Terrestrial Magnetism. (From Chambers's Edinburgh Journal.)
It is proposed in the following article to give the reader some idea of one of the greatest and most extensive scientific works going on at the present time in this country—namely, the examination of the phenomenon of the earth's [pg 652] magnetism; but before doing so, it will be necessary to make a few prefatory observations respecting magnetism generally.
The attractive power of the natural magnet or loadstone over fragments of iron seems to have been known from the remotest antiquity. It is distinctly referred to by ancient writers, and Pliny mentions a chain of iron rings suspended from one another, the first being upheld by a loadstone. It is singular that although the common properties of the loadstone were known, and even studied, during the dark ages, its directive power, or that of a needle touched or rubbed by it, seems to be the discovery of modern times, notwithstanding the claims of the Chinese and Arabians to an early acquaintance with this peculiarity.
There is no doubt that the mariner's compass was known in the twelfth century, for several authors of that period make special allusion to it; but centuries elapsed before its variation from pointing precisely to the poles became noticed. If a magnet be suspended by a thread, in such a manner as to enable it to move freely, it will, when all other magnetic bodies are entirely removed from it, settle in a fixed position, which, in this country, is about 25° to the west of north; this deviation of the needle from the north is called its variation. Again, if, in place of suspending a magnetized needle, making it move horizontally on a pivot, we balance it upon a horizontal axis, as the beam of a pair of scales, we shall find that it no longer remains horizontal, but that one end will incline downward, or, as it is called, dip, and this dip or inclination from a horizontal line is about 70° in this country.
Thus we are presented with two distinct magnetical phenomena: 1. The variation or declination of the needle; 2. Its dip or inclination; and to these we may add the intensity or force which draws the needle from pointing to the north, and which varies in different latitudes. These phenomena constitute what has been called terrestrial magnetism.
Recent writers, and among them the great philosopher Humboldt, have shown that in all probability the declination or variation of the magnet was known as early as the twelfth century; but this important discovery has been generally ascribed to Columbus. His son Ferdinand states that on the 14th September 1492, his father, when about 200 leagues from the island of Ferro, noticed for the first time the variation of the needle. “A phenomenon,” says Washington Irving, “that had never before been remarked.” “He perceived,” adds this author, “about nightfall that the needle, instead of pointing to the north star, varied half a point, or between five and six degrees, to the northwest, and still more on the following morning. Struck with this circumstance, he observed it attentively for three days, and found that the variation increased as he advanced. He at first made no mention of this phenomenon, knowing how ready his people were to take alarm; but it soon attracted the attention of the pilots, and filled them with consternation. It seemed as if the laws of nature were changing as they advanced, and that they were entering another world, subject to unknown influences. They apprehended that the compass was about to lose its mysterious virtues; and without this guide, what was to become of them in a vast and trackless ocean? Columbus tasked his science and ingenuity for reasons with which to allay their terrors. He told them that the direction of the needle was not the polar star, but to some fixed and invisible point: the variation was not caused by any failing in the compass, but because this point, like the heavenly bodies, had its changes and revolutions, and every day described a circle round the pole. The high opinion that the pilots entertained of Columbus as a profound astronomer gave weight to his theory, and their alarm subsided.”
Thus, although it is possible that the variation of the needle had been noticed before the time of Columbus, it is evident that he had discovered the amount of the variation, and that it varied in different latitudes. The great philosopher Humboldt observes on this point, that “Columbus has not only the incontestible merit of having first discovered a line without magnetic variation, but also of having, by his considerations on the progressive increase of westerly declination in receding from that line, given the first impulse to the study of terrestrial magnetism in Europe.”
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.
In the magnetic atlas which accompanies M. Hansteen's work there is a variation chart for 1787, showing the magnetic force at that period. In this chart the western line of no variation, or that which passes through all places on the globe when the needle points to the true north, begins in latitude 60° to the west of Hudson's Bay; proceeds in a southeast direction through the North American Lakes, passes the Antilles and Cape St. Roque, till it reaches the South Atlantic Ocean, when it cuts the meridian of Greenwich in about 65° of south latitude. This line of no variation is extremely regular, being almost straight, till it bends round the eastern part of South America, a little south of the equator. The eastern line of no variation is exceedingly irregular, being full of curves and contortions of the most extraordinary kind, indicating plainly the action of local magnetic forces. It begins in latitude 60° south, below New Holland; crosses that island through its centre; extends through the Indian Archipelago with a double sinuosity, so as to cross the equator three times—first passing north of it to the east of Borneo, then returning to it, and passing south between Sumatra and Borneo, and then crossing it again south of Ceylon, from which it passes to the east through the Yellow Sea. It then stretches along the coast of China, making a semicircular sweep to the west, till it reaches the latitude of 71°, when it descends again to the south, and returns northwards with a great semicircular bend, which terminates in the White Sea. Thus it is demonstrated that in the northern hemisphere the general motion of the variation lines is from west to east, in the southern hemisphere from east to west.
A great impetus was given to the study of terrestrial magnetism by the publication of M. Hansteen's labors; and the various arctic expeditions sent out by the country did much toward making us acquainted with the laws of magnetism in the northern regions. One of these expeditions led to the discovery of the north magnetic pole, or that point where the dipping-needle assumes a vertical position. The discovery was made by Captain Sir James Ross, who sailed with his uncle Sir John Ross, in a voyage undertaken in search of a northwest passage. He left his uncle's ship with a party for the sole purpose of reaching this interesting magnetical point, which a series of observations assured him could not be very far distant. The following extract from his journal communicating his discovery will be read with interest. Under the date of the 31st of May 1831, he writes: “We were now within fourteen miles of the calculated position of the magnetic pole, and my anxiety, therefore, did not permit me to do or endure any thing which might delay my arrival at the long wished-for spot. I resolved, therefore, to leave behind the greater part of our baggage and provisions, and to take onward nothing more than was strictly necessary, lest bad weather or other accidents should be added to delay, or lest unforeseen circumstances, still more untoward, should deprive me entirely of the high gratification which I could not but look to in accomplishing this most-desired object. We commenced, therefore, a most rapid march, comparatively disencumbered as we now were; and persevering with all our might, we reached the calculated place at eight in the morning of the 1st of June. The amount of the dip, as indicated by my dipping-needle, was 89° 59', being thus within one minute of the vertical; while the proximity at least of this magnetic pole, if not its actual existence where we stood, was further confirmed by the total inaction of the several horizontal needles then in my possession. These were suspended in the most delicate manner possible, but there was not one which showed the slightest effort to move from the position in which it was placed—a fact which even the most moderately-informed of readers must know to be one which proves that the centre of attraction lies at a very small horizontal distance, if at any. The land at this place is very low near the coast, but it rises into ridges of fifty or sixty feet high about a mile inland. We could have wished that a place so important had possessed more of mark or note. But nature had here erected no monument to denote the spot that she had chosen as the centre of one of her great and dark powers. We had abundance of materials for building in the fragments of limestone that covered the beach, and we therefore erected a cairn of some magnitude, under which we buried a canister containing a record of the interesting fact, only regretting that we had not the means of constructing a pyramid of more importance, and of strength sufficient to stand the assaults of time and of the Esquimaux.” The latitude of this spot is 70° 5' 17", and its longitude 96° 46' 45" west. The reader may remember that during his late arctic voyage in search of Sir John Franklin, Sir James Ross was extremely anxious to revisit this interesting locality, which he was at one time not very distant from; but which, as the places of magnetic intensity are continually changing, he would no longer have found representing the north magnetic pole. It is not a little remarkable that during Sir John Ross's voyage, Mr. Barlow, who had been long engaged investigating the laws of magnetism, had constructed a magnetical map, in which he laid down a point which he described as that where, in all probability, the dipping-needle would be perpendicular, and which is the very spot where Sir James Ross ascertained the north magnetic pole to exist.
But valuable and interesting as were the observations [pg 655] made by navigators in different parts if the globe, yet philosophers began to perceive that, without some definite plan of proceeding, the mere multiplication of random observations made here and there at irregular periods was not the course most likely to lead to desired results, and to make us acquainted with the mysterious laws of magnetism. The establishment of national observatories for the registration of magnetical observations became absolutely necessary; and the illustrious Humboldt, to whom every branch of science owes so much, gave the first impulse to this great undertaking. During the course of his memorable voyages and travels in various parts of the globe, the observation of the magnetic phenomena in all their particulars occupied a large portion of his attention; and as the commencement of any great work is always an epoch of rare and lasting interest, we shall give the philosopher's own words on the subject: “When the first proposal to establish a system of observatories forming a network of stations, all provided with similar instruments, was made by myself, I could hardly entertain the hope that I should actually live to see the time when, thanks to the united activity of excellent physicists and astronomers, and especially to the munificent and persevering support of two governments—the Russian and the British, both hemispheres should be covered with magnetic observatories. In 1806 and 1807 my friend M. Altmanns and myself frequently observed the march of the declination needle at Berlin for five or six days and nights consecutively, from hour to hour, and often from half hour to half hour, particularly at the equinoxes and solstices. I was persuaded that continuous uninterrupted observations during several days and nights were preferable to detached observations continued during an interval of many months.”
Political disturbances, always ruinous to the calm researches of the man of science, for many years prevented Humboldt carrying his wishes into effect; and it was not until 1828 that he was enabled to erect a small observatory at Berlin, whose more immediate object was to institute a series of simultaneous observations at concerted hours at Berlin, Paris, and Freiburg. In 1829 magnetic stations were established throughout Northern Asia, in connection with an expedition to that country which emanated from the Russian government; and in 1832 M. Gauss, the illustrious founder of a general theory of terrestrial magnetism, established a magnetic observatory at Göttingen, which was completed in 1834, and furnished with his ingenious instruments.
In 1836 Baron Humboldt addressed a long and highly-interesting letter to the Duke of Sussex, then president of the Royal Society, urging the establishment of regular magnetical stations in the British possessions in North America, Australia, the Cape of Good Hope, and between the tropics, not only for the observation of the momentary perturbations of the needle, but also for that of its periodical and secular movements. This appeal was nobly responded to.
The Royal Society, in conjunction with the British Association, called on government to advance the necessary funds to establish magnetical observatories at Greenwich, and in various parts of the British possessions; and in 1839-40 magnetical establishments were in activity at St. Helena, the Cape of Good Hope, Canada, and Van Diemen's Land. The munificence of the directors of the East India Company founded and furnished, at the request of the Royal Society, magnetic observatories at Simla, Madras, Bombay, and Singapore, and the observations will be published in a similar form to those of the British observatories. We will now briefly describe the scheme of observations, and the manner of making them in the different observatories.
Each observatory is supplied with three magnetometers, or bars of magnetized steel, delicately suspended by threads of raw silk, which measure the magnetical declination, horizontal intensity, and vertical force—and such astronomical apparatus as is required for ascertaining the time and the true meridian. To these have also been added in each case a most complete and perfect set of meteorological instruments, carefully compared with the standards in possession of the Royal Society, not only for the purpose of affording the necessary corrections of the magnetic observations, but also with a view to obtaining at each station, at very little additional cost and trouble, a complete series of meteorological observations. In order that the observations may be made at the same periods of time, it was resolved that the mean time at Göttingen should be employed at all the stations, without any regard to the apparent times of day at the stations themselves. Each day is supposed to be divided into twelve equal portions of two hours each, commencing at all the stations at the same instants of absolute time, which are called the magnetic hours. At the commencement of each period of two hours throughout the day and night, with the exception of Sundays, the magnetometers are observed, and the meteorological instruments read off. Independently of these observations, others are made at stated periodical intervals every two minutes and a half during twenty-four hours. These are known by the name of “turn-day observations.” Printed forms for registering the observations have been prepared with great care, in order that a complete form of registry may be preserved—a point of great importance, when it is remembered that all the observations made at the different stations must eventually be reduced and analyzed. A singularly felicitous adaptation of photography has been carried into effect with the magnetometers. By means of mirrors attached to their arms, reflected light is cast on highly-sensitive photographic paper wound round a cylinder moved by clockwork, and the slightest variation of the magnets is registered with the greatest accuracy.
The period has not yet arrived for reaping [pg 656] the fruits of all the labor carried on in the magnetic observatories at home and abroad, but already certain results have been deduced from the observations which are highly interesting. It appears that if the globe be divided into an eastern and a western hemisphere by a plane coinciding with the meridians of 100° and 280°, the western hemisphere, or that comprising the Americas and the Pacific Ocean, has a much higher magnetic intensity distributed generally over its surface than the eastern hemisphere, containing Europe and Africa, and the adjacent part of the Atlantic Ocean. The distribution of the magnetic intensity in the intertropical regions of the globe affords evidence of two governing magnetic centres in each hemisphere. The highest magnetic intensity which has been observed is more than twice as great as the lowest. It had long been known that in Europe the north end of a magnet suspended horizontally (meaning by the north end that which is directed toward the north) moves to the east from the night until between seven and eight o'clock in the morning, when an opposite movement commences, and the north end of the magnet moves to the west. Recent observations have shown that a similar movement takes place at the same hours of local time in North America, and that it is general in the middle latitudes of the northern hemisphere; but to show the capricious nature of magnetism, it may be mentioned, that although in the southern portion of the globe the movement of the magnet in the contrary direction is constant throughout the year, yet at St. Helena the peculiar feature of the diurnal is, that during one half of the year the movement of the north end of the magnet corresponds in direction with the movement which is taking place in the northern hemisphere, while in the other half of the year the direction corresponds with that which is taking place in the southern hemisphere.
Another striking result of these investigations is the estimate of the total magnetic power of the earth as compared with a steel bar magnetized one pound in weight. This proportion is calculated as 8,464,000,000,000,000,000,000 to 1, which, supposing the magnetic force uniformly distributed, will be found to amount to about six such bars to every cubic yard of the earth's surface.
Thus measured, it will be seen how tremendously mysterious is the power of magnetism, and how potent an influence it must possess over animate and inanimate nature! And not one of its least wonderful mysteries is its singular exception to the character of stability and permanence. The configuration of our globe, the distribution of temperature in its interior, the tides and currents of the ocean, the general course of winds, and the affections of climate—all these are appreciably constant. But magnetism, that subtle, undefinable fluid, is perpetually undergoing a change, and of so rapid a nature, that it becomes necessary to assume epochs, which ought not to be more than ten years apart, to which every observation should be reduced. The extreme importance of knowing the exact amount of magnetic variation can scarcely be overrated for maritime purposes; and the establishment of a complete magnetical theory, based on an extensive series of observations, must be regarded as a desideratum by the first nautical country.
The numerous magnetical surveys that have been made by our government, taken in conjunction with those in progress on the continent of Europe, and particularly in the Austrian dominions, give a full promise of the speedy realization of M. Humboldt's wish, so earnestly expressed, that the materials of the first general magnetic map of the globe should be assembled; and even permit the anticipation, that the first normal epoch of such a map will be but little removed from the present year.