the influence of air forgotten, which is, as we have seen, so powerful in most phænomena. Lavosier, on the other hand, has proved, as you have already seen, that a portion of air combines with bodies that are either calcined or burned; and hence another class of chymists hath arisen, who have began to doubt of the presence of phlogiston, and who attribute to fixity, or disengagement of air, all the phænomena, which Stahl held to be owing to the combination, or separation, of phlogiston. "Upon this subject," says Fourcroy, " it must be confessed, Lavoisier's doctrine seems to have the advantage of Stahl's." But, in opposition again, or more properly speaking, in assimilation with all, M. Macquer thinks, that all the phænomena of chymistry cannot be accounted for by the principle of gaz, without at the same time admitting that of phlogiston; and therefore, he has substituted in the room of phlogiston, the matter of light. And yet, it has not been decisively demonstrated, that light is present in any of the bodies, which Stahl considered as phlogistic. Thus in the age of experiments, and when the keenest researches are, in glorious emulation, proceeding in the different parts of the European world, we see that certainty is by no means as yet established, and that much trial

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trial, and much perseverance, may be still wanting in the various branches of the sciences. One object alone should unceasingly actuate inquiry, and that is, utility to mankind.

LET

LETTER XXXVI.

WE now come to the consideration of a subject, which has perplexed the most powerful understandings, and which, though at last reduced to system, is yet perhaps defective, and not perfectly understood. The magnet, or loadstone, is a rich iron ore, found in large detached masses, of a dusky iron grey, often tinged brown or red, and when broken, appearing something like the common emery, but less sparkling. It is heavy, and very hard; and is generally found in places where there are iron mines. This stone was well known to the ancients. It is said to have derived its name from Magnes, a young man, who found himself supernaturally stopped by the iron nails of his shoes, as he walked over a mine. Scarcely any other property of it, however, was known, than that of its attracting iron, as we may see in Pliny. It appears, indeed, that they knew something of its communicating virtue, not only as it has been mentioned by Pythagoras and Homer, but particularly by Plato, in his description of that celebrated chain of iron direction nearly north and south. 3. The end of a needle, touched by the south pole of the magnet, will turn towards the north, and vice versa. 4. Needles touched by the magnet, will dip below the horizon, or be directed on the touched part, to a point within the earth's surface. 5. This virtue may be also communicated to iron, by strong attrition all one way. 6. Iron rods, or bars, acquire a magnetic virtue, by standing long in one position. 7. Fire, by making the loadstone, or iron, red hot, totally destroys this virtue. 8. This power is exerted sensibly to the distance of several feet. 9. It is sensibly continued through the substances of several contiguous bodies, or pieces of iron. 10. It pervades the pores of the hardest body. 11. It equally attracts the iron in vacuo as in open air. 12. The poles of a magnet may be changed, by applying them to the more vigorous parts of another. 13. The communication of the magnetic virtue, does not sensibly impair that of the loadstone, though it has been observed, that some magnets, from the impurity of their masses, have communicated a greater power than they had themselves. As to the activity of the attraction, and repulsion of magnets, some extend their powers fourteen feet, and others not more than nine inches. The sphere

sphere of their activity is also greater on some days than on others. The attraction of iron by the magnet, is greater than that of a magnet by a magnet.

From these, the sensible properties and phænomena of the loadstone, let us try if we can ascertain how they are produced, and whence we may deduce their origin. A great variety of hypotheses were formerly framed upon this subject. But that most generally admitted, is the hypothesis of the very learned and sagacious Dr. Halley. That able man supposed the globe of the earth to be one great magnet, having four magnetical poles or points of attraction; near each pole of the equator, two; and that in those parts of the world which lie adjacent to any one of those magnetic poles, the needle is chiefly governed by those poles, the nearest pole being always predominant over the more remote one. Of the north poles, that which is nearest to us, he supposes to be in the meridian of the land'send of England, which governs the variations in European Tartary and the North Sea; the other he places in a meridian passing through California, about fifteen degrees from the north pole of the world, which governs the needle in North America, and the oceans on either side. In like

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