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Hunc homines lapidem mirantur, quippe catenam
And next explain we by what curious law
Vast is the wonder, mid th' admiring crowd,
Upon this subject also, we must take leave so subjoin the learned Translator's note on the passage :
6. There is nothing in nature too recondite for the daring penetra. tion of our poetic philosopher. The timid mineralogist of modern days cannot, without surprize, behold him thus undauntedly endea. vouring to develope a bond, into whose mysterious union he him. self feels totally unable to penetrate : and if, in pursuing his hardy footsteps, he perceive the bold speculator, at times, bewildered in a wrong path, he will seldom be able to point out to him a truer.
“ Hence, Polignac, to whose negative declaration, neither our poet, nor any modern philosopher, will, probably, object :
ANTI.LUCR. V. 1156.
For not yet clearly are the wonders trac'd
"The ferruginous ore, here spoken of, for it is nothing else than ferruginous ore, with a saturation of magnetic aura, was denomi. nated, as Lucretius observes, magnet, from its having been first noticed among the Magnetes, or inhabitants of Magnesia, a region of Lydia. It is also often entitled Herculeus lapis; either because it was first traced by Hercules, or detected in the vicinity of Heraclea ; or, lastly, from the prodigious strength of its attraction. Lucretius, indeed, employs this latter term on no occasion, but Marchetti has introduced it into his version, with a view of varying his phraseology:
l'Erculla pietra Con incognita forza il ferro tragga. “ Whether the ancients were acquainted with that most useful nautical instrument to which the properties of this stone have given birth, the mariner's compass, is in some degree doubtful. In modern Europe, we have no decisive knowledge of the existence of this instrument anterior to its use by Marco Polo, in 1260. Among the Chinese, however, it appears to have been employed immemo. rially: from which circumstance, many scholars of high reputation, and among the rest my learned and indefatigable friend the Rev. T. Maurice, conceives that other ancient nations were in an equal degree acquainted with its utility. They contend, that it was known to the Droids, and that the cardinal points of what they call the Druidic temples at Stonehenge and Abury, were determined by the use of such a compass. In like manner, ascribing its name of lapis Heraclius, or Herculeus, to Hercules, as its inventor, they conjecture it was known also to the Greeks; and that the golden cup which Apollo, or the Sun under that denomi. nation, gave to Hercules, was nothing more or less than the mariper's compass-box, by which, not in which, the latter sailed over the vast ocean ; they add also, that the golden fleece and the golden scyphus in the temple of Jupiter Ammon in Lybia, were nothing more than types of this curious instrument. I am afraid
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 this 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 nor 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 Dear 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 6° 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° 30 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 8o te 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 trans. placed 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 defilection, and is become stationary, in 0s.
der to return, like the sun in the tropics. This at present, viz. 1692, is in 'he 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 adjoining, 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 Dear 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 or 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 ma, netical system has one, or perhaps more motions; that th' moving force is very great, as extending its eflects 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 produce very wonderful effects in changing the axis of diurnal rotation, and oc asion strange altera. tion in the surface of the sea, by inundations and recessions, such as liistory 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 maynetical, 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 avd 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 allixed to the tarth, the whole must necessarily move together,