MAGNETISM, is supposed to have been first rendered useful about the end of the twelfth, cr at least very early in the thirteenth century, by John de Gioja, a handicraft of Naples, who noticed the pe culiar attraction of metals, iron in parti cular, towards certain masses of rude ore ; the touch of which communicated to other substances of a ferruginous na ture, especially iron or steel bars, the same property of attraction : these touch ed bars he observed to have a peculiar and similar tendency towards one parti cular point ; that when suspended in equilibrio, by means of threads around their centres, they invariably indicated the same point ; and that, when placed in a row, however adversely directed, they soon disposed themselves in perfectly parallel order. In this instance, he im proved upon the property long known to, but not comprehended or applied to use by the ancients, who considered the load stone simply as a rude species of iron ore, and curious only so far as it might serve to amuse. Gioja being possessed of a quick understanding, and of a strong mind, was not long in further ascertain ing the more sensible purposes to which the magnet might be appropriated. He accordingly fixed various magnets upon pivots, supporting their centres in such manner as allowed the bars to traverse freely. Finding that, however situated within the reach of observation and com parison, they all had the same tendency, he naturally concluded them to be go verned by some attraction, which might be ultimately ascertained and acted up on. He therefore removed into various parts of Italy, to satisfy himself whether or not the extraordinary impulse which agitated these bars that had been mag netised by friction, existed only in the vi cinity of Naples, or was general. The result of his researches appears to be, that the influence was general, but that the magnets were rendered extremely variable, and fluctuated much, when near large masses of iron. The experiments of Gioja gave birth to many others, and at length to a trial of the magnetic in fluence on the surface of the water. To establish this, a vessel was moored out at sea, in a direction corresponding with that of the magnet.; and a boat, having a magnet equipoised on a pivot at its cell tre, was sent out at night in the exact line indicated thereby ; which, being du ly followed, carried them close to the vessel that was at anchor. Thus the ac tive power of attraction appeared to be established on both elements, and in the course of time the magnet was fixed to a card, marked with thirty-two points, whereby the mariner's compass was pre sented to us. The points to which the magnet always turned itself, being gene rally in correspondence with the meri dian of the place where it acted, occa sioned the extremities of the bars to be called poles. Succeeding experiments
proved, that the magnetic bar never re tained an exactly horizontal position ; but that one of its poles invariably formed an angle with any perfect level, over which it was placed : this was not so very mea surable in a short bar, but in one of a yard in length was found to give several degrees of inclination. This, which is called " The Dip of the Needle," (or magnet) seems to indicate that the at tracting power is placed within the earth. What that attracting power is we cannot determine ; some consider it to be a fluid, while others conjecture it to be an im mense mass of load-stone situated some where about the north pole. The diffi culty is, however, considerably increased by the known fact of the needles of com passes not always pointing due north ; but in many places varying greatly from the meridional lines respectively ; and from each other at different times and places. The facility with which a meri dional line may be drawn by solar obser vation, and especially by taking an azi muth, fortunately enables navigators to establish the variation between the true northern direction, and that indicated by the magnet attached to the card of the compass. Nevertheless, we have great reason to believe, that, for want either of accurate knowledge of the prevalent va riations, or from inattention thereto, ma ny vessels, of which no tidings were ever heard, have been cast away ; it being ob vious that a false indication of the north ern point, in many places amounting to nearly the extent of twenty-five de grees, must produce so important an er ror in a vessel's course, as to subject her to destruction on those very shoals, rocks, &c. which the navigator unhappily thinks he steers wide of. To obviate such danger, as far as possible, all modern sea-charts have the variations of the com pass in their several parts duly noted down ; and in reckoning upon the course steered by compass, an allowance is usually made for the difference between the apparent course, by the compass, and the real course, as ascertained by celes tial observation. Under circumstances so completely contradictory, the principle of magnetism must remain unknown : we know not of any hypothesis which strikes conviction on our minds, or which seems to convey any adequate idea of the ori gin, or modes operandi, of this wondrous influence. All we can treat of is the ef fect ; also of the appearances which guide our practice, and of the manner in which the attractive power may be gene rated and increased.