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phenomena--the place of the apparition and disappearance of each meteor, the time it continued visible, its brightness relatively to the fixed stars, whether accompanied with a train, &c., were carefully noted. The trajectories were then projected on a large planisphere. The extent of the trajectories described by the meteors was very different, varying from 80 to 70° of angular space, and the velocities appeared also to differ considerably; but the average velocity concluded by M. Wartmann was 25° per second. It was found, from the comparison of the simultaneous observations, that the average height above the ground was about 550 miles; and hence the relative velocity was computed to be about 240 miles in a second. But as the greater number moved in a direction opposite to that of the earth in its orbit, the relative velocity must be diminished by the earth's velocity (about nineteen miles in a second. This still leaves upwards of 220 miles per second for the absolute velocity of the meteor, which is more than eleven times the orbital velocity of the earth, seven and a half times that of the planet Mercury, and probably greater than that of the comets at their perihelia.
From the above results, it is obvious that the heights and velocities of the shooting-stars are exceedingly various and uncertain; but if the observations are in any respect worthy of confidence, they prove that many of these meteors (according to Wartmann's observations, by far the greater number) are, during the time of their visibility, far beyond the limits to which atmosphere is supposed to extend, and that their velocities greatly exceed that which is due to bodies moving at the same distance from the sun under the influence of solar gravitation.
It is, perhaps, impossible to form any correct estimate of the absolute magnitudes of the meteors. Their apparent magnitudes differ greatly; the greater number resembling stars of the third or fourth magnitude, while many are equal to stars of the first, and some even surpass Jupiter and Venus in brilliancy. It is remarkable that the largest are those which have the greatest altitudes, and only the smaller ones appear to come within twenty or even forty miles of the earth.
With respect to the casual observations of the phenomena, the accounts of which are very numerous, the most interesting conclusion which has been inferred from them is the periodical recurrence of shooting-stars in unusual numbers at certain epochs of the year. Of these epochs, the most remarkable is that of November, on account of the prodigious number of meteors which have been seen in some years at that time. The principal displays were in 1799, 1832, 1833, and 1834. On the 11th of November, 1799, thousands were observed within a few hours by Humboldt and Bon pland at Cumana; and on the same night by different persons over the whole continent of America, from the borders of Brazil to Labrador, and also in Greenland and Germany. Ou November 12th, 1832, they were seen over the whole of the north of Europe; and on November 12th, 1833, the stupendous exhibition took place in North America which has been so often described. From the accounts of this phenomenon collected by Prof. Olmsted, M. Arago computed that the number of meteors on this night amounted to 240,000. In 1834, a similar phenomenon recurred on the night of November 13th, but on this occasion the meteors were of a smaller size. In 1835, 1836, and 1838, shooting-stars were observed on the night of November 13th, in different parts of the world; but though diligently looked for one the same night in the last few years, they do not appear to have been more numerous than on other nights about the same season,-a circumstance which has shaken the faith of many in their periodicity.
The second great meteoric epoch is the 10th of August, first pointed out by M. Quetelet; and although no displays similar to those of the November period have been witnessed on this night, there are more instances of the recurrence of the phenomena. In the last three years shooting-stars have been observed in great numbers, both on the 9th and 10th ; but they appear in general to be unusually abundant during the two first weeks of August. The other periods which have been indicated are the 18th of October, the 23rd or 24th of April, the 6th and 7th of December, from the 15th to the 20th of June, and the end of January; and it is not improbable that further observations will add to the number.
The different theories which have been given to explain the origin and phenomena of the shooting-stars are next stated. The following are the principal :
1.—That the shooting-stars and fire-balls are substances projected from volcanoes in the moon. It is known that a body projected vertically from the moon with a velocity of about 8,500 feet in a second would not fall back upon the lunar surface, but would recede from it indefinitely ; and in order to reach the earth the projectile would only require, under the most favourable circumstances, to have a velocity of about 8,300 feet. Such a velocity, which is only about four or five times greater than that of a cannon-ball, is quite conceivable ; but the extraordinary exhibitions of 1799 and 1833, to say nothing of their supposed periodicity, is utterly irreconcilable with the theory of a lunar origin. Benzenberg, however, adopts this theory, and supposes the the shooting-stars to be small masses of stone, from one to five feet in diameter, which are projected from lunar volcanoes, and circulate about the earth or about the sun when their projectile velocity exceeds a certain limit.
2.—Dr. Olbers, and some other astronomers, have supposed the shooting-stars to be the dèbris, or fragments of a large planet, burst into pieces by some internal explosion, of which Ceres, Pallas, Juno, and Vesta, are the principal remaining portions. The smaller fragments continue to circulate about the sun in orbits of great eccentricity, and when they approach the region of space through which the earth is moving, they enter the atmosphere with great velocity, and by reason of the resistance and friction are rendered incandescent, and emit a vivid light so long as they remain within it.
3.-It has been suggested by Biot that the extraordinary displays observed in November may be explained by supposing the meteors to have their origin in the zodiacal light. The extent of this lens-shaped nebulosity is not well ascertained; but as the plane of its principal section is not parallel to the ecliptic, if the earth passes through it at one season, it must be remote from it at another. But shooting-stars are observed at all times of the year; and the November meteors differ from those of other seasons in no respect excepting in their greater multitude.
9.-The hypothesis first suggested by Chladni is that which appears to have met with most favour, having been adopted by Arago and other eminent astronomers of the present day to explain the November phenomena. It consists in supposing that, independently of the great planets, there exist in the planetary regions myriads of small bodies which circulate about the sun, generally in groups or zones, and that one of these zones intersects the ecliptic about the place through which the earth passes in November. The principal difficulties attending this theory are the following :—First, that bodies moving in groups in the circumstances supposed must necessarily move in the same direction, and consequently, when they become visible from the earth, would all appear to emanate from one point and move towards the opposite. Now although the observations seem to shew that the predominating direction is from north-east to south-west, yet shooting-stars are observed on the same nights to emanate from all points of the heavens, and to move in all possible directions. Secondly, their average velocity (especially as determined by Wartmann) greatly exceeds that which any body circulating about the sun can have at the distance of the earth. Thirdly, from their appearance and the luminous train which they generally leave behind them, and which often remains visible for several seconds, sometimes for whole minutes, and also from their being situated within the earth's shadow, and at heights far exceeding those at which the atmosphere can be supposed capable of supporting combustion, it is manifest that their light is not reflected from the sun; they must therefore be self-luminous, which is contrary to every analogy of the solar system. Fourthly, if masses of solid matter approached so near the earth as many of the shooting-stars do, some of them would inevitably be attracted to it; but of the thousands of shooting-stars which have been observed, there is no authenticated instance of any one having actually reached the earth. Fifthly, instead of the meteors being attracted to the earth, some of them are observed actually to rise upwards, and to describe orbits which are convex towards the earth; a circumstance of which, on the present hypothesis, it seems difficult to give any rational explanation.
5.–The most recent hypothesis is that of Capocci of Naples, who regards the aurora borealis, shooting-stars, aerolites, and comets, as having all the same origin, and as resulting from the aggregation of cosmical atoms, brought into union by magnetic attraction. He supposes that in the planetary spaces there exist bands or zones of nebulous particles, more or less fine, and endued with magnetic forces, which the earth traverses in its annual revolution; that the smallest and most impalpable of these particles are occasionally precipitated on the magnetic poles of our globe, and form polar auroras; that the particles a degree larger, in which the force of gravitation begins to be manifested, are attracted by the earth, and appear as shooting-stars ; that the particles in a more advanced state of concretion give rise in like manner to the phenomena of fire-balls, aerolites, &c.; that the comets, which are known to have very small masses, are nothing else than the largest of the aerolites, or rather uranolites, which in course of time collect a sufficient quantity of matter to be visible from the earth. This theory of Capocci differs from Chladni's only by the introduction of magnetic forces among the particles, and it is obvious that all the objections to the former theory apply with equal force to this. It may be remarked, however, that some physical connexion between the phenomena of shooting-stars and aurora had been already suspected, and the observations adduced by M. Quetelet afford reason to suppose that the latter phenomenon is also periodical.
From the difficulties attending every hypothesis which has hitherto been proposed, it may be inferred how very little real knowledge has yet been obtained respecting the nature of the shooting-stars. It is certain that they appear at great altitudes above the earth, and that they move with prodigious velocity ; but every thing else respecting them is involved in profound mystery. From the whole of the facts M. Wartmann thinks that the most rational conclusion we can adopt is, that the meteors probably owe their origin to the disengagement of electricity, or of some analogous matter, which takes place in the celestial regions on every occasion in which the conditions necessary for the production of the phenomena are renewed.
The concluding part of the paper contains an account of the different attempts which have been made to deduce differences of longitude from the observation of shooting-stars. That meteors which appear and are extinguished so suddenly, and which by reason of their great altitude and brilliancy are visible over considerable portions of the earth's surface, would afford excellent natural signals, provided they could be identified with certainty, was an obvious thought; but so long as they were regarded merely as casual phenomena, it could scarcely be hoped that they would be of much use, in this respect, to practical astronomy.
As soon, however, as their periodicity became probable, the observation of the phenomena acquired a new interest. In observing the meteors for this purpose, it is assumed that they appear instantaneously to observers stationed at a distance from each other, and that the meteors seen by different observers so placed are identically the same. These points are not altogether free from uncertainty ; but the results of the trials that have been already made may be regarded as favourable, and as shewing that among the other methods of determining astronomical positions, the observation of shooting-stars is not to be disregarded. At The November meeting of this Society, in 1839, an account was given of Professor Schumacher's observations at Altona, on the night of the 10th of August, 1838. On the same night, corresponding observations were made at several observatories in Germany; but those at Breslaw appear to have been the most successful. From twelve coincident observations at Altona and Breslaw, Professor Boguslawski computed the difference of longitude of the two places to be 28m 22:07, which differs less than a second from that which had been previously adopted. In Silliman's American Journal for October, 1840, an account is given of simultaneous observations made on the 25th of November, 1835, at Philadelphia, and at the College of New Jersey, at Princeton. Seven coincidences were observed, and the mean result gave a longitude differing only 1"-2 from the mean of other determinations; the whole difference being two minutes. This appears to have been the first actual determinination of a difference of longitude by meteoric observations. In the corresponding observations of Wartmann and Reynier at Geneva
and Planchettes, the differences of longitude deduced from three of the meteors, which were attended with peculiarities so remarkable as to leave no doubt of their identity, were respectively 2", 2" 3', 2m 5*, whence it would seem that a single observation may be in error to the amount of several seconds of time. In the Bibliothèque Universelle de Genèra for August 1840, there is given an account of the determination by this method of the difference of longitude between Rome and Naples. The corresponding observations were begun in November 1838, and were continued at intervals under the direction of Father Vico at Rome, and of Capocci and Nobili at Naples. The apparent paths of the meteors were traced on a celestial globe, and the times of appearance and extinction compared with clocks regulated by astronomical observations. The observed times of the extinction of the phenomena presented a very satisfactory agreement, inasmuch as it is stated that there was in general a difference of only a few tenths of a second of time between the partial results for a difference of longitude amounting to 70 597.
The merit of first suggesting the use of shooting-stars and fire-balls as signals for the determination of longitudes is claimed by Dr. Olbers and the German astronomers for Benzenberg, who published a work on the subject in 1802. Mr. Bailey, however, has pointed out a paper published by Dr. Maskelyne twenty years previously, in which that illustrious astronomer calls attention to the subject, and distinctly points out this application of the phenomena. The paper which is printed on a single sheet, is entitled “A Plan for observing the Meteors called Fire-balls, by Nevil Maskelyne, D.D., F.R.S., and Astronomer-royal,” and is dated Greenwich, November 6th, 1783. After recounting some observations, from which he infers that such meteors appear more frequently than is commonly imagined, and stating the particulars to be attended to in observing them, he adds :
" It would be well if those persons who happen to see a meteor would put down the time by their watch when it first appeared, or was at its greatest altitude, or burst, or disappeared, and again when they hear the sound; and as common watches are liable to vary much in a few hours, that they would, as soon after as may be, find the error of their watch by a good regulator ; for if the exact time could be had at different places, the absolute velocity of the meteor, the velocity of the sound propagated to us from the higher regions of the atmosphere, and the longitudes of places, might be determined.”
0x THE LONGITUDES OF THE PRINCIPAL MARITIME POINTS OF THE
GLOBE.—By Lieut. Raper, R.N., Sec. R.A.S.
(Continued from p. 113.)
152. Acapulco. Fort St. Diego, (formerly, it appears San Carlos.) Malasp. 1791. Feb. 19, and Apr. 15, occult. v Scorp. calculated by M. Oltmanns, mean
99° 52' 13'' Jup. Satell.
99 51 5 ENLARGED SERIES.NO. 4.-VOL, FOR 1841.