MAGNET. The natural magnet, loadstone, or lodestone, is known in mineralogy as magnetic iron ore. It consists of one equivalent of protoxida and one equivalent of sesquioxide of iron (FeO, Fe,0,), and possesses the remarkable property of attracting iron and some other metals. It is said to have been found abundantly near Magnesia in Lydia, from which circumstance its name may have been derived from the Greek parses. The attracting power of the magnet was known at a very early period, as references are made to it by Aristotle, and more particularly by Pliny, who states that ignorant people called it ferrunt virem, or quick-iron. The same author appears to have been acquainted with the power of the magnet to communicate properties similar to its own to other bodies.
The universal law, that re-action is co-existent with action, implies, that iron must re-act on the magnet, and we find in fact, that if a piece of ion be fixed, and a small magnet be suspended by an untwisted line near it, the magnet will then be moved towards the iron ; thus all the iron in the mass of the globe acts upon a magnet. It is also known that electrical currents influence magnetic) bodies [Etecxnee3Inexexissi] while heat has an influence on magnetic intensity. Hence it follows as a mechanical consequence, that if a magnetic needle or cylinder be sus pended by its centre of gravity, so as to be free to move in any direction round that point, it will not take an arbitrary position like unmag netised bodies, but must take a specific direction, namely, that which represents the resultant of all the magnetic forces to which it is subject. Its position in a given place can be defined by two angles, the one called the variation or dedinatien, the other the inclination or dip. The first is the angle formed by tho vertical plane in which the needle lies with the plane of the meridian ; the second is the inclination of the line of the needle to the plane of the horizon. The latter is avoided in the compass-needle by sustaining it horizontally on a point which is necessarily different from its centre of gravity, and the variation is then the anglo made by the direction of the needle with that of an exact and horizontal north-and-south line. This property of the needle in called its polarity, and is a consequence of its other properties above noticed; the fact, however, escaped the notice of tho Greeks and Romans of antiquity, but tbo Chinese appear to havo been acquainted with it from a very remote data The Chinese name it, according to its most valuable property, or the "directing stone." In
Tonkinin it is called the " stone which shows the south." In Swedish it is segelaten, or the " seeing stone ;" in Icelandic, leiderstein, or tho " loading stone," after the Saxon ladan," to load," whence the English lodestone. In a similar way is derived the term or "guiding star," as applied to the polo-star, and the term lode, the "leading vein," in a mine. Polarity is the most useful of the known properties of the magnet, being of the most essential importance to the mariner, when the magnet is constructed in the form of the compa.ss.neetlle. [Costeass, HISTOFIT OF TILE.] Dr. Gilbert, who was physician in ordinary to Queen Elizabeth, states that P. Venctus brought a compass from China in 1260. Gilbert bestowed much attention on the subject of magnetism, and to some extent Inculcated the doctriue of gravitation, by comparing the earth to a great magnet. His theory on this subject is given in a work entitled Traetatus sive Physiologia nova de Magnete ' (1600), and the term "poles of a magnet" arose from that theory, which is remarkably consonant with the notions of the present day : for the north pole of the niagnet he denominated the south pole, in connection with his theory, while Poisson, in his elegant Memoirs on Magnetism,' calls the magnetic fluid at that extremity of the magnet the austral fluid, because, as like electricities repel [ELECTRICITY], so, on his hypothesis of the magnetic fluids, that occupying the north end of a magnetised needle is repelled by the boreal fluid of terrestrial magnetism.
The application the compass to the purposes of navigation must speedily have led to the discovery of its variation, and in the' Life of Columbus,' written by his son, it is distinctly assigned to that cele brated man ; and though its amount in 1492 must have been small in France, Spain, &c., yet it was doubtless a very observable quantity in many of the regions visited by Columbus. Some have carried back the date of this discovery to the year 1269, but on very doubtful grounds. When its amount came to be observed with some accuracy, we find it at Paris, in 1541, 7i* E.; in 1550, 84* E. ; in 1580, E.; in 1630, 43* E.; and at Rome, in 1670, r W. The following table shows the variation at London in different years :—