Imágenes de páginas

manner he accounts for the variations in the southern hemisphere. This system was ingenious, especially in its developements. *

But, it has long been dropped as too visionary.

Froin imaginary speculations, therefore, it now comes generally to be believed, that the magnetism of the earth arises from the magnetism of all the magnetic substances contained in the earth, and blended with other bodies. The

mag. netic poles of the earth may be considered as the centers of the polarities of all the particular aggregates of the magnetic substances; and these poles must change place, relatively to the surface of the earth, according as the particular ag. gregates of magnetic substances within the earth, are in some manner or other altered, so as to have their power diminished, increased, approached, or removed from the principal poles. In regard to the variation of the needle, in England we may suppose it one degree in every seven years; and more or less in other parts, ac"cording to circumstances. But, with respect to the dipping, that is not so easily accounted for. Captain Cooke, in the southern hemisphere, had the dipping needle almost perpendicular ; and consequently, we may venture to suppose


* Philosophical Transactions, No. 143.

[ocr errors]

that he was at, or at least was very near, the
southern magnetic pole. For, in a familiar in-
stance, take a bar properly imbued with the
magnetic virtue, and move a -small needle in a

the surface of it ; the needle, as it approaches the extremities, or poles, will assume a perpendicular direction; but, when in the center or equator, its position will become horizontal

Beccaria, indeed, with that sublimity of genius for which he has been so justly celebrated, observing that lightning gave polarity to the magnetic needle, and to all bodies that have any thing of iron in them; and further observing, that from the positions in which they lie, it was to be ascertained with the utmost certainty, in what direction the stroke had passed, boldly conjectured, that a regular and constant circulation of the whole mass of the fluid, from north to south, may be the original cause of magnetism in general. That this etherial current being invisible to us, he conceives to be no proof of its non-existence, since we ourselves are involved in it. This current, he would not suppose to arise irom one source, but from several, in the northern hemnisphere of the earth. The aberation of the common center of all these currents from the north point, may be the cause of the variation

of the needle ; the period of the declination of the center of the currents, may be the period of the variation; and the obliquity with which the currents strike into the earth, may be the cause of the dipping of the needle; and also why bars of iron more easily receive the magnetic virtue in one particulardirection, than in another.

That magnetism is in certain cases produced by electricity, is very rue. Doctor Franklin proved particularly, by electricity, the effects of the reversal of the poles of magnets, as caused by lightning. Yet, notwithstanding this, and that there is, it must be confessed, a surprizing analogy between electricity and magnetism, they virtually seem to be two very different powers in nature. in nature. The

power of electricity is of two sorts, the positive and the negative; and it is an invariable law, that bodies possessed of the same sort of electricity repel each other, whereas those which are possessed of different electricities, attract each other. Thus in magnets, there is a north and a south pole ; those parts of magnetic bodies which are possessed of the same polarity, repel each other; but those which are possessed of different polarities, attract each other. One sort of electricity cannot be produced by itself, but is always ac


companied by the other. In the same manner, the magnetic poles are always together; nor was there ever a ferruginous substance produced which had one polarity and not the other. The electric virtue can be retained and confined by certain bodies, as glass, amber, resin, and others called electrics; but it easily pervades other substances, called conductors, or non-electrics. The magnetic virtue is retained by ferruginous substances, especially those of a hard nature, like steel; but it pervades easily, and without the least perceivable impediment, all other sorts of substances.

But, on the other hand, the magnetic power differs from the electric; first, in its "not affecting our senses with any light, smell, taste, or noise ; secondly, magnetism attracts only iron, (nickel excepted), or those bodies which contain some of that inetal, whereas the electric power attracts bodies of various denominations; thirdly, the electric virtue is supposed to reside on the surface of electrified bodies, whereas the magnetic is quite internal; lastly, a magnét loses nothing of its power by communicating its strength to other substances; but an electrified body loses part of its electricity by electrifying other subsiances. These differences, therefore, shew that


F 3

these two powers, if not entirely distinct, are yet in

very essential properties dissimilar.

The power of the loadstone on the face of the globe, is affected by a variety of circumstances. Thus, it is increased by cooling, by a regeneration of iron, or phlogistication of its calx, and by the action of acids


iron It is diminished by heating, and by the decomposition of iron. Hence, these, together with volcanos, earthquakes, &c. alter the direction, or polarity of the needle. “On this account,” says De Saussure, “ I tried if the loadstone had not a different direction on the tops of mountains from that on plains; and if its attractive force did not diminish, like that of gravity, on receding from the earth. The result was, the needle continued

precisely in the same situation, and I consequently thence concluded, that the magnet preserves the same direction on the summit as in the valley. When it is otherwise, it must be supposed to arise from iron in some parts of the mountains ; or from the greater, or less dephlogistication of the ores, or the stones in which it is contained.”

Many philosophers have supposed the existence of a magnetic nucleus; of the earth's contain

« AnteriorContinuar »