The chemical elements of which these meteoric masses consist, are the same as those distributed throughout the Earth's crust, and are fifteen in number, namely, iron, nickel, cobalt, manganese, chromium, copper, arsenic, zinc, potash, soda, sulphur, phosphorus, and carbon; constituting altogether nearly one-third of all the known simple bodies. Notwithstanding this similarity with the primary elements into which inorganic bodies are chemically reducible, the aspect of aerolites, owing to the mode in which their constituent parts are compounded, presents, generally, some features foreign to our telluric rocks and minerals. The pure native iron, which is almost always found incorporated with aerolites, imparts to them a peculiar but not consequently a selenic character; for in other regions of space, and in other cosmical bodies besides our moon, water may be wholly absent, and processes of oxidation of rare oc

currence.

ZODIACAL LIGHT.

In order to complete our view of all that we have learnt to consider as appertaining to our solar system, which now, since the discovery of the small planets, of the interior comets of short revolutions, and of the meteoric asteroids, is so rich and complicated in its form, it remains for us to speak of the ring of Zodiacal Light, to which we have already alluded. Those who have lived for many years in the zone of palms must retain a pleasing impression of the mild radiance with which the Zodiacal Light, shooting pyramidally upwards, illumines a part of the uniform length of tropical nights. I have seen it shine with an intensity of light equal to the Milky Way in Sagittarius, and that not only in the rare and dry atmosphere of the summits of the Andes, at an elevation of from thirteen to fifteen thousand feet, but even on the boundless grassy plains, the Llanos of Venezuela, and on the sea-shore beneath the ever clear sky of Cumana. This phenomenon was often rendered especially beautiful by the passage of light fleecy clouds which stood out in picturesque and bold relief from the luminous background. A notice of this aerial spectacle is contained in a passage in my journal, while I was on the voyage from Lima to the western coasts of Mexico:-"For three or four nights (between 10° and 14°

1 Of lunar origin.

N. lat.) the Zodiacal Light has appeared in greater splendour than I have ever observed it. The transparency of the atmosphere must be remarkably great in this part of the Southern Ocean, to judge by the radiance of the stars and nebulous spots. From the 14th to the 19th of March a regular interval of three quarters of an hour occurred between the disappearance of the Sun's disc in the ocean and the first manifestation of the Zodiacal Light, although the night was already perfectly dark. An hour after sunset it was seen in great brilliancy between Aldebaran and the Pleiades; and on the 18th of March it attained an altitude of 39° 5'. Narrow elongated clouds are scattered over the beautiful deep azure of the distant horizon, flitting past the Zodiacal Light as before a golden curtain. Above these other clouds are from time to time reflecting the most brightly variegated colours. It seems a second sunset. On this side of the vault of heaven the lightness of the night appears to increase almost as much as at the first quarter of the moon. Towards ten o'clock the Zodiacal Light generally becomes very faint in this part of the Southern Ocean, and at midnight I have scarcely been able to trace a vestige of it. On the 16th of March, when most strongly luminous, a faint reflection was visible on the east." In our gloomy, so called "temperate" northern zone, the Zodiacal Light is only distinctly visible in the beginning of spring, after the evening twilight, in the western part of the sky, and at the close of autumn, before the dawn of day, above the eastern horizon.

TRANSLATORY MOTION OF THE SOLAR SYSTEM.

We have hitherto considered that which belongs to our solar system-that world of material forms governed by the Sun-which includes the primary and secondary planets, comets of short and long periods of revolution, meteoric asteroids, which move thronged together in streams, either sporadically or in closed rings, and finally a luminous nebulous ring, that revolves round the Sun in the vicinity of the earth, and for which, owing to its position, we may retain the name of Zodiacal Light. Everywhere the law of periodicity governs the motions of these bodies, however different may be the amount of tangential velocity, or the quantity

1 Tangent, the line which touches a curve without intersecting it: the line in which

of their agglomerated material parts; the meteoric asteroids which enter our atmosphere from the external regions of universal space, are alone arrested in the course of their planetary revolution, and retained within the sphere of a larger planet. In the solar system, whose boundaries determine the attractive force of the central body, comets are made to revolve in their elliptical orbits at a distance of forty-four times greater than that of Uranus; nay, in those comets, whose nucleus appears to us, from its inconsiderable mass, like a mere passing cosmical cloud, the Sun exercises its attractive force on the outermost parts of the emanations radiating from the tail over a space of many millions of miles. Central forces, therefore, at once constitute and maintain the system.

Our Sun may be considered as at rest when compared to all the large and small, dense and almost vaporous cosmical bodies that appertain to and revolve around it; but it actually rotates round the common centre of gravity of the whole system, which occasionally falls within itself, that is to say, remains within the material circumference of the Sun, whatever changes may be assumed by the positions of the planets. A very different phenomenon is that presented by the translatory motion of the Sun, that is the progressive motion of the centre of gravity of the whole solar system in universal space. Its velocity is such that, according to Bessel, the relative motion of the Sun, and that of the star 61 Cygni, is not less in one day than 3,336,000 geographical miles. This change of the entire solar system would remain unknown to us, if the admirable exactness of our astronomical instruments of measurement, and the advancement recently made in the art of observing, did not cause our advance towards remote stars to be perceptible, like an approximation to the objects of a distant shore in apparent motion. The proper motion of the star 61 Cygni, for instance, is so considerable, that it has amounted to a whole degree in the course of 700 years.

The amount or quantity of these alterations in the fixed stars (that is to say, the changes in the relative position of self-luminous stars towards each other) can be determined with a greater degree of certainty than we are able to attach the centrifugal force would carry any body, if the centripetal gravitating power were suspended. 1 The stars in the constellations are distinguished by the letters in the Greek alphabet, and by the Arabic numerals; Cygnus, the northern constellation, the Swan.

to the genetic explanation of the phenomenon. After taking into consideration what is due to the precession of the equinoxes, and the nutation of the Earth's axis, produced by the action of the Sun and Moon on the spheroidal figure of our globe, and what may be ascribed to the transmission of light -that is to say, to its aberration, and to the parallax formed by the diametrically opposite position of the Earth in its course round the Sun, we still find that there is a residual portion of the annual motion of the fixed stars due to the translation of the whole solar system in universal space, and to the true proper motion of the stars. The difficult problem of numerically separating these two elements, the true and the apparent motion, has been effected by the careful study of the direction of the motion of certain individual stars, and by the consideration of the fact that, if all the stars were in a state of absolute rest, they would appear, perspectively appear, to recede from the point in space towards which the Sun was directing its course. But the ultimate result of this investigation, confirmed by the calculus of probabilities, is that our solar system and the stars both change their places in space.

*

The aspect of the starry heavens, the relative position of stars and nebulæ, the distribution of their luminous masses, the picturesque beauty, if I may so express myself, of the whole firmament, depend, in the course of ages, conjointly upon the proper motion of the stars and nebulæ, the translation of our solar system in space, the appearance of new stars, and the disappearance or sudden diminution in the intensity of the light of others; and, lastly and specially, on the changes which the Earth's axis experiences from the attraction of the Sun and Moon. The beautiful stars in the constellation of the Centaur and the Southern Cross will, at

1 Precession of the equinoctial points; their backward movement along the ecliptic contrary to the order of the signs, throwing the stars of the signs forward with respect to the intersecting points of the ecliptic and equator. This movement, caused by the conical motion of the Earth's axis round the ecliptic pole is at the rate of 51′ 10′′ annually, and produces the progressive movement of any diameter of the orbit, as the line of apsides, round the whole circle in 25,868 years. Its results in Uranography are to alter the longitudes of the heavenly bodies, so that the Polar Star has not always been and will not always be the Cynosure. The movement of the axis causing precession is termed nutation, and has been compared to that of the axis of a spinning te-totum; and its results in the apparent position of the stars, to the effect on land objects when viewed from the deck of a rolling vessel.

Aberration of light; the alteration of apparent place in the celestial bodies, arising from the velocity of light, combined with that of the Earth.

Parallax; the apparent alteration of position among bodies arising from the spectator's motion with respect to them, or from their motion with respect to him.

For a distinct and popular view of these subjects consult Herschel's Astronomy.

some future time, be visible in our northern latitudes, whilst other stars, as Sirius and the stars in the belt of Orion, will in their turn disappear below the horizon. The places of the North Pole will successively be indicated by the stars ẞ and a Cephei, and d Cygni,1 until, after a period of 12,000 years, Vega in Lyra will shine forth as the brightest of all possible pole-stars. These data give us some idea of the extent of the motions which, divided into infinitely small portions of time, proceed without intermission in the great chronometer of the universe. If for a moment we could yield to the power of fancy, and imagine the acuteness of our visual organs to be made equal with the extremest bounds of telescopic vision, and bring together that which is now divided by long periods of time, the apparent rest that reigns in space would suddenly disappear. We should see the countless host of fixed stars moving in thronged groups in different directions; nebula wandering through space, and becoming condensed and dissolved like cosmical clouds; the veil of the Milky Way separated and broken up in many parts, and motion ruling supreme in every portion of the vault of heaven, even as on the Earth's surface, where we see it unfolded in the germ, the leaf, and the blossom, the organisms of the vegetable world. The celebrated Spanish botanist Caranilles was the first who entertained the idea of "seeing grass grow," and he directed the horizontal micrometer threads of a powerfully magnifying glass at one time to the apex of the shoot of a bambusa, and at another on the rapidly growing stem of an American aloe, (Agave Americana,) precisely as the astronomer places his cross of network against a culminating star. In the collective life of physical nature, in the organic as in the sidereal world, all things that have been, that are, and will be, are alike dependent on motion. HUMBOLDT.

FIXED STARS, CENTRES OF SYSTEMS.

Shall we say, then, of these vast luminaries, that they were created in vain? Were they called into existence for no other purpose than to throw a tide of useless splendour over the solitudes of immensity? Our Sun is only one of

these luminaries, and we know that he has worlds in his

1 See note 2, p. 20. Individual stars in constellations are also characterized by names, as Vega, in the Lyre; Alruccabah (the Pole Star) in the Little Bear; Regulus, the Lion's heart; Aldebaran, in the Bull.