With respect to the dip or inclination of the magnetic needle, which must be regarded as the other element of magnetic direction, there is little doubt that it was known long before the period usually assigned as the date of its discovery—namely, in 1576; for it is difficult to conceive how the variation of the needle should be observed and noted, and not its deviation from a horizontal line. In the above year a person of the name of Robert Norman, who styled himself “hydrographer,” published a book containing an account of this phenomenon. The title of this work is sufficiently curious to be quoted. It runs: “The New Attractive; containing a short Discourse of the Magnes or Loadstone, and amongst others his Virtues, of a neue discovered Secret and Subtill Propertie, concerning the Declination of the Needle touched therewith under the Plaine of the Horizon, now first found out by Robert Norman, Hydrographer.” In the third chapter we are told “by what meanes the rare and straunge declyning of the needle from the plaine of the horison was first found.”
“Having made many and diuers compasses, and using always to finish and end them before I touched the needle, I found continually that after I had touched the yrons with the stone, that presently the north point thereof would bend or declyne downwards under the horison in some quantity, insomuch that I was constrained to putt some small piece of waxe in the south parts thereof, to counterpoise this declyning, [pg 653] and to make it equal againe. Which effecte hauing many times passed my hands without any greate regarde thereunto, as ignorant of any such properties in the stone, and not before hauing heard or read of any such matter, it chanced at length that there came to my handes an instrument to be made with a needle of sixe inches long, which needle, after I had polished, cutt off at full length, and made it to stand leuel upon the pinn, so that nothing rested but only the touching of it with the stone. When I hadde touched the same, presently the north part thereof declyned down in such sort, that being constrained to cut away some of that part to make it equall againe in the end, I cut it too short, and so spoiled the needle wherein I had taken so much paines.
“Hereby being straken into some cholar, I applyed myself to seek farther into this effecte; and making certain learned and expert men, my friends, acquainted in this matter, they advised me to frame some instrument to make some exact triall how much the needle touched with the stone would declyne, or what greatest angle it would make with the plaine of the horison.”
The author then proceeds to give a number of experiments which he made with his instrument, and which may be regarded as the dipping-needle in its first and rudest form. By it he found the inclination or dip to be 71° 50'.
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.”
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 [pg 654] 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.