took up distinctly, with the same apparatus as before, 6 oz. 18 dwt. 3 gr.; since which time it has also several times been found to lift nearly the same quantity.

Mr. Knight further, at the same time, showed the president the following instances of his ability to invert or change the direction of the poles in natural loadstones.

Such a stone belonging to Mr. Francis Hauksbee, weighing about 5 oz. 14 dwts. of an irregular cylindrical form, with two of the sides somewhat flatted, on which armour had formerly been ap plied, had the direction of its polarity from one of these flatted sides to the other, notwithstanding the stone had a distinct grain running at right angles to that direction. It was tried, and observed that one of these flatted sides strongly attracted the north end, and repelled the south; and that the other attracted the south, and repelled the north end of the magnetic needle. The end of the stone, attracting the south end of the needle, was then marked, by the rubbing of a piece of silver upon it, as on a touchstone: after which, Mr. Knight carried the stone into his study; and reproducing it in about a minute, showed that the poles were then directly inverted; and that the same end, which before attracted the south end of the needle, now attracted the north, and repelled the south, and vice versâ.

After this, Mr. Knight, again taking the stone, brought it back in as short a time as before, with the direction of its polarity turn. ed at right angles to its former direction, and into the direction of the natural grain of the stone, the poles now lying in the flat ends of the cylinder; one of which, being the smoother end, attracted the south end of the needle, while the other, which was of a rougher texture, attracted the north end, and repelled the south end of the same when it was also observed, that the polarity appeared stronger in this case, than either of the former.

Lastly, Mr. Knight, in about the same time, inverted this last direction of the poles, keeping it still parallel to the axis of the cylinder, but causing the smooth end of the stone to attract the north end of the magnetic needle, and the rough end to attract the south, and repel the north end of the same needle.

After this report, Mr. Knight proceeded to show, at the meeting, some of the same artificial magnets there mentioned; and it was found, that the compound magnet, consisting of twelve steel bars, and which had, in the experiment made before the president,

Lifted 23 lb. 2oz. Troy weight, did here, under all the inconve. niencies and disadvantages of a crouded room, still lift a weight amounting to 21 lb. 11 oz.

It was also found, that the single armed block of steel, which had before lifted 14 lb. 2 oz. did here, under the same disadvan. tages as the former, lift 131b. 7 oz.

And lastly, Mr. Knight produced to the company the abovementioned natural loadstone belonging to Mr. Hauksbee, but with the direction of its polarity again altered from what it was, when it was last seen by the president.

P. S. Since the artificial magnets mentioned in the foregoing paper, Mr. Knight has caused some others to be made of a less size, but of a very great lifting power: and one of these, weighing, without its armour, just an ounce, and with the armour, cramps, and rings, 1 oz. 17 dwt. lifted 6 lb. 10 oz.

This magnet consisted of three plates of steel, each two inches long, 7-10ths of an inch in breadth, and not above 6.100ths of an inch in thickness: they were laid flat on each other, and screwed together by two small brass screws going through the three plates. After which, the little parallelopiped block so made up, was armed with iron at the two ends, cramped together with silver, and fitted with a double ring of the same metal, for the convenient holding of it.

[Phil. Trans 1744.

3. By Mr. Benjamin Wilson.

Containing an account of Dr. Knight's method of making artificial Loadstones.

The method was this: having provided himself with a large quan. tity of clean filings of iron, Dr. K. put them into a large tub that was more than one-third filled with clean water; he then, with great labour, worked the tub to and fro for many hours together, that the friction between the grains of iron by this treatment might break off such smaller parts as would remain suspended in the water for some time: the obtaining of which very small particles in sufficient quantity seemed to him to be one of the principal desiderata in the experiment. The water being by this treatment rendered very muddy, he poured it into a clean earthen vessel, leaving the filings behind; and when the water had stood long enough to

become clear, he poured it out carefully, without disturbing such of the iron sediment as still remained, which now appeared reduced almost to impalpable powder. This powder was afterwards removed into another vessel, in order to dry it; but as he had not obtained a proper quantity of it by this first step, he was obliged to repeat the process many times.

Having at last procured enough of this very fine powder, the next thing to be done was to make a paste of it, and that with some vehicle which would contain a considerable quantity of the phlogistic principle; for this purpose he had recourse to linseed oil in preference to all other fluids With these two ingredients only, he made a stiff paste, taking a particular care to knead it well before he moulded it into convenient shapes. Sometimes, while the paste continued in its soft state, he would put the impression of a seal on the several pieces; one of which is in the British Museum. This paste was then put upon wood, and sometimes on tiles, in order to bake or dry it before a moderate fire, at about a foot distance. The doctor found, that a moderate fire was most proper, because a greater degree of heat made the composition frequently crack in many places.

The time required for the baking or drying of this paste was generally five or six hours, before it attained a sufficient degree of hardness. When that was done, and the several baked pieces were become cold, he gave them their magnetic virtue in any direc tion he pleased, by placing them between the extreme ends of his large magazine of artificial magnets for a few seconds or more, as he saw occasion. By this method the virtue they acquired was such, that when any one of those pieces was held between two of his best ten guinea bars, with its poles purposely inverted, it im mediately of itself turned about to recover its natural direction, which the force of those very powerful bars was not sufficient to counteract.

[Phil. Trans. 1779.

393

CHAP. XLV.

AURORA BOREALIS AND AUSTRALIS.

SECTION I.

General History and Remarks.

THE reader who carefully attends to the different sections of the present chapter, will see the propriety of our having preceded it by a chapter explanatory of the general laws and phænomena of magnetism: since he will find sufficient reason, if we mistake not, for concurring in the general opinion of the day, that this splendid meteor is the result of a combination of the two powers of magnetism and electricity.

When the light or aurora appears chiefly in the north part of the heavens, it is AURORA BOREALIS, or NORTHERN LIGHTS; and when chiefly in the south part, AURORA AUSTRALIS, OF SOUTHERN LIGHTS. Where the corruscation is more than ordinarily bright and streaming, which, however, seldom occurs except in the north, it is denominated LUMEN BOREALE; and where these streams have assumed a decided curvature, like that of the rain-bow, they are distinguished by the name of LUMINOUS ARCHES.

The Aurora is chiefly visible in the winter season and in frosty weather. It is usually of a reddish colour, inclining to yellow, and sends out frequent corruscations of pale light which seem to rise from the horizon in a pyramidical, undulating form, shooting with great velocity up to the zenith. This meteor never appears near the equator, but of late years has frequently been seen to. ward the south pole, and when in that situation is, as above, called Aurora Australis, or southern lights; though this is to use the same term in two different senses.

It seems that the aurora borealis has appeared at some periods more frequently than at others. They were so rare in England, or else were so little regarded, that none are recorded in our annals

between a remarkable one observed on the 14th November 1754, and a very brilliant one on the 6th of March, 1716, and the two succeeding nights, but much the strongest on the first night, except that five small ones were noticed in the years 1707 and 1708. Hence it may be inferred, that either the air or earth, or perhaps both, are not at all times in such a state as tends to produce this phenomenon.

The extent of these appearances are also amazingly great; that in March, 1716, was visible from the west of Ireland to the confines of Russia, and the east of Poland, extending over, at least, thirty degrees of longitude, and from about the fiftieth degree of latitude over almost all the northern part of Europe; and in all places, at the same time, it exhibited the like wonderful features. Father Boscovich determined the height of an aurora borealis observed by the Marquis of Polini, 16th December, 1737, and found it 825 miles high; and M. Bergman, from a mean of thirty computations, makes the average height to be 70 Swedish, or 469 English miles. Euler supposes the aurora to be more than double that height; but in this opinion he stands alone; for M. Mairan, in a treatise which he wrote expressly upon this phenomenon, entitled, "Traité Physique et Historique de l'Aurore Boreale," fixes the height, on an average, at 175 leagues from the earth, which is equal to 464 English miles.

Many conjectural opinions have been formed concerning the cause of this phenomenon; Dr. Halley imagined that the watery vapours, or effluvia, exceedingly rarefied by subterraneous fire, and singed with sulphurous streams, which many naturalists have supposed to be the cause of earthquakes, might also be the cause of this appearance; or that it is produced by a kind of subtile matter freely pervading the pores of the earth, and which entering into it near the southern pole, passes out again, with some force, into the ether, at the same distance from the northern. The subtile matter, by becoming more dense, or having its velocity in. creased, may perhaps be capable of producing a small degree of light, after the manner of effluvia from electric bodies, which by a strong and quick friction emits light in the dark, to which sort of light the aurora seems to bear a great affinity. Phil. Trans. No. 347.

M. de Mairan, in the treatise above quoted, supposed its cause