From Chambers' Journal. Ono of those periods is now approaching when the earth thrilled through her whole frame by magnetic throes of lusuil intensity. Physicists will be able to trace in the lent indications of the suspended magnet the action of the ost remarkable of all the forces to-which the earth is sub-Bt3tL In telegraphic offices, the occurrence of these dis-.vbanco will be mado apparent by the interruption of com-uiication for longer or shorter intervals of time. And the lf-rocorJinjf instru nentsat Kew and other such observa-, ries will indicate by unusual movements the progress of | ??? mysterious electric convulsions known as magnetic ; crm?. But except ior suA indications as these, and ont or | 7o others which have only of late years been referred to the rsncy of terrestrial magnetism, the inhabitants of this earth il1 not bo made sensibly aware that anything unusual is in ogress. For ages these magnetic disturbances have thrilled trough the earth's framework without being recognized ; id even now it seems almost by an accident that our physi-sts have been led to understand the significance of one of ic most remarkable of all terrestrial phenomena. The facts which have been ascertained respecting terrestrial agnetism are so interesting and so little known, that we ay confidently claim the attention of the reader while we ate s une of the most striking and noteworthy of them. The most generally recognized property of the magnet, its nvor of indicating the north point, was discovered by the iiinssa many ages before it became known to European ob-irvors. We learn that the Chinese, when journeying over ?? great plains of Central Asia, used a magnetic car, in front which a il sating needle bore a figure, whose outstretched t? pointed continually southwards. The Greeks and Ro-iaus were aware that iron could be magnetized : but it never ippcned that a suitably balanced fragment of magnetizud on exhibited to* them the earth's directive force. Humboldt smirks thai " on this accidental circumstance alone the great isoovery dpende:.!." It must be remarked, however, that ich accidents have been common in the history of discovery ul invention. OR;! the western nations discovered the magnet's principal roporty so early as the Chinese, we should probably have ained valuable information respecting the next property h ich has to be considered—the fact, namely, that the magot do is not commonly point due north. It is not likely that le Chinese discovered this property, because over the whole L Eastern Asia the magnetic compass points very nearly wards the north. But even if they had, it is not so much in divergence of the compass from the north point which '9aid have rendered the discovery interesting to us, as the nowkttjrc! which ancient observations might have given us specting tue laws on which the changes of that divergence ? pend. In. Europe, as we shall presently see, these changes re very conspicuous. I It was in the thirteenth century that European observers rst detected the fact that the magnetic needle does not point ue north. It may be well to notice a certain peculiarity !out the nomenclature of this deviation. Seamen always ill it the needle's variation; but among scientific men it is ill id the declination. For a long time it was supposed that so direction of the needle was the same for all places ; but uring the first voyage of Christopher Columbus across the , tlantic it was found that this is not the case. He had trav-i\l six hundred miles from the most westerly of the Canary stands, when he notice], that the compass, which had been ointing towards the oast of north when he was in Europe, as now pointing due north. The actual day on which the is-every was made was September 18, 1492. As he sailed irther west he found that the westerly declination gradually creased. But here we have at otice to call attention to another pe-uliarity of the magnetic compass, otherwise the reader would irai a mistaken notion of the present nature of the needle's eclination. We have spoken of the needle as pointing to j lie cast of north in 1492. This is no longer a true descrip-ion of the declination in Europe. The needle now points far : the west of north. It is a peculiarity of the science of ter-sstrial magnetism thai variations are thus mixed up with ariations, until it has become a matter of exceeding difficulty t prs mt all the facts of the science in such a sequence that lie student shalj not be in any risk of befhg led astray, 'roperly speaking, the change of the needle's declination ?om time to time should be kept wholly separate from the hanges which are noticed as the needle is changed from lace to place. Yet, if this were done in describing the orig-aal discovery of the latter change, erroneous impressions ronld be given respecting the present state ot the needle's eclination in various countries. At present, the terrestrial globe may be looked upon as ividedinto two vast but unequal portions, which may be ailed the region of westerly magnets and the region of east-rly magnets. In the former must be included all Europe, xcept the extreme north-easterly parts of Eussia, the whole i Africa, Turkey, Arabia, the greater part of the Indian eean, and the western parts of Australia. Returning westwards, we must add to the region of westerly magnets the greater part of the Atlantic 0.;ean, the north-eastern parts of razil, the eastern parts of Canada, and the whole of Green-and. All the rest of the world belongs to the region of asterly magnets except an oval space, which is situated in he very middle of the region, yet has a contrary character, "his soace includes the eastern part of China, Manchooria, ,ud the islands of Japan. Such is the present arrangement of the two divisions ; but rfty years ago, the description would have been incorrect, and fifty years hence it will again be so ; for over the whole world the declination is steadily changing—here in one direction; there in the contrary ; quickly at some places, almost imperceptibly at others. And we may mention in passing, that, as a general rule, where the declination is least either westward or eastward, there it is changing most rapidly ; and where it is greatest, it is hardly changing at all. But there appear to be some places where the range of change is so small, that, though the declination is never large, it does not change rapidly—as in other places of small declination. As yet, however, much remains to be learned respecting the progress of these strange changes in countries where magnetic observations have been only commenced in recent times. Some idea of the complexity of the question will be suggested by comparing the changes which have occurred in two places so near to each other as London and Paris. We shall see that not only are the declinations different in these cities, but their range of variation indifferent, both as to extent and as to the period in which a complete oscillation of the needle is effected. The easterly declination of the needle in London was observed to disappear in about the year 1657. From that epoch, the needle continually traveled westwards, until it began to be thought that it would move ever in that direction, and so come at length to point southwards. In Paris, the easterly declination had not disappeared before the year 1663, and th ere also the needle traveled continually westwards, though not quite so rapidly as in London. In 1814, the needle pointed about 22i degrees towards the west in Paris, and some two degrees further west in London. In that year, however, Arago startled the scientific world by announcing that in his opinion the needle's westerly motion was flagging, and he asserted his belief that that motion would presently give place to an easterly movement. Only three years passed before the prediction was fulfilled ; and on the 10th of April, 1817, Arago was able to announce that the needle had begun to return towards the north. But observers in London pronounced against this view. The London needles were still traveling westward, though with a slowly diminishing motion. It was not until the spring of 1819 that the London observers admitted that the needles had really leached the limit of their westerly oscillation. And whereas in Paris the needles had not traveled more than 22i degrees towards the west, in London they had passed no less than 25 degrees from the north point. Corresponding to this circumstance, we see also that the duration of the half-oscillation (for the needles had not been watched from their greatest easterly declinations) was a hundred and sixty-two years in London, and a hundred and fifty-four years in Paris. It gives a grand idea of the nature of those ever-acting forces to which terrestrial magnetism is due, to consider that the sway of the magnetic needle from limit to limit of its range should occupy so long an interval as three centuries in both these instances. Conceive the scale on which a pendulum should be constructed in order that its oscillations might have a period of as many seconds ! It was while they were engaged in tracking the progress of this long oscillation, that physicists detected minute oscillations superposed, so to speak, upon the main one, and even more singular in th eir character. The case is somewhat as though to the bob of a long pendulum there were attached a short one, and that it was to the motions of this short pendulum (beating with its own rapid swing, while carried slowly backward and forward by the main movement) that attention was primarily directed. Each day the magnetic needle sways backward and forward twice across its mean position. Shortly before midnight, it begins to travel from west to east, reaching the limit of that motion soon after eight in the morning. Then it sweeps westward to its greatest westerly limit, which it reaches soon after one. Then back again towards the east, until half-past eight, and so to its original position at about eleven o'clock. It must be understood that these motions are so minute in comparison with the great secular oscillation, that they never affect the general direction of the magnet to any noteworthy extent. For instance, we have just spoken of the two easterly limits of the daily swing, but throughout the day the magnet always points far to the west of north. The mean declination, in fact, is (roughly) about 20 degrees, whereas the daily swing never ranges over more than the fifth part of a degree. It will be noticed that the oscillations above described correspond closely with the diurnal motions of the sun. They are such, in fact, as the needle would exhibit on the supposition that it tries to follow the sun during his complete apparent revolution round the celestial sphere. It is believed that the daily motions of flowers, and in particular that class of motion which has given the sun-flower its distinctive appellation, are due to the same magnetic properties which cause the diurnal swing of the suspended needle. But besides the daily sway of the magnetic needle, there is an annual oscillation of a somewhar different character. In fact, properly speaking, the annual change is not oscillatory, though it has a regularly recurrent character. The daily swing is variable. Now this variability would be somewhat confusing, on account ot its general irregularity ; therefore, physicists consider the mean of several days, and thus get rid of what for the present we may term accidental variations. When this has been done, it is found that the average daily swing of the needle is subject to a slow progressive increase, followed by an equally slow diminution ; and the period of these slow changes is a year. The peculiarity of this annual change is that its progress is the same for both hemispheres. It might have been expected that it would attain its maximum in summer, when the solar influence is strongest ; but this is not the case. It attains its maximum in January, which is indeed near midsummer for the southern hemisphere, but nearly the least sunny of our northern months. The secret of this peculiar ,ly lies in the fact that the sun is nearest to the earth in January. The peculiarity is a very meaning one, as showing that the magnetic influence is not a local matter, however variable the magnetic declination may be-as we shift from place to place. The real fact pointed to by this, as by many other phenomena, is, that the earth must be looked upon as a single gigantic magnet, gaining or losing power throughout its whole frame simultaneously. The consideration of the power of the great earth-magnet must be for a moment laid on one side, while we deal with a form of deviation as remarkable as the declination. We refer to the dip of the needle. The ordinary compass is, we know, suspended horizontally, and, for anything which appears to the contrary when we examine such an instrument, that might be the needle's position of rest. But when a needle is so suspended by a silken thread as to be free to assume an inclined position, it is found that the northern end dips perceptibly. We are assuming, of course, that in its non-magnetized state the needle would rest horizontally. In our latitudes, the dip or inclination is so great that the needle is inclined only about 22 degrees to the vertical. When we travel northwards, the dift increases ; when southwards, it diminishes, until we reach a place near the equator (traveling always, it is assumed, in the longitude of London) where the needle becomes horizontal. Alter passing that point, the southern end dips, and the inclination continues to increase as we travel southwards. The same is true for other longitudes, only the place of " no dip " is differently situated. The line along which there, is no inclination lies near the equator, crossing that circle at two opposite points, one in west longitude 8 degrees, the other in east longitude 177 degrees. The magnetic equator is not a strictly circular curve, however ; it is noteworthy that it departs most from the figure of a true circle where it traverses the Atlantic Ocean. We have seen the variations which are exhibited in the declination of the magnet, not only at different places, but at different times in the same place. Changes of precisely the same character are exhibited in the dip of the magnet; in London, for example, the dip has diminished four degrees in less than a century ; in Paris, during the last two centuries, the dip has diminished about seven degrees. Seeing this, we must accept with some little question the locales usually assigned to the magnetic poles ; because we have every reason for supposing that these poles must be continually shifting their position. In fact, the motion of the magnetic equator, which is continually sweeping from east to west along the true equator, suffices of itself to demonstrate that the magnetic poles are continually traveling around the true poles. What the laws of this motion may bo, it would not be easy to determire in the present state of our knowledge ; but it is worthy of notice that the same motion would serve to account at once for the change of dip and the change of declination. For example, in 1663 the magnetic pole may be reasonably supposed to have been due north of Paris. In the latter year, the inclination was 75 degrees in Paris, so that we can judge that the magnetic pole was on the nearer side of the true pole. As the magnetic pole passed away from this position, traveling westward, there would naturally result both a westerly declination and a gradual diminution of dip. And the fact that when Sir J. C. Boss determined the position of the northern pole in 1837, it was found to be somewhat more than 90 degrees west of the longitude- of Paris—in other words, the fact that it had traversed somewhat more than a quarter of a complete revolution soon after the westerly declination at Paris had attained its maximum value—seems strikingly confirmatory of this view. If this theory is correct, the inclination will continue to diminish until the magnetic pole has completed half a revolution, so as to be again due north of Paris, but on the further side of the %rue pole. Then the declination will be nothing, and it will afterwards become easterly. It must be admitted, however, thatthere is much more complexity in the laws according to which the declination Varies, than the above view, taken alone would imply. Doubtless, the peculiarities of the earth's structure, the arrangement of land and water, mountain-ranges, table-lands, and valleys, have much to do with the matter. The variations of the intensity of magnetic action, either from time to time, or as we proceed from place to place, are among the most interesting of all the phenomena of terrestrial magnetism. The latter class of change is associated so obviously with the changes of declination and dip, that we need not enter on its consideration. The former, however, points to problems of extreme interest in connection with the probable character and source of the whole range of forces included under the subject we are dealing with. We hav'e seen already that from hour to hoar, and from day to day, there are changes in the extent of the minute oscillations of the suspended magnet, and that these changes indicate variations in the intensity of the magnetic force under diurnal and annual solar influences. When we add to these variations a change which has a period corresponding to the motions of the moon, it becomes evident that it is to an influence as subtile and as prevading in its character as gravitation itselt, that the terrestrial magnet owts its powers. But there are other variations still more significant. A long series of researches had convinced Colonel Sabine, one ot our leading authorities on the subject of terrestrial magnetism, that the intensity of the magnetic action is subject to a process of change having a period of somewhat more than ten years. Scarcelyhad this law been established, when the results of a long and elaborate series of solar observations exhibited to the world the strange fact, that the spots which stain the sun's face vary in frequency according to a precisely similar relation. It was found that the changes of solar spottiness, and of magnetic intensity of action, arc not merely characterized by an equality ot period, but that the maximum effect under one period is absolutely coincident with the maximum effect under the other. We mifht have looked upon this as merely a very singular coincidence, had we not ind pendent evidence of an association between the sun's action and the intensity of terrestrial magnetism. Part of this evidence has been already referred to. But the evidence founded on the exact coincidence of magnetic storms, thrilling in a moment through the whole frame of the earth, with solar disturbances actually witnessed by astronomical observers, is even more striking. Thus, no room is left to question the dependence of terrestrial magnetism on solar action, and the relation between the sun's spots and the vibrations of the needle—a relation which, when first propounded, was received even by eminent physicists with ridicule—has been accepted as one of the most well established of all the circumstances known respecting terrestrial magnetism. Ot the meaning of this singular relation, we have not at present space to speak ; indeed, wa should be led into a variety of considerations, which would be out of place in such a paper as the present. The appearance presented by the solar spots, the processes by which they are formed, the laws on which their changes depend—all these, and many other questions of the sort would have to be dealt with, to say nothing of the planetary movements on which, according to modern researches, the habitudes of the solar atmosphere are dependent. We may note, in conclusion, tha. the solar face has recently presented all the signs which we have learned to associate with the intenser phases of terrestrial magnetic action. Enormous spots and clusters of spots have broken ont during the past few months ; and probably the spots which will shortly make their appearance will be yet larger, since the epoch of maximum disturbance has not yet been fully reached.
This article was originally published with the title "Terrestrial Magnetism" in Scientific American 21, 21, 322-323 (November 1869)