It is remarkable, that until within the last seventy years, it appears to have been the received opinion that the intensity of terrestrial magnetism was the same at all parts of the earth’s surface; or, in other words, that in all countries the needle was similarly affected. And yet few things are more inconstant; for, not only is the magnetic force widely different in various parts of our globe, but the magnetic condition itself is one of swift and ceaseless change. The first person who attempted to collect and generalize observations on the variation of the needle, was Robert Halley, who constructed a chart, showing a series of lines drawn through the points or places where the needle exhibited the same variation. This chart was published in 1700, and was preceded by some exceedingly curious papers, communicated to the Royal Society, in which he expresses his belief that he has put it past doubt that the globe of the earth is one great magnet, having four magnetic poles or points of attraction, two near each pole of the equator; and that in those parts of the world which lie adjacent to any one of those magnetical poles, the needle is chiefly governed thereby, the nearest pole being always predominant over the more remote. The great importance of collecting as much information as possible respecting the laws of magnetism, with a view to the proper understanding of its effects, was fully understood by Halley, as the following passage, taken from one of his papers, read before the Royal Society in 1692, singularly attests: “The nice determination of the variation, and several other particulars in the magnetic system, is reserved for a remote posterity. All that we can hope to do is, to leave behind us observations that may be confided in, and to propose hypotheses which after-ages may examine, amend, or refute; only here I must take leave to recommend to all masters of ships, and all others, lovers of natural truths, that they use their utmost diligence to make, or procure to be made, observations of these variations in all parts of the world, as well in the north as south latitude, after the laudable custom of our East India commanders; and that they please to communicate them to the Royal Society, in order to leave as complete a history as may be to those that are hereafter to compare all together, and to complete and perfect this abstruse theory.”