The peculiarity13 that the magnet needle does not, in general, point to the north, is the first of a series of peculiarities14 which I now propose briefly15 to describe. The irregularity is called by sailors the needle’s variation, but the term more commonly used by scientific men is the declination of the needle. It was probably discovered a long time ago, for 800 years before our era the Chinese applied16 the magnet’s directive force to guide them in journeying over the great Asiatic plains, and they must soon have detected so marked a peculiarity. Instead of a ship’s compass, they made use of a magnetic car, on the front of which a floating needle carried a small figure, whose outstretched arm pointed southwards. We have no record, however, of their discovery of the declination, and know only that they were acquainted with it in the twelfth century.16 The declination was discovered, independently, by European observers in the thirteenth century.
As we travel from place to place, the declination of the needle is found to vary. Christopher Columbus was the first to detect this. He discovered it on the 13th of September, 1492, during his first voyage, and when he was six hundred miles from Ferro, the most westerly of the Canary Islands. He found that the declination, which was towards the east in Europe, passed to the west, and increased continually as he travelled westwards.
But here we see the first trace of a yet more singular peculiarity. I have said that at present the declination is towards the west in Europe. In Columbus’s time it was towards the east. Thus we learn that the declination varies with the progress of time, as well as with change of place.
The genius of modern science is a weighing and a measuring one. Men are not satisfied nowadays with knowing that a peculiarity exists; they seek to determine its extent, how far it is variable—whether from time to time or from place to place, and so on. Now the results of such inquiries17 applied to the magnetic declination have proved exceedingly interesting.
We find, first, that the world may be divided into two unequal portions, over one of which the needle has a westerly, and over the other an easterly, declination. Along the boundary line, of course, the needle points due north. England is situated18 in the region of westerly magnets. This region includes all Europe,17 except the north-eastern parts of Russia; Turkey, Arabia, and the whole of Africa; the greater part of the Indian Ocean, and the western parts of Australia; nearly the whole of the Atlantic Ocean; Greenland, the eastern parts of Canada, and a small slice from the north-eastern part of Brazil. All these form one region of westerly declination; but, singularly enough, there lies in the very heart of the remaining and larger region of easterly magnets an oval space of a contrary character. This space includes the Japanese Islands, Manchouria, and the eastern parts of China. It is very noteworthy also, that in the westerly region the declination is much greater than in the easterly. Over the whole of Asia, for instance, the needle points almost due north. On the contrary, in the north of Greenland and of Baffin’s Bay, the magnetic needle points due west; while still further to the north (a little westerly), we find the needle pointing with its north end directly towards the south.
In the presence of these peculiarities, it would be pleasant to speculate. We might imagine the existence of powerfully magnetic veins19 in the earth’s solid mass, coercing20 the magnetic needle from a full obedience21 to the true polar summons. Or the comparative effects of oceans and of continents might be called into play. But unfortunately for all this, we have to reconcile views founded on fixed22 relations presented by the earth with the process of change indicated above. Let us consider the declination in England alone.
In the fifteenth century there was an easterly decli18nation. This gradually diminished, so that in about the year 1657 the needle pointed due north. After this the needle pointed towards the west, and continually more and more, so that scientific men, having had experience only of a continual shifting of the needle in one direction, began to form the opinion that this change would continue, so that the needle would pass, through north-west and west, to the south. In fact, it was imagined that the motion of the needle would resemble that of the hands of a watch, only in a reversed direction. But before long observant men detected a gradual diminution23 in the needle’s westerly motion. Arago, the distinguished24 French astronomer25 and physicist26, was the first (I believe) to point out that ‘the progressive movement of the magnetic needle towards the west appeared to have become continually slower of late years’ (he wrote in 1814), ‘which seemed to indicate that after some little time longer it might become retrograde.’ Three years later, namely, on the 10th of February, 1817, Arago asserted definitively27 that the retrograde movement of the magnetic needle had commenced to be perceptible. Colonel Beaufoy at first oppugned Arago’s conclusion, for he found from observations made in London, during the years 1817-1819, that the westerly motion still continued. But he had omitted to take notice of the circumstance, that London and Paris are two different places. A few years later the retrograde motion became perceptible at London also, and it has now been established by the observations of forty years. It appears, from a19 careful comparison of Beaufoy’s observations, that the needle reached the limit of its western digression (at Greenwich) in March 1819, at which time the declination was very nearly 25°. In Paris, on the contrary, the needle had reached its greatest western digression (about 22?°) in 1814. It is rather singular that although at Paris the retrograde motion thus presented itself five years earlier than in London, the needle pointed due north at Paris six years later than in London, viz., in 1663. Perhaps the greater amplitude28 of the needle’s London digression may explain this peculiarity.
‘It was already sufficiently29 difficult,’ says Arago, ‘to imagine what could be the kind of change in the constitution of the globe which could act during one hundred and fifty-three years in gradually transferring the direction of the magnetic needle from due north to 23° west of north. We see that it is now necessary to explain, moreover, how it has happened that this gradual change has ceased, and has given place to a return towards the preceding state of the globe.‘ ‘How is it,’ he pertinently30 asks, ‘that the directive action of the globe, which clearly must result from the action of molecules31 of which the globe is composed, can be thus variable, while the number, position, and temperature of these molecules, and, as far as we know, all their other physical properties, remain constant?’
But we have considered only a single region of the earth’s surface. Arago’s opinion will seem still juster when we examine the change which has taken place20 in what we may term the ‘magnetic aspect’ of the whole globe. The line which separates the region of westerly magnets from the region of easterly magnets now runs, as we have said, across Canada and eastern Brazil in one hemisphere, and across Russia, Asiatic Turkey, the Indian Ocean, and West Australia in the other, besides having an outlying oval to the east of the Asiatic continent. These lines have swept round a part of the globe’s circuit in a most singular manner since 1600. They have varied32 alike in direction and complexity33. The Siberian oval, now distinct, was in 1787 merely a loop of the eastern line of no declination. The oval appears now to be continually diminishing, and will one day probably disappear.
We find here presented to us a phenomenon as mysterious, as astonishing, and as worthy12 of careful study as any embraced in the wide domains34 of science. But other peculiarities await our notice.
If a magnetic needle of suitable length be carefully poised35 on a fine point,—or better, be suspended from a silk thread without torsion,—it will be found to exhibit each day two small but clearly perceptible oscillations. M. Arago, from a careful series of observations, deduced the following results:—
At about eleven at night, the north end of the needle begins to move from west to east, and having reached its greatest easterly excursion at about a quarter-past eight in the morning, returns towards the west to attain37 its greatest westerly excursion at a quarter-past one. It then moves again to the east, and having reached its21 greatest easterly excursion at half-past eight in the evening, returns to the west, and attains38 its greatest westerly excursion at eleven, as at starting.
Of course, these excursions take place on either side of the mean position of the needle, and as the excursions are small, never exceeding the fifth part of a degree, while the mean position of the needle lies some 20° to the west of north, it is clear that the excursions are only nominally39 eastern and western, the needle pointing throughout, far to the west.
Now, if we remember that the north end of the needle is that farthest from the sun, it will be easy to trace in M. Arago’s results a sort of effort on the part of the needle to turn towards the sun—not merely when that luminary40 is above the horizon, but during his nocturnal path also.
We are prepared, therefore, to expect that a variation having an annual period, shall appear, on a close observation of our suspended needle. Such a variation has been long since recognised. It is found that in the summer of both hemispheres, the daily variation is exaggerated, while in winter it is diminished.
But besides the divergence41 of a magnetised needle from the north pole, there is a divergence from the horizontal position which must now claim our attention. If a non-magnetic needle be carefully suspended so as to rest horizontally, and be then magnetised, it will be found no longer to preserve that position. The northern end dips very sensibly. This happens in our hemisphere. In the southern, it is the southern end22 which dips. It is clear, therefore, that if we travel from one hemisphere to the other we must find the northern dip of the needle gradually diminishing, till at some point near the equator the needle is horizontal; and as we pass thence to southern regions, a gradually increasing southern inclination42 is presented. This has been found to be the case, and the position of the line along which there is no inclination (called the magnetic equator) has been traced around the globe. It is not coincident with the earth’s equator, but crosses that circle at an angle of twelve degrees, passing from north to south of the equator in long. 3° west of Greenwich, and from south to north in long. 187° east of Greenwich. The form of the line is not exactly that of a great circle, but presents here and there (and especially where it crosses the Atlantic) perceptible excursions from such a figure.
At two points on the earth’s globe the needle will rest in a vertical43 position. These are the magnetic poles of the earth. The northern magnetic pole was reached by Sir J. G. Ross, and lies in 70° N. lat. and 263° E. long., that is, to the north of the American continent, and not very far from Boothia Gulf44. One of the objects with which Ross set out on his celebrated45 expedition to the Antarctic Seas was the discovery, if possible, of the southern magnetic pole. In this he was not successful. Twice he was in hopes of attaining46 his object, but each time he was stopped by a barrier of land. He approached so near, however, to the pole, that the needle was inclined at an angle of nearly23 ninety degrees to the horizon, and he was able to assign to the southern pole a position in 75° S. lat., 154° E. long. It is not probable, we should imagine, that either pole is fixed, since we shall now see that the inclination, like the declination of the magnetic needle, is variable from time to time, as well as from place to place; and in particular, the magnetic equator is apparently47 subjected to a slow but uniform process of change.
Arago tells us that the inclination of the needle at Paris has been observed to diminish year by year since 1671. At that time the inclination was no less than 75°; in other words, the needle was inclined only 15° to the vertical. In 1791 the inclination was less than 71°. In 1831 it was less than 68°. In like manner, the inclination at London has been observed to diminish, from 72° in 1786 to 70° in 1804, and thence to 68° at the present time.
It might be anticipated from such changes as these that the magnetic equator would be found to be changing in position. Nay48, we can even guess in which way it must be changing. For since the inclination is diminishing at London and Paris, the magnetic equator must be approaching these places, and this (in the present position of the curve) can only happen by a gradual shifting of the magnetic equator from east to west along the true equator. This motion has been found to be really taking place. It is supposed that the movement is accompanied by a change of form, but more observations are necessary to establish this interesting point.
24
Can it be doubted that while these changes are taking place, the magnetic poles also are slowly shifting round the true pole? Must not the northern pole, for instance, be further from Paris now that the needle is inclined more than 23° from the vertical, than in 1671, when the inclination was only 15°? It appears obvious that this must be so, and we deduce the interesting conclusion that each of the magnetic poles is rotating around the earth’s axis49.
But there is another peculiarity of the needle which is as noteworthy as any of those I have mentioned. I refer to the intensity50 of the magnetic action—the energy with which the needle seeks its position of rest. This is not only variable from place to place, but from time to time, and is further subject to sudden changes of a very singular character.
It might be expected that where the dip is greater, the directive energy of the magnet would be proportionately great. And this is found to be approximately the case. Accordingly, the magnetic equator is very nearly coincident with the ‘equator of least intensity,’ but not exactly. As we approach the magnetic poles we find a more considerable divergence, so that instead of there being a northern pole of greatest intensity nearly coincident with the northern magnetic pole, which we have seen lies to the north of the American continent, there are two northern poles, one in Siberia nearly at the point where the river Lena crosses the Arctic circle, the other not so far to the north—only a few degrees north, in fact, of Lake Superior. In25 the south, in like manner, there are also two poles, one on the Antarctic circle, about 130° E. long., in Adélie Land, the other not yet precisely51 determined52, but supposed to lie on about the 240th degree of longitude53, and south of the Antarctic circle. Singularly enough, there is a line of lower intensity running right round the earth along the valleys of the two great oceans, ‘passing through Behring’s Straits and bisecting the Pacific, on one side of the globe, and passing out of the Arctic Sea by Spitsbergen and down the Atlantic, on the other.’
Colonel Sabine discovered that the intensity of the magnetic action varies during the course of the year. It is greatest in December and January in both hemispheres. If the intensity had been greatest in winter, one would have been disposed to have assigned seasonal54 variation of temperature as the cause of the change. But as the epoch55 is the same for both hemispheres, we must seek another cause. Is there any astronomical56 element which seems to correspond with the law discovered by Sabine? There is one very important element. The position of the perihelion of the earth’s orbit is such that the earth is nearest to the sun on about the 31st of December or the 1st of January. There seems nothing rashly speculative57, then, in concluding that the sun exercises a magnetic influence on the earth, varying according to the distance of the earth from the sun. Nay, Sabine’s results seem to point very distinctly to the law of variation. For, although the number of observations is not as yet very great and26 the extreme delicacy58 of the variation renders the determination of its amount very difficult, enough has been done to show that in all probability the sun’s influence varies according to the same law as gravity—that is, inversely59 as the square of the distance.
That the sun, the source of light and heat, and the great gravitating centre of the solar system, should exercise a magnetic influence upon the earth, and that this influence should vary according to the same law as gravity, or as the distribution of light and heat, will not appear perhaps very surprising. But the discovery by Sabine that the moon exercises a distinctly traceable effect upon the magnetic needle seems to me a very remarkable60 one. We receive very little light from the moon, much less (in comparison with the sun’s light) than most persons would suppose, and we get absolutely no perceptible heat from her. Therefore it would seem rather to the influence of mass and proximity61 that the magnetic disturbances62 caused by the moon must be ascribed. But if the moon exercises an influence in this way, why should not the planets? We shall see that there is evidence of some such influence being exerted by these bodies.
More mysterious, if possible, than any of the facts I have discussed is the phenomenon of magnetic storms. The needle has been exhibiting for several weeks the most perfect uniformity of oscillation. Day after day, the careful microscopic64 observation of the needle’s progress has revealed a steady swaying to and fro, such as may be seen in the masts of a stately ship at27 anchor on the scarce-heaving breast of ocean. Suddenly a change is noted65; irregular jerking movements are perceptible, totally distinct from the regular periodic oscillations. A magnetic storm is in progress. But where is the centre of disturbance63, and what are the limits of the storm? The answer is remarkable. If the jerking movements observed in places spread over very large regions of the earth—and in some well-authenticated cases over the whole earth—be compared with the local time, it is found that (allowance being made for difference of longitude) they occur precisely at the same instant. The magnetic vibrations thrill in one moment through the whole frame of our earth!
But a very singular circumstance is observed to characterise these magnetic storms. They are nearly always observed to be accompanied by the exhibition of the aurora66 in high latitudes67, northern and southern. Probably they never happen without such a display, but numbers of auroras escape our notice. The converse68 proposition, however, has been established as an universal one. No great display of the aurora ever occurs without a strongly marked magnetic storm.
Magnetic storms sometimes last for several hours or even days.
Remembering the influence which the sun has been found to exercise upon the magnetic needle, the question will naturally arise, Has the sun anything to do with magnetic storms? We have clear evidence that he has.
On the 1st of September, 1859, Messrs. Carrington28 and Hodgson were observing the sun, one at Oxford69 and the other in London. Their scrutiny70 was directed to certain large spots which, at that time, marked the sun’s face. Suddenly a bright light was seen by each observer to break out on the sun’s surface, and to travel, slowly in appearance, but in reality at the rate of about 7,000 miles in a minute, across a part of the solar disc. Now it was found afterwards that the self-registering magnetic instruments at Kew had made at that very instant a strongly marked jerk. We learned, also, that at that moment a magnetic storm prevailed at the West Indies, in South America, and in Australia. The signalmen in the telegraph stations at Washington and Philadelphia received strong electric shocks; the pen of Bain’s telegraph was followed by a flame of fire; and in Norway the telegraphic machinery71 was set on fire. At night great auroras were seen in both hemispheres. It is impossible not to connect these startling magnetic indications with the remarkable appearance observed upon the sun’s disc.
But there is other evidence. Magnetic storms prevail more commonly in some years than in others. In those years in which they occur most frequently, it is found that the ordinary oscillations of the magnetic needle are more extensive than usual. Now when these peculiarities had been noticed for many years, it was found that there was an alternate and systematic72 increase and diminution in the intensity of magnetic action, and that the period of the variation was about eleven years. But at the same time, a diligent73 observer29 had been recording74 the appearance of the sun’s face from day to day and from year to year. He had found that the solar spots are in some years more freely displayed than in others. And he had determined the period in which the spots are successively presented with maximum frequency to be about eleven years. On a comparison of the two sets of observations, it was found (and has now been placed beyond a doubt by many years of continued observation) that magnetic perturbations are most energetic when the sun is most spotted75, and vice76 versa.
For so remarkable a phenomenon as this, none but a cosmical cause can suffice. We can neither say that the spots cause the magnetic storms nor that the magnetic storms cause the spots. We must seek for a cause producing at once both sets of phenomena77. There is as yet no certainty in this matter, but it seems as if philosophers would soon be able to trace in the disturbing action of the planets upon the solar atmosphere the cause as well of the marked period of eleven years as of other less distinctly marked periods which a diligent observation of solar phenomena is beginning to educe36.
(From the Cornhill Magazine, June 1868.)
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implicit
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| a.暗示的，含蓄的，不明晰的，绝对的 | |
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thither
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| adv.向那里；adj.在那边的，对岸的 | |
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artistic
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| adj.艺术(家)的，美术(家)的；善于艺术创作的 | |
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poising
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vibrations
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| n.摆动( vibration的名词复数 )；震动；感受；（偏离平衡位置的）一次性往复振动 | |
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unwilling
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proceeding
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precedent
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thoroughly
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confiscated
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pointed
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worthy
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peculiarity
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peculiarities
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briefly
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applied
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inquiries
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situated
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veins
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| n.纹理；矿脉( vein的名词复数 )；静脉；叶脉；纹理 | |
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coercing
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obedience
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fixed
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diminution
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distinguished
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astronomer
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physicist
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definitively
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amplitude
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sufficiently
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pertinently
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molecules
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varied
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complexity
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domains
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poised
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educe
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attains
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| （通常经过努力）实现( attain的第三人称单数 )； 达到； 获得； 达到（某年龄、水平、状况） | |
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nominally
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luminary
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divergence
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inclination
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| n.倾斜；点头；弯腰；斜坡；倾度；倾向；爱好 | |
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vertical
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celebrated
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attaining
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| （通常经过努力）实现( attain的现在分词 )； 达到； 获得； 达到（某年龄、水平、状况） | |
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apparently
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nay
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axis
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intensity
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precisely
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determined
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| adj.坚定的；有决心的 | |
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longitude
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seasonal
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epoch
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astronomical
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| adj.天文学的，（数字）极大的 | |
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speculative
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| adj.思索性的，暝想性的，推理的 | |
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delicacy
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| n.精致，细微，微妙，精良；美味，佳肴 | |
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inversely
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| adj.相反的 | |
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remarkable
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| adj.显著的，异常的，非凡的，值得注意的 | |
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proximity
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| n.接近，邻近 | |
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disturbances
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| n.骚乱( disturbance的名词复数 )；打扰；困扰；障碍 | |
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disturbance
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| n.动乱，骚动；打扰，干扰；(身心)失调 | |
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microscopic
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| adj.微小的，细微的，极小的，显微的 | |
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noted
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| adj.著名的，知名的 | |
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aurora
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| n.极光 | |
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latitudes
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| 纬度 | |
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converse
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| vi.谈话，谈天，闲聊；adv.相反的，相反 | |
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Oxford
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| n.牛津（英国城市） | |
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scrutiny
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| n.详细检查，仔细观察 | |
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machinery
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| n.(总称)机械，机器；机构 | |
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systematic
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| adj.有系统的，有计划的，有方法的 | |
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diligent
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| adj.勤勉的，勤奋的 | |
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recording
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| n.录音，记录 | |
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spotted
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| adj.有斑点的，斑纹的，弄污了的 | |
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vice
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| n.坏事；恶习；[pl.]台钳，老虎钳；adj.副的 | |
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phenomena
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| n.现象 | |
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