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precedes Jupiter, (the greatest in size of all the planetary bodies,) if we consider them with regard to distance from the Sun; and yet the discs of these small asteroids, which scarcely admit of measurement, have an aërial surface not much more than half that of France, Madagascar, or Borneo. However striking may be the extremely small density of all the colossal planets, which are furthest removed from the Sun, we are yet unable in this respect to recognise any regular succession. Uranus appears to be denser than Saturn;' and notwithstanding the inconsiderable difference of density observed in the innermost planetary group, we find both Venus and Mars less dense than the Earth, which lies between them. The time of rotation certainly diminishes with increasing solar distance, but yet it is greater in Mars than in the Earth, and in Saturn than in Jupiter. The elliptic orbits of Juno, Pallas, and Mercury, have the greater degree of eccentricity, and Mars and Venus, which immediately follow each other, have the least. Mercury and Venus exhibit the same contrasts that may be observed in the four smaller planets, or asteroids, whose paths are so closely interwoven.

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The hitherto discovered principal planets that revolve round our Sun, are attended certainly by fourteen, and probably by eighteen, secondary planets, (moons or satellites.) The principal planets are therefore themselves the central bodies of subordinate systems. We seem to recognise in the fabric of the universe the same process of arrangement so frequently exhibited in the development of organic life, where we find in the manifold combinations of groups of plants or animals, the same typical form repeated in the subordinato classes. The secondary planets or satellites are more frequent in the external region of the planetary system, lying beyond the intersecting orbits of the smaller planets or asteroids ; in the inner region none of the planets are attended by satellites, with the exception of the Earth, whose moon is relatively of great magnitude, since its diameter is equal to a fourth of that of the earth; whilst the diameter of the largest of all known secondary planets--the sixth satellite of Saturn-is probably about one-seventeenth, and the largest of Jupiter's moons, the third, only about one-twentysixth part that of the primary planet or central body. The planets which are attended by the largest number of satellites are most remote from the Sun, and are at the same time the largest, most compressed at the poles, and the least dense.

1 Comparative densities-Earth, 1. ; Sun, 0.25 ; Mercury, 2.94 ; Venus, 0.92; Mars, 0 25; Jupiter, 0.24; Saturn, 0.14; Herschel, 0.24 ; Neptune, 0.25.

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It now remains for us to speak of the unnumbered host of comets, which constitute a portion of the cosmical bodies revolving in independent orbits round the Sun. If we assume an equable distribution of their orbits, and the limits of their perihelia, or greatest proximities to the Sun, and the possibility of their remaining invisible to the inhabitants of the Earth, and base our estimates on the rules of the calculus of probabilities, we shall obtain, as the result, an amount of myriads perfectly astonishing. Kepler, with his usual animation of expression, said that there were more comets in the regions of space than fishes in the depths of the ocean. As yet, however, there are scarcely one hundred and fifty whose paths have been calculated, if we may assume at six or seven hundred the number of comets, whose appearance and passage through known constellations have been ascertained by more or less precise observations.

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COS

Although comets have a smaller mass than

any

other mical bodies, being, according to our present knowledge, probably not equal to 1-5000th part of the Earth's mass, yet they occupy the largest space, as their tails, in several instances, extend over many millions of miles. The cone of luminous

vapour

which radiates from them has been found in some cases (as in 1680 and 1811) to equal the length of the Earth's distance from the Sun, forming a line that intersects both the orbits of Venus and Mercury. It is even probable that the vapour of the tails of comets mingled with our atmosphere in the years 1819 and 1823.

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Although, as a rule, the tails of comets increase in magnitude and brilliancy in the vicinity of the Sun, and are directed away from that central body, yet the comet of 1823 offered the remarkable example of two tails, one of which was turned towards the Sun, and the other away from it, forming with each other an angle of 160°. Modifications of polarity and the unequal manner of its distribution, and of the direction in which it is conducted, may, in this rare instance, have occasioned a double, unchecked, continuous emanation of nebulous matter.l-HUMBOLDT.

AEROLITES.

Shooting stars, fire-balls, and meteoric stones are, with great probability, regarded as small bodies moving with planetary velocity, and revolving in obedience to the laws of general gravity in conic sections? round the Sun. When these masses meet the Earth in their course, and are attracted by it, they enter within the limits of our atmosphere in a luminous condition, and frequently let fall more or less strongly heated stony fragments covered with a shining black crust. When we enter into a careful investigation of the facts observed at those epochs when showers of shooting stars fell periodically in 1799, and in North America during the years 1833 and 1834, we shall find that fire-balls cannot be considered separately from shooting stars. Both these phenomena are frequently not only simultaneous and blended together, but they likewise are often found to merge into one another, the one phenomenon gradually assuming the character of the other alike with respect to the size of their discs, the emanation of sparks, and the velocities of their motion. Although exploding smoking luminous fire-balls are sometimes seen even in the brightness of tropical daylight, equalling in size the apparent diameter of the moon, innumerable quantities of shooting stars have, on the other hand, been observed to fall in forms of such extremely small dimensions, that they appear only as moving points, or phosphorescent lines.

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The connexion of meteoric stones with the grander phenomenon of fire-balls—the former being known to be projected from the latter with such force as to penetrate from ten to fifteen feet into the earth—has been proved in many instances. Meteoric stones are in some instances thrown from dark clouds suddenly formed in a clear sky, and fall with a noise resembling thunder. Whole districts have thus occa

? In the Kosmos (i. p. 94) will be found a catalogue of historical comets, and some pretty observations on the alteration of feeling with which the progress of knowledge has caused men to regard their appearance.

“ In the German vineyards,” says M. Humboldt, " in the beautiful valleys of the Rhine and Moselle, a belief has arisen, ascribing to these once ill-omened bodies a beneficial influence in the ripening of the

2 The curves of all celestial motions are sections of the cone.

vine."

sionally been covered with thousands of fragmentary masses, of uniform character, but unequal magnitudes, that have been hurled from one of these moving clouds. In less frequent cases, as in that which occurred on the 16th of September 1843 at Kleinwenden, near Mülhausen, a large aerolite fell with a thundering crash, while the sky was clear and cloudless. The intimate affinity between fire-balls and shooting stars is further proved by the fact, that fire-balls, from which meteoric stones have been thrown, have occasionally been found, (as at Angers on the 9th of June 1822,) having a diameter scarcely equal to that of the small fireworks called Roman candles.

The formative power, and the nature of physical and chemical processes involved in these phenomena, are questions all equally shrouded in mystery, and we are as yet ignorant whether the particles composing the dense mass of meteoric stones are originally, as in comets, separated from one another in the form of vapour, and only condensed within the fiery ball when they become luminous to our sight; or whether, in the case of smaller shooting stars, any compact substance actually falls ; or, finally, whether a meteor is composed only of a smoke-like dust, containing iron and nickel; whilst we are wholly ignorant of what takes place within the dark cloud from which a noise like thunder is often heard for many minutes before the stones fall.

It is very probable that a large number of these cosmical bodies traverse space undestroyed by the vicinity of our atmosphere, and revolve round the Sun without experiencing any alteration but a slight increase in the eccentricity of their orbits, occasioned by the attraction of the Earth's mass. We may, consequently, suppose the possibility of these bodies remaining invisible to us during many years and frequent revolutions.

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Shooting stars fall either separately and in considerable numbers—that is, sporadically, or in swarms of many

thousands. The latter, which are compared by Arabian authors to swarms of locusts, are periodic in their occurrence, and move in streams, generally in a parallel direction. Amongst periodic falls, the most celebrated are that known as the November phenomenon, occurring from about the 12th to the 14th of November, and that of the festival of St. Law

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rence, (the 10th of August,) whose “ fiery tears noticed in former times in a church calendar of England, no less than in old traditionary legends, as a meteorological event of constant recurrence. Notwithstanding the great quantity of shooting stars and fire-balls of the most various dimensions, which, according to Klöden, were seen to fall at Potsdam on the night between the 12th and 13th of November 1822, and on the same night of the year in 1832 throughout the whole of Europe, from Portsmouth to Orenbury on the Ural River, and even in the southern hemisphere, as in the Isle of France, no attention was directed to the periodicity of the phenomenon, and no idea seems to have been entertained of the connexion existing between the fall of shooting stars and the recurrence of certain days, until the prodigious swarm of shooting stars which occurred in North America between the 12th and 13th of November 1833, and was observed by Olmstead and Palmer. The stars fell, on this occasion, like flakes of snow, and it was calculated that at least 240,000 had fallen during a period of nine hours. Palmer of Newhaven, Connecticut, was led, in consequence of this splendid phenomenon, to the recollection of the fall of meteoric stones in 1799, first described by Ellicot and myself, and which, by the comparison of the facts I had adduced, showed that the phenomenon had been simultaneously seen in the New Continent, from the equator to New Herrnhut in Greenland, (64° 14' lat.) and between 46° and 82o long. The identity of the epochs was recognised with astonishment. The stream which had been seen from Jamaica to Boston (40° 21' lat.) to traverse the whole vault of heaven on the 12th and 13th of November 1833, was again observed in the United States in 1834, on the night between the 13th and 14th of November, although, on this latter occasion, it showed itself with somewhat less intensity. In Europe the periodicity of the phenomenon has since been manifested with great regularity.1

1 Arago makes the following observations in reference to the November phenomena :" We thus become more and more confirmed in the belief that there exists a zone composed of millions of small bodies, whose orbits cut the plane of the ecliptic nearly at the point which our Earth annually occupies between the 11th and 13th of November. It is a new planetary world beginning to be revealed to us." —Annuaire, 1836, p. 296.

The acute mind of Olbers led him almost to predict that the next appearance of the phenomenon of shooting stars and fire-balls intermixed, falling like flakes of snow, would not occur until between the 12th and 14th November 1867.

The stream of the November asteroids has occasionally only been visible in a small section of the Earth. Thus, for instance, a very splendid meteoric shower was seen in England in the year 1837.

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