perses the beams in some degree, which collect again after the electric circulation ceases.

Many of my readers, I make no doubt, will be surprised to find, after having formed a conception that the relation betwixt the aurora and magnetism was to be explained and demonstrated, chiefly if not solely, from the observations on the disturbance of the needle during the aurora, that no mention or use whatever is made of those observations, in the preceding sections. In fact, the relation above mentioned is demonstrable without any reference to them; notwithstanding which, they not only corroborate the proof of it, but almost establish the truth of the hypothesis we are here advancing.

The variations of the needle during the aurora, as may be seen in the observations, are so exceedingly irregular, that after considering them awhile, one would conclude this is the only fact ascertained by these observations. However, I think we may deduce the following:

1. When the aurora appears to rise only about 5, 10, or 15° above the horizon, the disturbance of the needle is very little, and often insensible.

2. When it rises up to the zenith, and passes it, there never fails to be a considerable disturbance.

3. This disturbance consists in an irregular oscillation of the horizontal needle, sometimes to the eastward, and then to the westward of the mean daily position, in such sort that the greatest excursions on each side are nearly equal, and amount to about half a degree each, in this place.

4. When the aurora ceases, or soon after, the needle returns to its former station.

Now, from these facts alone. independent of what is contained in the preceding sections. I think we cannot avoid inferring, that there is something magnetic constantly in the higher regions of the atmosphere, that has a share at least in guiding the needle; and that the fluctuations of the needle during the aurora are occasioned by some mutations that then take place in this magnetic matter in the incumbent atmosphere; for, it is certainly improbable, if not absurd, to suppose that the aurora produces this magnetic matter, at its commencement, and destroys it at its termination. Moreover, abstracting from a chemical solution of the metal, nothing is known to affect the magnetism of steel, but heat and electricity; heat weakens or destroys it; electricity does more, it sometimes changes the pole of one denomination to that of another, or inverts the magnetism. Hence, we are obliged to have recourse to one of these two agents, in accounting for the mutations above mentioned. As for heat, we should find it difficult, I believe, to assign a reason for such sudden and irregular productions of it in the higher regions of the atmosphere, without introducing electricity as an agent in those productions; but rather than make such a supposition, it would be more philosophical to suppose electricity to produce the effect on the magnetic matter immediately. Hence then were we obliged to form an hypothesis of the aurora

borealis, without any other facts relative to it than the four above mentioned, we ought to suppose it a phenomenon produced in some manner by the united agency of magnetism and electricity.

It appears then, that the disturbance of the needle during an aurora equally countenances the conclusions drawn in the last section, and the hypothesis adopted in this; and it may be accounted for on the hypothesis, as follows.

The beams of the aurora, being magnetic, will have their magnetism weakened, destroyed, or inverted, pro tempore, by the several electric shocks they receive during an aurora; or perhaps the temporary dispersion and diffusion of the magnetic matter thereby, may considerably alter its influence; when, therefore, the alterations on each side of the magnetic meridian do not balance each other, the consequence will be a disturbance of the needle.*

In fine, the conclusions in the last section, and the hypothesis in this, afford a very plausible reason for the appearance of the aurora being so much

* I conceive that a beam may have its magnetism inverted, and exist so for a time, because the repulsion, acting longitudinally upon it, will only impel it in that direction, and not turn it round; just as the north pole of a magnet may be applied to the north pole of a magnetic needle, without turning it round, by keeping the magnet exactly in the same line with the needle, and thus making the needle act upon the And I further conceive, that when the beam is restored to its natural position of the north pole downward, it is effected, not by inverting the beam, wholly as a beam, (for this is never observed in an aurora,) but by inverting the constituent particles, which may easily be admitted of a fluid,

centre.

more frequent now than formerly in these parts; if the earth's magnetic poles be like the centres of the aurora, as the phenomena indicate, it is plain the aurora must move along with them, and appear or disappear at places, according as the magnetic poles approach or recede from them; and hence it may be presumed that the earth's magnetic pole in the northern hemisphere is nearer the west of Europe in this century than it was in the last or preceding. The observations upon the dip of the needle, however, if they have been accurately made, seem to indicate the approach of the magnetic pole to have been very little; the dip at London, according to Mr. CAVALLO, was 71° 50′ in 1576, and 72° 3' in 1775; but there is reason to suspect the accuracy of the instruments at so early a period as 1576; besides, we do not know in what proportion the dip of the needle keeps pace with the approach of the pole.

It may perhaps be necessary here, before the subject is dismissed, to caution my readers not to form an idea that the elastic fluid of magnetic matter, which I have all along conceived to exist in the higher regions of the atmosphere, is the same thing as the magnetic fluid or effluvia of most writers on the subject of magnetism. This last they consider as the efficient cause of all the magnetic phenomena; but it is a mere hypothesis, and the existence of the effluvia has never been proved. My fluid of magnetic matter is, like magnetic steel, a substance possessed of the properties of magnetism, or, if

these writers please, a substance capable of being acted upon by the magnetic effluvia, and not the magnetic effluvia themselves.

Whether any of the various kinds of air, or elastic vapour, we are acquainted with, is magnetic, I know not, but hope philosophers will avail themselves of these hints to make a trial of them.

SECTION FIFTH.

An investigation of the supposed effect of the Moon in producing the Aurora Borealis.*

SOME time after the author began his observations on the aurora borealis, it occurred to him that the phenomenon had more frequently happened about the change of the moon than at any other time; this produced the suspicion that the aerial tides occasioned by the moon might have some influence upon it. Granting this to be the case, it was obvious, the full moon must have an equal share with the new, though the phenomenon may often be

* An essay on this subject was first published by the author in the beginning of 1789, in Mr. DAVISON's Mathematical and Philosophical Repository.