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however, there is more ingenuity in such conjectures than solid ar. gument or historic fact : and in addition to the observa!ions ad. vanced on the other side of the question by Sir William Temple, Dr.Wotton, and Mr. Clarke, I cannot avoid remarking, that had tbis instrument been known in the time of Lucretius, he would not have failed to have adverted to it on the present occasion. But it is neither mentioned by Lucretius inor by Suidas."
On the Cause of the Change in the Variation of the Magnetic
Needle; with an Hypothesis of the structure of the internal parts of the Earth.
By Mr, Edmund Halley.
Having published, in the Transactions, No. 148, a theory of the variation of the magnetic needle, in which, by comparing many observations, I came at length to this general conclusion, viz. That the globe of the earth might be supposed to be one great magnet, having four magnetical poles or points of attraction, two of them near each pole of the equator: and that in those parts of the world, which lie near any of those magnetical poles, the needle is chiefly governed thereby, the nearest pole being always predominant over the more remote. And I there endeavoured to state and limit the present position of those poles on the surface of our globe. Yet I found two difficulties not easy to surmount: the one was, that no magnet, I had ever seen or heard of, had more than two opposite poles; whereas the earth had visibly four, and perhaps more. And secondly, it was plain that these poles were not, at least all of them, fixed in the earth, but shifted from place to place, as appeared by the great changes in the needle's direction within this last cen. tury of years, not only at London, where this great discovery was first made, but almost all over the globe of the earth; whereas it is not known, or observed, that the poles of a loadstone ever shifted their place in the stone, nor, considering the compact hardness of that substance, can it easily be supposed.
These difficulties made me quite despair of ever being able to ac. count for this phænomenon, when in an accidental conversation I stumbled on the following hypothesis. It is sufficiently known and allowed, that the needle's variation changes ; and that this change is gradual and universal, will appear by the following examples. At London, in the year 1580, the variation was observed by Mr. Bur. rows to be 11° 15' to the east; in 1622, the same was found by Mr. Gunter to be only 6o O to the east; in 1634, Mr. Gellibrand found it at 4° 5' to the east; in 1657, Mr. Bond observed that there was no variation at London ; anno 1762, I observed it 2° 3U to the west ; and this present year 1692, I again found it 6° to the west. So that, in 112 years, the direction of the needle has changed no less than seventeen degrees.
At Paris, Orontius Finæus, about the year 1550, reckoned it about 8° or go east variation; in 1640, it was found 3° to the east; in 1666 there was no variation there; and in 1681, I found it to be 2° 30' to the west.
At Cape d'Agulhas, the most southerly promontory of Africa, about the year 1600, the needle pointed due north and south with. out variation, whence the Portuguese gave it that name; in 1622 there was 2° west variation ; in 1675 it was go to the west; and this year 1691, it was accurately observed to be not less than 11° degrees to the west.
At St. Helena, about the year 1600, the needle declined so to the east; in 1623, it was but 6° to the east; in 1677, when I was there, I observed it accurately on shore to be 40 east; and now this year it was about 1° to the westward of the north.
At Cape Comorin, in India, in the year 1620, there were 14° 20' west variation; in 1680, there was 8° 48'; but in 1668, it was no more than 7° 30'; so that here the needle has returned to the east about 7° in seventy years.
In all the other examples the needle has gradually moved towards the west, and the places are too far asunder to be influenced by the removal of any magnetical matter, which may by accident be transplaced within the bowels or on the surface of the earth. From these, and many other observations, it is evident that the direction of the needle is in no place fixed and constant, though in some it changes faster than in others. And where for a long time it has continued as it were unaltered, it is there to be understood that the needle has its greatest deflection, and is become stationary, in or.
der to return, like the sun in the tropics. This at present, viz. 1692, is in the Indian sea, about the island Maurtius, where is the highest west variation, and in a tract tending from thence to the N.N.W. towards the Red Sea and Egypt. And in all places to the westward of this tract, all over Africa and the seas adjoiding, the west variation will be found to have increased ; and to the eastwards thereof, as in the example of Cape Comorin, to have decreased, viz. all over the East Indies and the islands near it.
In like manner, in that space of east variation, which, beginning near St. Helena, is found all over South America, and which at present is highest about the mouth of Rio de la Plata, it has been observed, that in the eastern parts of it, the variation of the needle gradually decreases; but whether, on the contrary, it increases in those places which lie more westerly than that tract wherein the highest east variation is found ; or how it may be in the vast Pa. cific Sea, we have not experience enough to ascertain ; only we may by analogy infer, that both the east and west variations gra. dually increase and decrease after the same rule.
These phænomena, being well understood and duly considered, sufficiently evince, that the whole magnetical system has one, or perhaps more motions; that th moving force is very great, as ex. tending its effects from pole to pole; and that its motion is not per saltum, but a gradual and regular motion.
Now considering the structure of our terraqueous globe, it can. not be well supposed that a very great part of it can move within it, without notably changing its centre of gravity, and the equili. brium of its parts, which would prociuce very wonderful effects in changing the axis of diurnal rotation, and ocasion strange altera. tion in the surface of the sea, by inundations and recessions, such as history never yet mentioned. Besides the solid parts of the earth are not to be supposed permeable by any other than fluid substances, of which we know none that are any ways magnetical, So that the only way to render this motion intelligible and possible, is, to suppose it to turn about the centre oi the globe, having its centre of gravity fixed aud immoveable in the same common centre of the earth: and there is yet required, that this moving internal substance be loose, and detached from the external parts of the earth, whereon we live; for otherwise, were it affixed to the earth, the whole must necessarily move together,
So then the external parts of the globe may well be considered as the shell, and the internal as a nucleus, or inner globe, included within ours, with a fluid medium between. Which having the same common centre and axis of diurnal rotation, may turn about with our earth each twenty-four hours; only this outer sphere having its turbinating motion some small matter either swifter or slower than the internal ball. And a very minute difference in length of time, by many repetitions becoming sensible, the internal parts will by degrees recede from the external, and not keeping pace with each other, will appear gradually to move either to the east or west by the difference of their motions,
Now supposing such an internal sphere, having such a motion, we day solve the two great difficulties in my former hypothesis. For if this exterior shell of earth be a magnet, having its poles at a distance from the poles of diurnal motion ; and if the internal nucleus be likewise a magnet, having its poles in two other places distant also from the axis ; and these latter, by a gradual and slow motion, change their place in respect of the external, we may then give a reasonable account of the four magnetical poles, as also of the changes of the needle's variations.
The period of this motion being wonderfully great, and there being hardly a century since these variations have been duly observed, it will be very hard to bring this hypothesis to a calculus, especially since, though the variations increase and decrease regularly in the same place, yet in different places, at no great distance, there are found such casual changes of it, as can nowise be ac. counted for by a regular hypothesis ; as depending on the unequal and irregular distribution of the magnetical matter within the substance of the external shell or coat of the earth, which deflects the needle from the position it would acquire from the effect of the general magnetism of the whole. Of this, the variations at London and Paris afford a notable instance ; for the needle has been con. stantly about 10 more easterly at Paris than at London ; though it be certain that, according to the general effect, the difference onght to be the contrary way. Notwithstanding which, the va. riation in both places changes alike.
Hence, and from some other of the like nature, I conclude, that the two poles of the external globe are fixed in the earth, and that if the needle were wholly governed by them, the variations would
be always the same, with some little irregularities on the account just now mentioned: but the internal sphere, having such a gradual translation of its poles, influences the needle, and directs it variously, according to the result of the attractive or directive power of each pole ; and consequently there must be a period of the revolution of this internal ball, after which the variations will return again as before. But if it shall in future ages be observed otherwise, we must then conclude, that there are more of these in. ternal spheres, and more magnetical poles than four, which at pre. sent we have not a sufficient number of observations to determine, and particularly in that vast Mar del Zur, which occupies so great a part of the whole surface of the earth.
If then two of the poles be fixed, and two moveable, it remains to ascertain, which they are that keep their place: and though I could wish we had the experience of another century of years to found our conclusions upon, yet I think we may safely determine, that our European north pole, supposed to be near the meridian of the Land's End, and about 7 from it, is that which is moveable of the two northern poles, and which has chiefly influenced the va. riations in these parts of the world : for in Hudson's Bay, which is under the directiou of the American pole, the change is not ob. served to be near so fast as in these parts of Europe, though that poie be much farther removed from the axis.
As to the south poles, from the like observation of the slow decrease of the variation on the coast of Java, and near the meridian of the Asian pole, I take the Asiatic pole, which I place about the meridian of the island of Celebes, to be the fixed one, and consequently the American pole to be moveable. If this be allowed, it is plain that the fixed poles are the poles of this external shell or cortex of the earth, and the other two the poles of a magnetical nucleus, included and moveable within the other. It likewise fol. lows, that this motion is westwards, and by consequence that the aforesaid nucleus has not precisely attained the same degree of ve. locity with the exterior parts in their diurnal revolution : but so very nearly equals it, that in 365 revolves the difference is scarcely sensible. This I conceive to arise from the impulse by which this diurnal motion was impressed on the earth, being given to the ex. ternal parts, and from thence in time communicated to the internal; but not so as perfectly to equal the velocity of the first motion