imperceptible, and at other times invisible, may now and then, by the concourse of several causes very rarely coincident, and to us as yet unknown, be capable of producing a small degree of light; perhaps from the greater density of the matter, or the greater velocity of its motion: after the same manner as we see the effluvia of electric bodies, by a strong and quick friction, emit light in the dark to which sort of light this seems to have a great affinity.

This being allowed, I think we may readily assign a cause for se veral of the strange appearances we have been treating of, and for some of the most difficult to account for otherwise; as, why these lights are rarely seen any where else but in the north, and never, that we hear of, near the equator: as also why they are more frequently seen in Iceland and Greenland, than in Norway, though nearer the po'e of the world. For the magnetical poles, in this age, are to the westward of our meridian, and more so of that of Norway, and not far from Greenland; as appears by the variation of the needle this year, 1716, observed, full 12 degrees at London to the west.

The erect position of the luminous beams or striæ so often repeated that night, was occasioned by the rising of the vapour or lucid matter nearly perpendicular to the earth's surface. For any line erected perpendicularly on the surface of the globe, will appear erect to the horizon of an eye placed any where in the same spherical superficies; as Euclid demonstrates in a plane, that any line erected at right angles to it, will appear to be perpendicular to that plane from any point of it. That it should be so in the sphere is a very pretty proposition, not very obvious, but demon strated from Prop. 5, Lib. i, Theodosii Sphæric. For by it all lines erected on the surface pass through the centre, where meeting with those from the eye, they form the planes of vertical circles to it. And by the converse hereof, it is evident that this luminous matter arose nearly perpendicular to the earth's surface, because it appeared in this erect position. And whereas in this appearance (and perhaps in all others of the kind) those beams which arose near the east and west, were farthest from the perpendicular on both sides, inclining towards the south, while those in the north were directly upright: the cause of which may well be explained by the obliquity of the magnetical curves,

making still obtuser angles with the meridians of the terella, as they are farther from its poles.

Hence also it is manifest how that wonderful corona, that was seen to the southward of the vertex, in the beginning of the night, and so very remarkable for its tremulous and vibrating light, was produced; viz. by the concourse of many of those beams rising very high out of the circumjacent regions, and meeting near the zenith their effluvia mixing and interfering with one another and so occasioning a much stronger but uncertain wavering light. And since it is agreed by all our accounts that this corona was tinged with various colours, it is more than probable that these vapours were carried up to such a height, as to emerge out of the shadow of the earth, and to be illuminated by the direct beams of the sun: whence it might come to pass that this first corona was seen coloured, and much brighter, than what appeared afterwards in some places, where the sight of it was more than once repeated, after the sun was gone down much lower under the horizon. Hence also it will be easily understood, that this corona was not one and the same in all places, but was different in every differing horizon; exactly after the same manner as the rainbow, seen in the same cloud, is not the same bow, but different to every several eye.

Nor is it to be doubted, but the pyramidical figure of these ascending beams is optical: since probably they are parallel. sided, or rather tapering the other way. But by the rules of perspective, their sides ought to converge to a point, as we see in pictures the parallel borders of straight walks, and all other lines parallel to the axis of vision, meet as in a centre. Therefore those rays which rose highest above the earth, and were nearest the eye, seemed to terminate in cusps sufficiently acute, and have been for that reason supposed by the vulgar to represent spears. Others seen from afar, and perhaps not rising so high as the former, would terminate as if cut off with plains parallel to the horizon, like truncated cones or cylinders: these have been taken to look like the battlements and towers on the walls of cities fortified after the ancient manner. While others yet further off, by reason of their great distance, good part of them being intercepted by the interposition of the convexity of the earth, would

only shew their pointed tops, and because of their shortness have been called swords.

Next, the motion of these beams furnishes us with a new, and most evident argument, to prove the diurnal rotation of the earth: though that be a matter which, at present, is generally taken by the learned to be past dispute. For those beams which rose up to a point, and did not presently disappear, but continued for some time, had most of them a sensible motion from east to west, contrary to that of the heavens; the largest and tallest of them, as being nearest, swiftest; and the more remote and shorter, slower. By which means, the one overtaking the other, they would sometimes seem to meet and jostle; and at other times to separate, and fly one another. But this motion was only optical, and occasioned by the eye of the spectator being carried away with the earth into the east; while the exceedingly rare vapour which those beams consisted of, being raised far above the atmos. phere, was either wholly left behind, or else followed with but part of its velocity, and therefore could not but seem to recede and move the contrary way. And after the same manner as the stars that go near the zenith, pass over those vertical circles which border on the meridian, much swifter than those stars which are more distant from it; so these luminous rays would seem to recede faster from east to west, as their bases were nearer the eye of the spectator; and è contra, slower as they were further off.

Nor are we to think it strange, if after so great a quantity of luminous vapour had been carried up into the ether, out of the pores of the earth, the cause of its effervescence at length abating, or perhaps the matter consumed, these effluvia should at length subside, and form those two bright luminations which we have de. scribed; and, whose edges being turned to us, were capable to emit so much light that we might read by them. I choose to call them luminations, because, though they were but thin, doubtless they spread horizontally over a large tract of the earth's surface, And while this luminous matter dropped down from the upper plate to the under, the many white columns were formed between them by its descent, only visible for the moment of their fall, These by the swiftness with which they vanished, and their great number, shewing themselves and disappearing without any order,

exhibited a very odd appearance; those on the right seeming some times to drive and push those on the left, and vice versa.

These are the principal phænomena; of whose causes I should have more willingly and with more certainty given my thoughts, if I had had the good luck to have seen the whole from beginning to end; and to have added my own remarks to the relations of others and especially if we could by any means have come at their distances. If it shall by any be thought a bold supposition, that I assume the effluvia of the magnetical matter for this pur. pose, which in certain cases may themselves become luminous, or rather may sometimes carry with them out of the bowels of the earth a sort of atoms proper to produce light in the ether: I answer, that we are not as yet informed of any other kinds of effluvia of terrestrial matter which may serve for our purpose, than those we have here considered, viz. the magnetical atoms, and those of water highly rarefied into vapour. Nor do we find any thing like it in what we see of the celestial bodies, unless it be the effluvia projected out of the bodies of comets to a vast height, and which seem by a vis centrifuga to fly with an incredible swiftness, the centres both of the sun and comet, and to go off into tails of a scarcely conceivable length. What may be the constitution of these cometical vapours, we the inhabitants of the earth can know but little, and only that they are evidently excited by the heat of the sun; whereas this meteor, if I may so call it, is seldom seen except in the polar regions of the world, and that most commonly in the winter months. But whatever may be the cause of it, if this be not, I have followed the old axiom of the schools, Entia non esse temere neque absque necessitate multiplicanda.

Lastly, I beg leave on this occasion to mention what, near 25 years since, I published in No. 195 of these Transactions, viz. That supposing the earth to be concave, with a less globe included, in order to make that inuer globe capable of being inhabited, there might not improbably be contained some luminous medium between the balls, so as to make a perpetual day below. That very great tracts of the etherial space are occupied by such a shining medium, is evident from the instances given in the first paper of this Transaction; and if such a medium should be thus inclosed within us; why may we not be allowed to suppose that some parts

of this lucid substance may, on very rare and extraordinary occa. sions, transude through and penetrate the cortex of our earth, and being got loose may afford the matter of which this our me. teor consists. This seems favoured by one considerable circumstance, viz. that the earth, because of its diurnal rotation, being necessarily of the figure of a flat spheroid, the thickness of the cortex, in the polar parts of the globe, is considerably less than towards the equator; and therefore more likely to give passage to these vapours; whence a reason may be given why these lights are always seen in the north. But I desire to lay no more stress on this conceit than it will bear.

It having been noted that in the years 1575 and 1580, when this appearance was frequent, that it was seen not far from the lines of the two equinoxes; it may be worth while for the curious to bestow some attention on the heavens in the months of September and October next; and in case it should again happen, to endeavour to observe, by the method I have here laid down, what may de. termine, with some degree of exactness, its distance and height; without which we can scarcely come to any just conclusion. [Phil. Trans. 1716.

SECTION III.

Observations on the Lumen Boreale, or Streaming, Oct. 8, 1726.

By the Rev. W. Derham, F.R.S.

THERE are two sorts of streamings, which have been noticed; one, by way of explosion from the horizon; the other, by opening and shutting, without shootings up, and swift dartings. Of the latter sort chiefly was that of October 8, 1726, in which, although the streams or spires, or lances, or cones, or whatever else they inay be called, were as large and remarkable as in the year 1715-6; yet they exhibited themselves principally by the vaporous matter opening and shutting, as if a curtain had been drawn and withdrawn before them. It began about eight o'clock, and soon streamed all round in the south, east and west, as much, or nearly as much, as in the north; which was a thing not observed before in these phænomena.