Abbildungen der Seite
PDF
EPUB

SECTION V.

Luminous Meteor, seen at Peckham, Dec. 11, 1741.

By Thomas Milner, M.D.

Dec. 11, 1741, at seven minutes past one in the afternoon, a globe of light, somewhat larger thau the horizontal full moon, and as bright as the moon appears at any time while the sun is above the horizon, instantaneously appeared, in a clear blue sky, about the S.S.E. moving towards the east with a continual equable motiou, and leaving behind it a narrow streak of light, whiter than the globe itself, throughout its whole course. Towards the end it appeared less than at the beginning of its motion; and within three, or at most four, seconds, it suddenly vanished. Its apparent velocity was nearly equal to half the medium velocity of those usual meteors commonly called falling or shooting stars.

The narrow luminous streak remained very distinct after the globe was gone; and gave a fair opportunity for taking the elevation of this phenomenon above the horizon, at the beginning and end of its motion, &c. which was found to be twenty degrees. This lumi. nous track, or path, seemed a right line, not quite parallel, but a little inclined to the plane of the horizon, viz. highest towards the east. It was at first very narrow, and pointed at each extremity: but soon grew broader, and within twenty minutes after the appearance, it was exactly like a long bright rare cloud, discontinued in two places, above three times its first breadth, and a little more inclined to, and elevated above, the horizon, than it was immediately after the motion of the globe.

[Phil. Trans. 1742.

SECTION VI.

Account of some late Fiery Meteors, with Observations.

By Charles Blagden, M.D. Sec. R. S.

This account respects chiefly the two most remarkable of the meteors that had lately appeared, and is founded partly on private communications, and partly on such accounts as were publisbed in the newspapers. These meteors were of the kind known to the ancients by the names of saunades, II.801, Bolides, Faces, Globi, &c. from particular differences in their shape and appearance, and sometimes, it seems, under the general term of comets: in the Philosophical Transactions they are called indiscriminately fire-balls, or fiery meteors; and names of a similar import have been applied to them in the different languages of Europe. The most material circumstances observed of such meteors may be brought under the following heads. 1. Their general appearance. 2. Their path. 3. Their shape or figure. 4. Their light and colours. 5. Their height. 6. Their noise. 7. Their size. 8. Their duration. 9. Their velocity.

Dr. B. begins with the first of these meteors, which was seen August 18, 1783.

§ 1. Its general appearance in these parts of Great Britain was that of a luminous ball, which rose in the N.N.W. nearly round, became elliptical, and gradually assumed a tail as it ascended, and in a certain part of its course seerned to undergo a remarkable change, compared to bursting ; after which it proceeded no longer as an entire mass, but was apparently divided into a great number, or a cluster of balls, some larger than the others, and all carrying a tail or leaving a train behind; under this form it continued its course with a nearly equable motion, dropping or casting off sparks, and yielding a prodigious light, which illuminated all objects to a surprising degree; till having passed the east, and verging consider. ably to the southward, it gradually distended, and at length was lost out of sight. The time of its appearance was 9h. 16 min. P. M. mean time of the meridian of London, and it continued visible about half a miuute.

$ 2. How far north the meteor may have begun there are no materials to determine with precision ; but, as it was seen in Shetland, and at sea between the Lewes and Fort William, and appeared to persons at Aberdeen and Blair, in Athol, ascending from the northward; and to an observer in Edinburgh as rising like ibe planet Mars; there can be little doubt but its course commenced beyond the farthest extremity of this island, somewhere over the northern ocean. General Murray, F.R S. being then at Athol House, saw it pass over bis head as nearly vertical as he could judge, tracing it from about 45 degrees of elevation north-north-westward to 30 or 20 degrees south-south-eastward, where a range of buildings intercepted it from his view. From near the zenith of Athol House, it passed on a little westward of Perth, and probably a little eastward of Edinburgh ; and continuing its progress over the south of Scof. Jand, and the western parts of Northumberland, and the bishoprie of Durham, proceeded almost through the middle of Yorkshire, Jeaving the capital of that county somewhat to the eastward. Hitlerto its path was as nearly S.S.e. as can be ascertained; but somewhere near the borders of Yorkshire, or in Lincolnshire, it appears to have gradually deviated to the eastward; and in the course of that deviation to have suffered a very remarkable change in the nature of its appearance, and to have separated into two parts. After this division the compact cluster of smaller meteors seems to bave moved for some time almost S.E. thus traversing Cambridgeshire, and perhaps the western confines of Suffolk; but gradually recore!. ing its original direction, it proceeded over Essex and the Straits of Dover, entering the continent probably not far from Dunkirk, where, as well as at Calais and Ostend, it was thought to be vertical. AR terwards it was seen at Brussels, Paris, and Nuits in Burgundy, still holding on its course to the southward ; nay, there is an intimation, though of doubtful authority, that it was perceived at Rome. Dar information of its progress over the continent is indeed very defective and obscure; yet we bave sufficient proof that it traversed in all thirteen or fourteen degrees of latitude, describing a track of 1000 miles at least over the surface of the earth ; a length of course far exceeding the utmost that has been hitherto ascertained of any similar phenomenon.

§ 3. This meteor was described by most spectators under three different forms, and is so represented by Mr. Sandby in his beautiful drawing; but the first two of those do not imply any real variation in its shape, depending only on a difference in the point of view. Accordingly, in the first part of its course over Scotland, it was seen to have a tail, and is thus described by General Murray when it passed Athol House. I'wo causes concur in this deception; first, the fore-shortening, and even occultation, of the tail, when the object is seen nearly in front; and, 2dly, that the light of most part of the tail is of so inferior a kind, as to be difficultly perceived at a great distance, especially when the eye is dazzled by the overpowering brilliancy of the body. The length and shape of the tail, howerer, were perpetually varying; nor did the body continue always of the same magnitude and figure ; but was sometimes round, at other times elliptical, with a blunt or a pointed protuberance behind. From such changes of figure in this and other meteors it is, that they have been compared to columns or pyramids of fire, comets, barrels, bottles, flasks, paper-kites, trumpets, tad-poles, glass drops, quoits, torches, javelins, goats, and many other objects; whence the multifarious appellations given to them by the ancients were borrowed.

Respecting the tails of meteors, it is here necessary to distinguish between two different parts of which they consist. The brightest portion seems to be of the same nature as the body, and indeed an elongation of the matter composing it; but the other, and that commonly the largest portion, might more properly be called the train, appearing to be a matter left behind after the meteor has passed; it is far less luminous than the former part, and often only of a dull or dusky red colour. A similar train or streak is not un, frequently left by one of the common falling stars, especially of the brighter sort; and vestiges of it sometimes remain for several minutes. It often happens, that even the large fire-balls have no other tail but this train, and this of the 18th of August appeared at times to be in that state; its tail was thought by some spectators to be spiral.

Under this changeable form, but still as a single body, it proceeded regularly till a certain period; when expanding with a great increase of light, it separated into a cluster of smaller bodies or ovals, each extended into a tail and producing a train. At the same time a great number of sparks appeared to issue from it in various directions, but mostly downward, some of wbich were so bright as also to leave a small train. Most fire-balls have suffered a bursting or explosion of this kind; but, in general, they have been thought to disappear immediately afterwards. This, however, con. tinued its course, becoming more compact, or perhaps re-uniting, and seems to have undergone other similar explosions before it left our island, and again on the continent. The different accounts tend to shew, that its first separation or bursting happened some. where over Lincolnshire, perhaps near the cominencement of the fens. It is observable, that the great change in this meteor corre. sponds with the period in which it suffered a deviation from its course, as if there were some connexion between those two circumstances; and there are traces of something of the same kind having happened to other meteors. If the explosion be any sort of effort, we cannot wonder that the body should be moved by it from a straight line; but, on the other hand, it seems equally probable, that if the meteor be forced, by any cause, to change its direction, the consequence should be a division or separation of its parts.

$ 4. Nothing relative to these meteors strikes the beholders with so much astonishment as the excessive light they afford, sufficient to render very minute objects visible on the ground in the darkest night, and larger ones to the distance of many miles from the eye. The illamination is often so great as totally to obliterate the stars, to make the moon look dull, and even to affect the spectators like the sun itself; nay, there are many instances in which such meteors have made a splendid appearance in full sun-shine. The colour of their light is various and changeable, but generally of a bluish cast, which makes it appear remarkably white. A curious effect of this was observed at Brussels, the 18th of August, that while the meteor was passing, “ the moon appeared quite red, but soon recovered its natural light." The brightness alone of the meteor, is not sufficient to explain this, for the moon does not appear red when seen by day; but it must have depended on the contrast of colour, and shows how large a proportion of blue rays enters into the composition of that light, which could make even the silver moon appear to have excess of red. Prismatic colours were also observed in the body, tail, and sparks of this meteor, variously by different per sons; some compared them to the hues of gems. The moment of its greatest brightness seems to have been when it burst the first time; but it continued long to be more luminous after that period than it was before.

The body of ihe fire-ball, even before it burst, did not appear of a uniform substance or brightness, but consisted of lucid and dull parts, which were perpetually changing their respective positions ; so that the whole effect was to some eyes like an internal agitation or boiling of the matter, and to others like moving chasins or aper. tures. Similar expressions have been used in the descriptiou of former meteors. The luminous substance was compared to burning brimstone or spirits, Chinese fire, the stars of a rocket, a pellucid ball or bubble of tire, liquid pearl, lightning and electrical fire ;

« ZurückWeiter »