1. Theories of TerrestrialMagnetism.—The earliest theory was that suggested by Gil bert, in which it is supposed that a magnet in the middle of the earth extends from one magnetic pole to the other. On this supposition, the general phenomena of terrestrial magnetism may be accounted for—a needle, both by declination and dip, must point to the poles. This must always remain, from its simplicity, the popular theory on the subject. In consistency with his theory, Gilbert considered the n. pole of the magnet to be a s. pole, as he took the n. pole of the earth for his standard n. pole. If this theory were correct, the magnetic equator would be a great circle of the earth, and the inag netic poles would be 90° from it, which is far from the case. It is only a rough' approximation to a just theory.
Halley endeavored to supplement Gilbert's theory by supposing two magnets of unequal strength crossing each other at the earth's center to be the cause of terrestrial magnetism. The theory of the two magnets or four poles was ably defended by Hansteen.
Barlow considered that the earth acted on the needle as if currents of electricity traversed it from e. to west. He imitated its action by wrapping a wire in parallel coils round a wooden globe, and causing a galvanic current to pass through it. Each turn of the wire represented a magnetic parallel, and the two ends of the coil the magnetic poles; and, to complete the analogy, the globe was movable on an axis, which stood in the same relation to the ends of the coil as the astronomical to the magnetic poles of the earth. When a small need16 was placed on the globe, its declination and dip bore a striking resemblance to those of a needle similarly situated on the earth's surface. The objection to this theory is the difficulty of accounting for the origin of such currents in the earth. To meet this, some suppose the earth to be a huge thermo-electrie pile; as the heat of the sun falls on one side of it, currents are there generated which travel round the globe. But how, again, it may be asked, are the conditions of thermoelec tricity implemented by the materials of the earth? This question still remains to be answered. The close connection between temperature and magnetism is shown by the diurnal variation of declination, the epochs of which closely correspond with those of the daily temperature, and by the fact that the isodynainic and isothermal lines manifest a marked correspondence. Sir David Brewster has also shown that there are two centers of maximum cold in the northern hemisphere, which are situated near to the two inten sity poles.
Gauss did not start from any simple supposition of one or two magnets giving rise to the magnetism of the earth, nor did he assert or deny its electric origin. Considering the whole earth as magnetic, he aimed at determining how it must act as a whole at the different points ou its surface. In order to make the equations be obtained theo
retically in this attempt express the distribution on the earth, the magnetic elements of eight stations at a sufficient distance from each other on the earth's surface had to be ascertained and substituted in these equations. This done from the longitude and latitude of any station he considered himself prepared to deduce its magnetic elements. The magnetic charts which he sketched, though founded on the imperfect observations to which he had access, are singularly in keeping with fact, and go far to establish the correctness of his reasonings.
The secular variations are as yet wholly unaccounted for. The cause of the diurnal variation is universally attributed to the sun. Secchi, who carefully studied the diurnal variation of the needle, considers that the sun, so far as they are concerned, acts upon the earth as'a powerful magnet at a distance.
Historical Sketch.—The discovery of the change in declination at different places is generally attributed to Columbus, and was one of the many important observations of his memorable voyage across the Atlantic. Robert Norman, an instrument maker in Lon don, first discovered the dip of the needle in 1576. He was led to it by finding that needles nicely balanced before magnetization bad to he slightly loaded on the s. end, to keep than horizontal after beino. magnetized. Gilbert (1600) gave the first theory of terrestrial magnetism, viz., that of the single magnet. Halley, the astronomer-royal, published his theory of the four poles in 1683. In 1688 and 1689, at the expense of government, he made two magnetic voyages, the results of which he embodied in his charts of the lines of equal declination, published in 1701, which were the first magnetic charts ever published. In 1722, the diurnal variation was discovered by Graham, the cele brated instrument-maker of London. The first inclination chart was published by Wilke at Stockholm, 1768. Humboldt inaugurated the present system of careful observations of terrestrial magnetism by comparative measurements of the magnetic elements at Peru and (1799-1803). Hansteen's work on the Magnetism of the Earth was published at Christiania, 1817; in 1826 he published the first isodynamic charts. Barlow (1831) suggested the electric origin of terrestrial magnetism. In 1831, captain Ross came upon the n. magnetic pole. In 1835, stations were established throughout Europe, and the observations were published by Gauss and Weber, 1836. Gauss (1833-1840) perfected his theory. In 1837, col. Sabine published an isodynamic chart of the whole globe. Observations were made (1840-1854) at stations throughout the British empire by British officers, under the direction of col. Sabine.