be attended to; for, though many may be found afterwards to be trivial, and of little or no moment in leading us towards the discovery, yet some one or other of them generally happens to be of importance. It will be seen hereafter, that the exact bearing and extent of the large, still, horizontal arch of the aurora, and the point in the heavens to which the coruscations tend, are amongst the circumstances of much importance in the investigation of its These circumstances, it must be confessed, were not accurately noticed, either at Kendal or Keswick, previous to the middle of October, 1792.

cause.

As for myself, the only minute I usually made upon the still aurora was, that it was situate in the NW. by which I meant that its centre was between the N. and the W. without once attempting to ascertain the exact bearing of the centre; and the corona, when there was one, is often mentioned in my notes, as being south of the zenith, but the number of degrees was not ascertained.

Mr. CROSTHWAITE, however, has been rather more particular, at times, with respect to the bearings, extent, &c. The centre of that on January 10th, 1788, he observes bore NNW.; that of the 28th of April, NW. b N.; the centres of all the rest are said to have been between the North and West, or else North; not one was observed to have its centre to the East of the meridian.

N. B. The additional observations on the Aurora, beginning with that on the 13th of October, 1792, will be given after the next Section.

60

SECTION ELEVENTH.

On Magnetism, and the variation of the Needle.

In order to understand the additional observations, and the subsequent Essay on the aurora borealis, a competent knowledge of magnetism is requisite ; and as the principal facts relating to that subject are few and simple, we have thought it would not be amiss to state them here, for the sake of such as may not be previously acquainted therewith.

The Loadstone, or natural Magnet, is a mineral production, found in the bowels of the earth, amongst rich iron ores, of which it is one itself; its distinguished property is that of attracting iron and steel. This property, which is called magnetism, is communicable to steel only, so as to be permanent; and to iron when within the influence of a magnet, but as soon as the magnet is withdrawn, the magnetism of iron ceases.

Every magnet has two opposite points or extremities, called its poles; the one is denominated its north pole, and the other its south pole; and the attraction of the magnet is strongest at its poles.

If an oblong bar of tempered steel (it will answer well if five inches long, half an inch broad, and a quarter of an inch thick) be rubbed over from one end to the other, always the same way, by either pole of the magnet, it will be converted into a

magnet itself; and that end to which the pole was first applied, will be a pole of the new magnet, of the same name as the generating pole. By rubbing the new magnet the contrary way, with the same pole, its magnetism will be first destroyed, and then fresh magnetism will be communicated; but the poles of the new magnet will be of contrary names to what they were before.

Either pole of a magnet attracts iron, or steel not magnetic; but the pole of one magnet repels the pole of another magnet, of the same name, and attracts the pole of a contrary name; the repulsion in the former case seems to be equal to the attraction in the latter.

Magnetism is sometimes communicated, destroyed, or inverted, by lightning, or by an electric shock, &c.

If a magnetic bar, or needle, be suffered to move freely in an horizontal plane, it will only rest in one position, when the north pole points northward, and the south pole southward.-Hence the common needle and compass, which was invented about the beginning of the fourteenth century.

If a plane perpendicular to the horizon be conceived to be drawn through the horizontal needle, when at rest, it is called the plane of the magnetic meridian; and the angle made by this plane, with the plane of the true meridian, is called the variation of the needle....

If a magnetic needle be nicely poised on an axis passing through its centre of gravity, or middle,

and suffered to move freely both horizontally and perpendicularly, it will rest only in one position, namely, when in the plane of the magnetic meridian, and having its north pole pointing towards the ground; the angle of deflection from the horizontal plane, is called the dip of the needle, and the needle itself in this case a dipping-needle; its position is the proper and natural one of every magnet that is suffered to be guided solely by the magnetic influence. From this phenomenon, and others of the same nature, it is inferred, that the earth itself is a magnet; whether its magnetism results from the united influences of the natural magnets it contains, or whether its magnetism may be in its atmosphere, is not certain; and as poles of unlike denominations attract each other, the south pole of the earth's magnetism must be in the northern hemisphere, because it attracts the north pole of the needle.

The variation of the needle is very different at different places of the globe, and even at the same place at different times; in these parts it is at present westerly, and is increasing every year; the variation at London in 1580 was 11° 15' E. in 1657 it was 0° 0'; at present, 1793, it is about 224° W. and increases nearly 10' each year. From the result of several observations I find it to be 25° W. at this time, at Kendal.

The dip of the needle too is very different at different places, and probably at the same place at different times; but, for various reasons, the ob

servations on this head are neither so numerous nor so accurate as those of the variation. It seems at present to be about 72° at London, according to Mr. CAVALLO; and there is reason to suppose, it is not many degrees different in any part of England; for want of proper instruments I have not been able to ascertain it at this place.

Besides the annual change in the variation of the needle, there is a daily change, or variation of the variations. According to Mr. CANTON, who made a series of observations on the daily variation for a long time, the north pole of the needle moves gradually westward till 2 or 3 P. M. and then returns gradually to its former station; the mean daily variation in winter is about 7', and in summer about 13. He moreover observed, that the needle was disturbed when an Aurora borealis was in the atmosphere.

I have myself made a like series of observations for some months, and find them in general to agree with his; but as it is not necessary for my purpose to relate the result of them, any further than what is contained in the subsequent pages, I shall not detain the reader longer on the subject.