Halley’s theory, or rather hypothesis, which regarded our globe as a great piece of clockwork, by which the poles of an internal magnet were carried round in a cycle of determinate but unknown period, was so far confirmed, that his variation chart had been hardly forty years completed, when, by the effect of these changes, it had already become obsolete; and to satisfy the requirements of navigation, it became necessary to reconstruct it. This was performed by the aid of various observations furnished by the Commissioners of the Navy, and the East India, Africa, and Hudson’s Bay Companies. But the chart was far from satisfactory, and, in consequence of the discordant nature of the observations, no dependence could be placed on it. No further steps were taken to ascertain the magnetism of the earth until the close of the last century, when the French government undertook the first comprehensive experimental inquiry on the subject. When the exploring expedition of La Pérouse was organized, the French Academy of Sciences prepared instructions for the expedition, containing a recommendation that observations with the dipping-needle should be made at stations widely remote, as a test of the equality or difference of the magnetic intensity; suggesting also, with a sagacity anticipating the result, that such observations should particularly be made at those parts of the earth where the dip was greatest, and where it was least. The experiments, whatever their results may have been, which, in compliance with this recommendation, were made in the expedition of La Pérouse, perished in its general catastrophe, neither ships nor navigators having ever been heard of; but the instructions survived. Our knowledge of the laws of magnetism was not increased until 1811, when, on the occasion of a prize proposed by the Royal Danish Academy, M. Hansteen, whose attention had for many years been turned to magnetic phenomena, undertook its re-examination. With indefatigable labor M. Hansteen traced back the history of the subject, and filled up the interval from Halley’s time, and even from an earlier epoch (1600). The results appeared in his very remarkable and celebrated work, published in 1819, entitled, “Upon the Magnetism of the Earth;” in which he clearly demonstrates, by a great number of facts, the fluctuation which the magnetical element has undergone during the last two centuries, confirming in great detail the position of Halley—that the whole magnetical system is in motion; that the moving force is very great, extending its effects from pole to pole; and its that motion is not sudden, but gradual and regular.