In like manner Dr. Young has observed, that the polarity of iron itself is destroyed by a mixture of antimony with this last metal. The Decade Philosophique, No. 21, contains an account of various experiments made before the French Imperial Institute, which seem to shew that all bodies are subject to the magnetic in. fluence, even in a degree which is capable of being measured. These experiments were made by Mr. Coulomb, and repeated by him before the Institute. He employed all the substances that he examined in the form of a cylinder, or a small bar; he suspended them by a thread of silk in its natural state, as spun by the worm, and placed them between the opposite poles of two magnets of steel. Such a thread can scarcely support more than two or three drachms without breaking; it was therefore necessary to reduce these needles to very small dimensious. Mr. Coulomb made them about a third of an inch in length, and about a thirtieth of an inch in thickness; and those of metal only one third as thick. In making the experirents, he placed the magnets in the same right line. Their opposite poles were separated about a quarter of an inch more than the length of the needle which was to oscil. late between them. The result was, that of whatever substance the needles were formed, they always ranged themselves accurately in the direction of the magnets; and if they were deflected from this direction, they returned to it with oscillations, which were often as frequent as thirty or more in a minute. Hence, the weight and figure of the needles being given, it was easy to deter. mine the force that produced these oscillations. The experiments were made in succession with small plates of gold, silver, copper, lead, and tin; with little cylinders of glass, with a bit of chalk, a fragment of bone, and different kinds of wood. In the course of his lecture on magnetism on the 30th of April, Dr. Young repeated some of these experiments with wires of differ. ent kinds one of them was of tin, and suspended within a cylin. drical glass jar by a single silk worm's thread: its oscillations were so slow as to occupy several minutes, and it was scarcely affected by turning the cross bar to which the thread was attached; so that the suspension must have been sufficiently delicate: under these circumstances the opposite poles of two strong magnets were ap. plied close to the jar, and at the distance of about twice the length of the suspended wire but the effect was absolutely impercep tible in the morning indeed, there had been an appearance of oscillations occupying about a minute, and tending to the direction of the magnets, perhaps derived from some superficial particles of iron which had lost their magnetic property by oxidation in the course of the day. There must at any rate be a doubt whether the presence of a quantity of iron, too small to be ascertained by chemical tests, might not have been the cause of the effects described by Mr Coulomb, although they indicate a force something greater, upon a rough calculation, than one 2000th of the weight of the substance. By farther experiments of Mr. Coulomb, however, related in No. 3, tom. iii. of the Bulletin de la Societé Philomathique, and which appear to have been made with great caution, it seems ob. vious that the greater part, if not the whole, of the magnetic influence observed in the preceding cases, was owing to the presence of iron. For it appears that, according to the method employed in the purification of the metals examined, their apparent magnetic power was very materially different. Mr. Coulomb observes that, upon this foundation, we may make the action of the magnet, upon a needle thus suspended, a very useful instrument in chemical exa. minations; for he finds that the attractive force is directly as the quantity of iron in any mixture; and, according to its magnitude, we may estimate that quantity, when it is so small as wholly to elude all chemical tests. Many of the properties of the magnet were known at a very early period to the Greek philosophers, and especially to Plato and Epicurus; the latter of whom endeavoured to account for them by an ingenious hypothesis, which is fully detailed by Lucretius. We have not space to enter into this hypothesis; but the poet's minute description of the well-known powers of what we now call the oad-stone or artificial magnet, as given at so early a period, cannot fail of being acceptable to our readers. It occurs in his very extraordinary poem De Rerum Natura, lib. vi. v. 906: Quod super est, agere incipiam quo fœdere fiat Hunc homines lapidem mirantur, quippe catenam Unus ubi ex uno dependet, subter adhærens ; And next explain we by what curious law Vast is the wonder, mid th' admiring crowd, The lowlier cleaving to the sphere above, And this to that, proclaiming, as it hangs, Its deep-felt conscience of the magnet's power. GOOD. Upon this subject also, we must take leave so subjoin the learned Translator's note on the passage: "There is nothing in nature too recondite for the daring penetration 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: Miracula nondum Omnia magnetis perspeximus; at mihi certum est Ferratus trahere, ac secum vincire catenas. ANTI LUCR. v. 1156. For not yet clearly are the wonders trac'd Of mutual love t' attract and clasp the steel. "The ferruginous ore, here spoken of, for it is nothing else than ferruginous ore, with a saturation of magnetic aura, was denominated, 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: : -I' Erculla pietra Con incognita forza il ferro tragga. In "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. 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 immemorially 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 Druids, 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, ascrib. ing 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 mariner'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 argument or historic fact: and in addition to the observations advanced 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." [EDITOR. SECTION II. 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 needie'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 |