EFFICE, ut interea fera munera militiaï O charming Goddess, whose mysterious sway, Do thou, what time thy fondling arms are thrown Do thou, thy Rome to save, thy power to prove, CHAPTER I. DISCOVERY OF LAWS OF MAGNETIC PHENOMENA. HE history of Magnetism is in a great degree same persons were employed in the two trains of research. The general fact, that the magnet attracts iron, was nearly all that was known to the ancients, and is frequently mentioned and referred to; for instance, by Pliny, who wonders and declaims concerning it, in his usual exaggerated style. The writers of the Stationary Period, in this subject as in others, employed themselves in collecting and adorning a number of extravagant tales, which the slightest reference to experiment would have disproved; as, for example, that a magnet when it has lost its virtue, has it restored by goat's blood. Gilbert, whose work De Magnete we have already mentioned, speaks with becoming indignation and pity of this bookish folly, and repeatedly asserts the paramount value of experiments. He himself, no doubt, acted up to his own precepts; for his work contains all the fundamental facts of the science, so fully examined indeed, that even at this day we have little to add to them. Thus, in his first Book, the subjects of the third, fourth, and fifth Chapters are,-that the magnet has poles,-that we may call these poles the north and the south pole, -that in two magnets the north pole of each attracts the south pole and repels the north pole of the other. This is, indeed, the cardinal fact on which our generalizations rest; and the reader will perceive at once its resemblance to the leading phenomena of statical electricity. But the doctrines of magnetism, like those of heat, have an additional claim on our notice from the manner 1 Hist. Nat. lib. xxxvi. c. 25. in which they are exemplified in the globe of the earth. The subject of terrestrial magnetism forms a very important addition to the general facts of magnetic attraction and repulsion. The property of the magnet by which it directs its poles exactly or nearly north and south, when once discovered, was of immense importance to the mariner. It does not appear easy to trace with certainty the period of this discovery. Passing over certain legends of the Chinese, as at any rate not bearing upon the progress of European science, the earliest notice of this property appears to be contained in the Poem of Guyot de Provence, who describes the needle as being magnetized, and then placed in or on a straw, (floating on water, as I presume:) 2 Puis se torne la pointe toute 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 found3 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. 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. 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 variation, as Thomas Hariot, Robert Hues, Edward Wright, Abraham Kendall, all Englishmen : others have invented magnetic instruments and con 3 Before 1269. 2 Enc. Met. art. Magnetism, p. 736. 5 c. 3. The 4 De Magnete, lib. iv. c. 6 Lib. i. c. I. venient 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 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 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 7 Enc. Met. p. 738. |