Harper's New Monthly Magazine, No. V, October, 1850, Volume I.

by Unknown

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.