if it be considered that, in consequence of daylight, fogs, and great southern declination, one comet out of two must be hid from us. According to M. Arago, more than seven millions of comets frequent the planetary orbits.

The different degrees of velocity with which the planets and comets were originally propelled in space is the sole cause of the diversity in the form of their orbits, which depends only upon the mutual relation between the projectile force and the sun's attraction...

When the two forces are exactly equal to one another, eircular motion is produced; when the ratio of the projectile to the central force is exactly that of 1 to the square root of 2, the motion is parabolic; any ratio between these two will cause a body to move in an ellipse, and any ratio greater than that of 1 to the square root of 2 will produce hyperbolic motion (N. 222).

The celestial bodies might move in any one of these four curves by the law of gravitation; but as one particular velocity is necessary to produce either circular or parabolic motion, such motions can hardly be supposed to exist in the solar system, where the bodies are liable to such mutual disturbances as would infallibly change the ratio of the forces, and cause them to move in ellipses in the first case, and hyperbolas in the other. On the contrary, since every ratio between equality and that of 1 to the square root of 2 will produce elliptical motion, it is found in the solar system in all its varieties, from that~ which is nearly circular, to such as borders on the parabolic from excessive ellipticity. On this depends the stability of the system; the mutual disturbances only cause the orbits to become more or less eccentric without changing their nature.

For the same reason the bodies of the solar system might have moved in an infinite variety of hyperbolas, since any ratio of the forces, greater than that which causes parabolic motion, will make a body move in one of these curves. Hyperbolic motion is however very rare; only two comets appear to move in orbits of that nature, those of 1771 and 1824; probably all such comets have already come to their perihelia, and consequently will never return.

The ratio of the forces which fixed the nature of the celestial orbits is thus easily explained; but the circumstances which determined these ratios, which caused some bodies to move nearly in circles and others to wander toward the limits of the solar attraction, and which made all the heavenly bodies to rotate and revolve in the same direction, must have had their origin in the primeval state of things; but as it pleases the Supreme Intelligence to employ gravitation alone in the maintenance of this fair system, it may be presumed to have presided at its creation.

The Fixed Stars-Their Numbers-Estimation of their Distances and Magnitudes from their Light-Stars that have vanished-New StarsDouble Stars-Binary and Multiple Systems-Their Orbits and Periods Orbitual and Parallactic Motions-Colors-Proper Motions-General Motions of all the Stars-Clusters-Nebula--Their Number and Forms -Double and Stellar Nebula-Nebulous Stars-Planetary Nebula-Constitution of the Nebule, and Forces which maintain them-Distribution-Meteorites-Shooting Stars.

GREAT as the number of comets appears to be, it is absolutely nothing when compared with the multitude of the fixed stars. About 2000 only are visible to the naked eye; but when we view the heavens with a telescope, their number seems to be limited only by the imperfection of the instrument. In one hour Sir Wil

liam Herschel estimated that 50,000 stars passed through the field of his telescope, in a zone of the heavens 2° in breadth. This, however, was stated as an instance of extraordinary crowding; but, on an average, the whole expanse of the heavens must exhibit about a hundred millions of fixed stars within the reach of telescopic vision.

The stars are classed according to their apparent brightness, and the places of the most remarkable of those visible to the naked eye are ascertained with great precision, and formed into a catalogue, not only for the determination of geographical positions by their occultations, but to serve as points of reference for marking the places of comets and other celestial phe

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nomena. The whole number of stars registered amounts to about 150,000 or 200,000. The distance of the fixed stars is too great to admit of their exhibiting a sensible disc; but in all probability they are spherical, and must certainly be so if gravitation pervades all space, which it may be presumed to do, since Sir John Herschel has shown that it extends to the binary systems of stars. With a fine telescope the stars appear like a point of light; their occultations by the moon are therefore instantaneous. Their twinkling arises from sudden changes in the refractive powers of the air, which would not be sensible if they had discs like the planets. Thus we can learn nothing of the relative distances of the stars from us, and from one another, by their apparent diameters. The annual parallax of all but a very few being insensible, shows we must be more than two hundred millions of millions of miles at least from them. Many of them, however, must be vastly more remote; for of two stars that appear close together, one may be far beyond the other in the depth of space. The light of Sirius, according to the observations of Sir John Herschel, is 324 times greater than that of a star of the sixth magnitude; if we suppose the two to be really of the same size, their distances from us must be in the ratio of 57.3 to 1, because light diminishes as the square of the distance of the luminous body increases.

sun.

Nothing is known of the absolute magnitude of the fixed stars, but the quantity of light emitted by many of them shows that they must be much larger than the Dr. Wollaston determined the approximate ratio • which the light of a wax candle bears to that of the sun, moon, and stars, by comparing their respective images reflected from small glass globes filled with mercury, whence a comparison was established between the quantities of light emitted by the celestial bodies themselves. By this method he found that the light of the sun is about twenty millions of millions of times greater than that of Sirius, the brightest and one of the nearest of the fixed stars. Since the parallax of Sirius is about half a second, its distance from the earth must be 592,200 times the distance of the sun from the earth; and therefore Sirius, placed where the sun is, would appear

to us to be 3-7 times as large as the sun, and would give 13.8 times more light. Many of the fixed stars must be infinitely larger than Sirius.

Many stars have vanished from the heavens; the star 42 Virginis seems to be of this number, having been missed by Sir John Herschel on the 9th of May, 1828, and not again found, though he frequently had occasion to observe that part of the heavens. Sometimes stars have all at once appeared, shone with a bright light, and vanished. Several instances of these temporary stars are on record; a remarkable instance occurred in the year 125, which is said to have induced Hipparchus to form the first catalogue of stars. Another star appeared suddenly near a Aquilæ in the year 389, which vanished, after remaining for three weeks as bright as Venus. On the 10th of October, 1604, a brilliant star burst forth in the constellation of Serpentarius, which continued visible for a year; and a more recent case occurred in the year 1670, when a new star was discovered in the head of the Swan, which, after becoming invisible, reappeared, and having undergone many variations in light, vanished after two years, and has never since been seen. In 1572 a star was discovered in Cassiopeia, which rapidly increased in brightness till it even surpassed that of Jupiter; it then gradually diminished in splendor, and having exhibited all the variety of tints that indicate the changes of combustion, vanished sixteen months after its discovery, without altering its position. It is impossible to imagine anything more tremendous than a conflagration that could be visible at such a distance. It is however suspected that this star may be periodical, and identical with the stars which appeared in the years 945 and 1264. There are probably many stars which alternately vanish and reappear among the innumerable multitudes that spangle the heavens; the periods of several have already been pretty well ascertained. Of these the most remarkable is the star Omicron, in the constellation Cetus. It appears about twelve times in eleven years, and is of variable brightness, sometimes appearing like a star of the second magnitude; but it does not always attain the same lustre, nor does it increase or diminish by the same degrees. Accord

ing to Hevelius, it did not appear at all for four years. y Hydræ also vanishes and reappears every 494 days: and a very singular instance of periodicity is given by Sir John Herschel, in the star Algol or B Persei, which is described as retaining the size of a star of the second magnitude for two days and fourteen hours; it then suddenly begins to diminish in splendor, and in about three hours and a half is reduced to the size of a star of the fourth magnitude; it then begins again to increase, and in three hours and a half more regains its usual brightness, going through all these vicissitudes in two days, twenty hours, and forty-eight minutes. a Cassiopeia is also periodical, accomplishing its changes in 225 days: the period of the star 34 Cygni is 18 years; and Sir John Herschel has discovered very singular variations in the star 7 of the constellation Argo. It is surrounded by a wonderful nebula, and from a star of little more than the second magnitude it suddenly increased between the years 1837 and 1838 to be a first-rate star of the first magnitude. At the latter period it was equal to Arcturus, and its brilliancy was then so great as to obliterate some of the details of the surrounding nebula. Afterward it decreased to the first magnitude, and then began to increase again. Sir John has also discovered that a Orionis may now be classed among the variable and periodic stars, a circumstance the more remarkable, as it is one of the conspicuous stars of our hemisphere, and yet its changes had never been remarked. The inferences Sir John draws from the phenomena of variable stars are too interesting not to be given in his own words. "A periodic change existing to so great an extent in so large and brilliant a star as a Orionis, cannot fail to awaken attention to the subject, and to revive the consideration of those speculations respecting the possibility of a change in the lustre of our sun itself which were put forth by my father. If there really be a community of nature between the sun and fixed stars, every proof that we obtain of the extensive prevalence of such periodical changes in those remote bodies adds to the probability of finding something of the kind nearer home. If our sun were ever intrinsically much brighter than at present, the mean temperature of the surface of our