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nute stars which give the idea of accompanying satellites. These nebulæ are of enormous dimen
sions. One of them, near
sible diameter of about 20",
Aquarii, has a senand another presents
a diameter of 12". Sir John Herschel has computed that, if these objects be as far from us as the stars, their real magnitude must, even on the lowest estimation, be such as would fill the orbit of Uranus. He concludes that, if they be solid bodies of a solar nature, their intrinsic splendour must be greatly inferior to that of the sun, because a circular portion of the sun's disc, subtending an angle of 20", would give a light equal to that of a hundred full moons, while, on the contrary, the objects in question are hardly, if at all, visible to the naked eye. From the uniformity of the discs of the planetary nebulæ, and their want of apparent condensation, he presumes that they may be hollow shells, only emitting light from their surfaces.
The existence of every degree of ellipticity in the nebulæ from long lenticular rays to the exact circular form,-and of every shade of central condensation-from the slightest increase of density to apparently a solid nucleus,-may be accounted for by supposing the general constitution of these nebula to be that of oblate spheroidal masses of every degree of flatness, from the sphere
to the disc, and of every variety in their density and ellipticity towards the centre. It would be erroneous however to imagine, that the forms of these systems are maintained by forces identical with those already described, which determine the form of a fluid mass in rotation; because, if the nebulæ be only clusters of separate stars, as in the greater number of cases there is every reason to believe them to be, no pressure can be propagated through them. Consequently, since no general rotation of such a system as one mass can be supposed, it may be conceived to be a quiescent form, comprising within its limits an indefinite multitude of stars, each of which may be moving in an orbit about the common centre of the whole, in virtue of a law of internal gravitation resulting from the compound gravitation of all its parts. Sir John Herschel has proved that the existence of such a system is not inconsistent with the law of gravitation under certain conditions.
The distribution of the nebulæ over the heavens is even more irregular than that of the stars. In some places they are so crowded together as scarcely to allow one to pass through the field of the telescope before another appears, while in other parts hours elapse without a single nebula occurring in the zone under observation. They are in general only to be seen with the very best
telescopes, and are most abundant in a zone whose general direction is not far from the hour circles Oh and 12h, and which crosses the milky way nearly at right angles. Where that zone crosses the constellations Virgo, Coma Berenices, and the Great Bear, they are to be found in multitudes.
Such is a brief account of the discoveries contained in Sir John Herschel's paper, which, for sublimity of views and patient investigation, has not been surpassed in any age or country. To him and to Sir William Herschel is due almost all that is known of sidereal astronomy; and in the inimitable works of that highly-gifted father and son, the reader will find this subject treated of in a style altogether worthy of it, and of them.
So numerous are the objects which meet our view in the heavens, that we cannot imagine a part of space where some light would not strike the eye;-innumerable stars, thousands of double and multiple systems, clusters in one blaze with their tens of thousands of stars, and the nebulæ amazing us by the strangeness of their forms and the incomprehensibility of their nature, till at last, from the imperfection of our senses, even these thin and airy phantoms vanish in the distance. If such remote bodies shine by reflected light, we should be unconscious of their existence; each star must then be a sun, and may be presumed
to have its system of planets, satellites, and comets, like our own; and, for aught we know, myriads of bodies may be wandering in space unseen by us, of whose nature we can form no idea, and still less of the part they perform in the economy of the universe; nor is this an unwarranted presumption; many such do come within the sphere of the earth's attraction, are ignited by the velocity with which they pass through the atmosphere, and are precipitated with great violence on the earth. The fall of meteoric stones is much more frequent than is generally believed; hardly a year passes without some instances occurring, and if it be considered that only a small part of the earth is inhabited, it may be presumed that numbers fall in the ocean or on the uninhabited part of the land, unseen by man. They are sometimes of great magnitude; the volume of several has exceeded that of the planet Ceres, which is about 70 miles in diameter. One which passed within 25 miles of us was estimated to weigh about 600000 tons, and to move with a velocity of about 20 miles in a second,-a fragment of it alone reached the earth. The obliquity of the
descent of meteorites, the peculiar substances they are composed of, and the explosion accompanying their fall, show that they are foreign to our system. Luminous spots, altogether in
dependent of the phases, have occasionally appeared on the dark part of the moon; these have been ascribed to the light arising from the eruption of volcanos; whence it has been supposed that meteorites have been projected from the moon by the impetus of volcanic eruption. It has even been computed that, if a stone were projected from the moon in a vertical line, with an initial velocity of 10992 feet in a second,— more than four times the velocity of a ball when first discharged from a cannon,-instead of falling back to the moon by the attraction of gravity, it would come within the sphere of the earth's attraction, and revolve about it like a satellite. These bodies, impelled either by the direction of the primitive impulse, or by the disturbing action of the sun, might ultimately penetrate the earth's atmosphere, and arrive at its surface. But from whatever source meteoric stones may come, it seems highly probable that they have a common origin, from the uniformity-we may almost say identity of their chemical composition.