not deviate far from the earth’s equator; although, from the best observations, it appears to be by no means a regular circle. And the phenomena, both of the dip and of the variation, in high northern latitudes, appear to indicate the existence of a pole below the surface of the earth to the north of Hudson’s Bay. In his second remarkable expedition into those regions, Captain Ross is supposed to have reached the place of this pole; the dipping-needle there pointing vertically downwards, and the variation-compass turning towards this point in the adjacent regions. We shall [hereafter] have to consider the more complete and connected views which have been taken of terrestrial magnetism.

[2] Enc. Met. art. Magnetism, p. 736.

[3] Before 1269. Enc. Met. p. 737.

[4] De Magnete, lib. iv. c. 1.

[5] c. 3.

[6] Lib. i. c. 1.

[7] Enc. Met. p. 738.

In 1633, Gellibrand discovered that the variation is not constant, as Gilbert imagined, but that at London it had diminished from eleven degrees east in 1580, to four degrees in 1633. Since that time the variation has become more and more westerly; it is now about twenty-five degrees west, and the needle is supposed to have begun to travel eastward again.

The next important fact which appeared with respect to terrestrial magnetism was, that the position of the needle is subject to a small diurnal variation: this was discovered in 1722, by Graham, a philosophical instrument-maker, of London. The daily variation was established by one thousand observations of Graham, and confirmed by four thousand more made by Canton, and is now considered to be out of dispute. It appeared also, by Canton’s researches, that the diurnal variation undergoes an annual inequality, being nearly a quarter of a degree in June and July, and only half that quantity in December and January.