The Fixed Stars—Their Number—The Milky Way—Double Stars—Binary Systems—Their Orbits and Periodic Times—Colours of the Stars—Stars that have vanished—Variable Stars—Variation in Sun’s Light—Parallax and Distances of the Fixed Stars—Masses of the Stars—Comparative Light of the Stars—Proper Motions of the Stars—Apparent Motions of the Stars—Motion and Velocity of the Sun and Solar System—The NebulÆ—Their Number—Catalogue of them—Consist of Two Classes—Diffuse NebulÆ—Definitely formed NebulÆ—Globular Clusters—Splendour of Milky Way—Distribution of the NebulÆ—The Magellanic Clouds—NebulÆ round ? ArgÛs—Constitution of NebulÆ, and the Forces that maintain them—Meteorites and Shooting Stars.

Great as the number of comets appears to be, it is absolutely nothing in comparison of the multitude of the fixed stars. About 2000 only are visible to the naked eye; but when the heavens are viewed through a telescope, their number seems to be limited only by the imperfection of the instrument. The number registered amounts to 200,000; their places are determined with great precision, and they are formed into a catalogue, not only for the purpose of ascertaining geographical positions by the occultations of the brightest among them, but also to serve as points of reference for marking the places of comets and other celestial phenomena. Sirius, a Centauri, and Arcturus are the brightest stars in the heavens; the others are classed according to their apparent lustre, from the first to the seventeenth magnitudes. Capella, a LyrÆ, Procyon, and twenty or twenty-one more, are of the first magnitude; a Persei, ? Orionis, a Cygni, and in all fifty or sixty, are of the second; and of the third there are about 200, such as ? Bootis and ? Draconis, the numbers increasing as the magnitude diminishes. Those of the eighth magnitude are scarcely visible to the naked eye, and it requires a very good telescope to see stars of the seventeenth. This sequence is perfectly arbitrary; but Sir John Herschel has ascertained by actual measurement the comparative lustre of a great many—for example, he found that the light of a star of the sixth magnitude is 100 times less than that of one of the first magnitude, and that Sirius would make between three and four hundred of such little stars. Were the photometric scale completed, it would be of the greatest importance with regard to the variable stars.

The three or four brightest classes of stars are scattered pretty equably over the sky, with the exception of a zone or belt following the course of the great circle passing through e Orionis and a Crucis, where they are very numerous, especially in the southern hemisphere. The stars of all magnitudes visible to the naked eye increase in numbers towards the borders of the Milky Way, which derives its lustre and name from the diffused light of myriads of stars; so numerous are they in some parts of it that more than 50,000 passed through the field of Sir William Herschel’s telescope in the course of an hour, in a zone only two degrees broad; in many places they are numerous beyond estimation, and most of them are extremely small on account of their enormous distances.

The Milky Way, which forms so conspicuous a part of the firmament, is a vast and somewhat flattened stratum or congeries of stars, encircling the heavens in a broad band, split through one part of its circumference into two streams of stars, bearing a strong resemblance to fig. 5, plate 5. It is contorted and broken in some places, and occasionally lengthened into branches stretching far into space. Its thickness is small compared with its length and breadth; yet in some places it is unfathomable even with the best telescopes; in others there is reason to believe that it is possible to see through it, and even beyond it, in its own plane. There is a gradual but rapid increase in the crowding of the stars on each side of the flat stratum towards the centre.

The solar system is deeply though excentrically plunged into this mass of stars, near that point where the circular stratum splits into two streams. Sir John Herschel’s description of the stars of the southern hemisphere shows that the Milky Way is a most magnificent object there. “The general aspect of the southern circumpolar regions (including in that expression 60° or 70° of south polar distance) is in a high degree rich and magnificent, owing to the superior brilliancy and large development of the Milky Way, which, from the constellation of Orion to that of Antinous, is a blaze of light, strangely interrupted, however, with vacant and entirely starless patches, especially in Scorpio, near a Centauri and the Cross, while to the north it fades away pale and dim, and is in comparison hardly traceable. I think it is impossible to view this splendid zone, with the astonishingly rich and evenly distributed fringe of stars of the 3rd and 4th magnitude, which forms a broad skirt to its southern border like a vast curtain, without an impression amounting almost to conviction, that the Milky Way is not a mere stratum, but annular, or at least that our system is placed within one of the poorer or almost vacant parts of its general mass, and that eccentrically, so as to be much nearer to the region about the Cross than to that diametrically opposite to it.”

Those dark vacuities called “Coal Sacks” by the ancient navigators, which are so numerous between a Centauri and a Antaris, are among the most extraordinary phenomena in the southern hemisphere; they are of intense blackness, though by no means void of extremely small telescopic stars; the darkness arises from the contrast these nearly vacant spaces form with the excessive brilliancy of the surrounding part of the Milky Way, and the sudden sharp transition from light to darkness. The largest and most conspicuous of them is a pear-shaped vacuity close to the Southern Cross. That portion of the Milky Way that is split longitudinally through its centre lies between a Centauri and the constellation of Cygnus: the two bands are joined here and there by narrow bridges of condensed stars, stretching across the darker space between them. In Scorpio and Sagittarius Sir John Herschel describes the Milky Way as composed of definite clouds of light running into clusters of extremely minute stars like sand, not strewed evenly as with a sieve, but as if thrown down by handfuls, and by both hands at once, leaving dark intervals. In this astonishing profusion the stars are of all sizes, from the 14th to the 20th magnitude, and even down to nebulosity. After an interval the same profusion is renewed, the stars being inconceivably minute and numerous beyond description—they are in millions and millions. Thus there is great irregularity in their diffusion as well as magnitude—in some places intensely crowded, in others the deep blackness of the sky, over which they are thinly scattered, irresistibly led to believe that in these regions the power of our telescopes fairly penetrates through the starry stratum, and beyond it. Sometimes we look through a sheet of stars nearly of the same size, of no great thickness compared with their distance from us, and not unfrequently there is a double stratum, one of large stars spread over another of very small ones.

The most southerly of the two streams of stars which form the Milky Way in this part of the firmament maintains an unbroken course of extreme brilliancy, containing some of the finest clusters of stars in the heavens. One round ? Sagittarii is an intense aggregate of stars, in some parts of which they are so crowded as to exceed enumeration; at a very moderate estimate Sir John Herschel thinks this group cannot contain fewer than a hundred thousand stars. Other two groups between the constellations of the Shield and Ophiuchus stand out like promontories of intense brilliancy in the dark space that separates the starry streams of the Milky Way.

The distance of the fixed stars is too great to admit of their exhibiting a sensible disc, but they must be spherical if gravitation pervades all space, as there is every reason to believe it does. With a powerful telescope the stars are like points of light: their occultations by the moon are therefore instantaneous. Their twinkling arises from sudden changes in the refractive power of the air, which would not be sensible if they had discs like planets. Thus nothing can be known of their distance from us or from one another by their apparent diameters. Although from the appearance of the stars no inference can be drawn as to their distance, yet among the multitudes in the heavens a few are found near enough to exhibit distinct parallactic motions arising from the revolution of the earth in its orbit, from whence their distance from the sun has been computed: a Centauri, the brightest star in the southern hemisphere, is a very remarkable instance. Professor Henderson at the Cape of Good Hope determined its parallax to be 1 by a series of observations on its position at opposite periods of the year, that is, from opposite points in the earth’s orbit. The result was afterwards confirmed by Mr. Maclear, who found the amount to be 0·913. The difference between the two is wonderfully small, considering the many unavoidable sources of error in the determination of such minute quantities (N.230).

Since no star in the northern hemisphere has so great an amount of parallax, an arc of 1 is assumed as the parallactic unit. Now radius is to the sine of 1 as 206,265 is to 1; hence, a Centauri is 206,265 times more distant from the sun than the sun is from the earth. Light flying at the rate of 192,000 miles in a second must take 3 years and 83 days to come to us from that star.

One or two tenths of a second becomes a very great error when the maximum amount of parallax is only 1, and on that account, with the exception of a Centauri, it has been found impracticable to determine the annual changes in the apparent motions of single stars affected by precession, nutation, aberration, and the variations of temperature of the instruments used in observing. However, as two stars in juxtaposition are equally affected by all of these; the difference in their motions is independent of them. Of two stars apparently in close approximation, one may be far behind the other in space. They may seem near to one another when viewed from the earth in one part of its orbit, but may separate widely when seen from the earth in another position, just as two terrestrial objects appear to be one when viewed in the same straight line, but separate as the observer changes his position. In this case the stars would not have real, but only apparent motion. One of them would seem to oscillate annually to and fro in a straight line on each side of the other, a motion that could not be mistaken for that of a binary system where one star describes an ellipse about the other; or if the edge of the orbit be turned towards the earth, where the oscillations require years for their accomplishment. The only circumstances that can affect the stars unequally, and which must be eliminated, are the proper motion of the stars in space, and specific aberration, a very minute quantity arising from peculiarities in the star’s light. This method of finding the distances of the fixed stars was proposed by Galileo and attempted by Dr. Long without success. Sir William Herschel afterwards applied it to some of the binary groups; and although he did not find the thing he sought for, it led to the discovery of the orbital motions of the double stars.

M. Struve was the first to apply this method, and that in a very difficult case. He perceived that a very small star is close to a LyrÆ, and by a series of most accurate differential measurements from 1835 to 1838 he found that a LyrÆ has a parallax of 0·261, which was afterwards corroborated by the observations of M. Peters; hence a LyrÆ is 789,600 times more distant from the sun than the earth is.

It was natural to suppose that in general the large stars are nearer to the earth than the small ones; but there is now reason to believe that some stars, though by no means brilliant, are nearer to us than others which shine with greater splendour. This is inferred from the comparative velocity of their proper motions; all the stars have a general motion of translation, which tends ultimately to mix those of the different constellations; but none that we know of moves so rapidly as 61 Cygni, and on that account it was reckoned to be nearer to us than any other, for an object seems to move more quickly the nearer it is. Now M. Bessel saw that two minute and probably very remote stars are very near 61 Cygni, their directions from that star being at right angles to one another; so that, during the revolution of the earth, one of these distances was a maximum and the other a minimum alternately every three months. This alternation, although it indicated a parallax or difference of parallaxes of only 0·348, was maintained with such perfect regularity every three months, that it leaves not a doubt of its accuracy, which was afterwards confirmed by the observations of M. Peters at Polkova. It follows from that small parallax that 61 Cygni must be 592,700 times farther from the earth than the sun is—a distance that light would not pass over in less than nine years and three months.

Mr. Henderson found the parallax of Sirius, the brightest of all the stars, to be only 0·230; it is consequently more distant than 61 Cygni, though the latter is but of the 6th magnitude.

M. Argelander has calculated that the apparent magnitude of the stars depends upon their distance. Supposing them all to be of the same size, the smallest visible in Sir William Herschel’s 20 feet reflecting telescope, namely those of the 17th magnitude, would be 228 times farther off than those of the first magnitude; and M. Peters of Polkova from the annual parallax of thirty-five, seven of which are now very accurately determined, has ascertained the distance of the nearest of them to be such, that light flying at the rate of 95 millions of miles in a second would take 15 years and a half to come from them to the earth, and that a star of the 17th magnitude might be extinguished for 3541 years before we should be aware of it. (N.231.)

The great gulfs that separate the stars from the sun, and probably from one another, no doubt maintain the stability of the stellar system, in the same manner that in the solar system the distances of the planets from the sun and the satellites from their primaries are so arranged as to preserve their mutual disturbances within due limits. The stars supposed to be nearest the sun are probably in a great zone which crosses the Milky Way between ? ArgÛs and a Crucis. It comprises the bright stars of the constellations Orion, Canis Major, the Southern Cross, Centaur, Lupus, and Scorpio. The axis of the zone is inclined at an angle of 20° to the medial line, or circle, passing through the centre of the Milky Way.

A very great number of stars undergo periodical changes of lustre, varying in some cases from complete extinction to their original brilliancy, strongly suggesting the idea that they are temporarily obscured, and sometimes completely hid, by opaque bodies revolving round them in regular periodic times, as the planets do about the sun.

The star Mira, or ? Ceti, which was first noticed to be periodical by Fabricius, in 1596, appears about twelve times in eleven years, or in periods of 331^d 8^h 4^m 16^s; it remains at its greatest brightness about a fortnight, being then on some occasions equal to a large star of the second magnitude; then it decreases during about three months, till it becomes completely invisible to the naked eye, in which state it remains about five months; after that it continues increasing during the remainder of its period. Such is the general course of its changes; but it does not always return to the same degree of brightness, nor increase and diminish by the same gradations, neither are the successive intervals of its maxima equal. From the observations and investigations of M. Argelander, the mean period given is subject to fluctuation, embracing 88 such periods, and having the effect of gradually lengthening and shortening alternately those intervals to the extent of 25 days one way and the other. The irregularities in the degree of brightness attained at the maximum are probably also periodical. For four years previous to 1676 it did not appear at all; and on October 5, 1839, it exceeded a Ceti, and equalled AurigÆ, in lustre. These irregularities may be occasioned by periodical perturbations among opaque bodies revolving about the star. Algol, or Persei, is another very remarkable instance of a variable star. It has the size of a star of the second magnitude for two days and thirteen and a half hours, and then suddenly begins to diminish in splendour, till, in about three hours and a half, it 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 brightness, going through all these vicissitudes in 2^d 20^h 48^m 54^s·7. Sir John Herschel and Mr. Goodricke, by whom the variable nature of this star was discovered in 1782, considered this to be a case strongly indicative of the revolution of an opaque body, which, coming between us and Algol, cuts off a large portion of the light. This star has been constantly observed, and the more recent observations, compared with the ancient ones, indicated a diminution in the periodic time. It is even proved that this decrease is not uniformly progressive, but is actually proceeding with accelerated rapidity, which, however, will probably not continue, but will by degrees relax, and then be changed into increase, according to the laws of periodicity, which, as well as their causes, remain to be discovered. The first minimum of this star, in 1844, happened on January 3rd, at 4^h 14^m Greenwich time. ? HydrÆ also vanishes and reappears every 494 days. LyrÆ was discovered to be variable, in 1784, by Mr. Goodricke, and its period was ascertained by Argelander to be 12^d 21^h 53^m 10^s, in which time a double maximum and minimum takes place, the two maxima being nearly equal, but the two minima unequal; besides these semi-periods, there is a slow aberration of period, which appears to be periodical itself: from its discovery to 1840 the time was continually lengthening, but more and more slowly, till, in 1840, it ceased to increase, and has since been slowly on the decrease.

The stars d Cephei and ? AquilÆ, or Antinoi, were discovered to be variable in 1784; their respective periods, being 5^d 8^h 47^m 39^s and 7^d 4^h 13^m 53^s, have since been accurately determined. Besides these, the variations of between 30 and 40 have been approximately ascertained, and a great many more among the smaller stars have been discovered to be variable by Mr. Hind, who has remarked that many of those stars which continue visible at their minimum appear hazy and indistinct, as though some cloudy or nebulous medium intervened. Some of the variable stars are red, and others present successive changes through blue, yellow, and red. When the brightness is increasing the star has a blueish tinge, when it is past its maximum lustre it assumes a yellow tint, and while on its decrease it becomes ruddy with flashes of bright red light. These changes are very marked in a small star near the star 77, at the extremity of the south wing of Virgo.

Sir John Herschel, after having described the glory of the starry heavens, asks, “For what purpose are we to suppose such magnificent bodies scattered through the abyss of space? Surely not to illuminate our nights, which an additional moon of the thousandth part the size of our own would do much better, nor to sparkle as a pageant void of meaning and reality, and bewilder us with vain conjectures. Useful, it is true, they are to man as points of exact and permanent reference; but he must have studied astronomy to little purpose who can suppose man to be the only object of his Creator’s care, or who does not see in the vast and wonderful apparatus around us provision for other races of animated beings. The planets, we have seen, derive their light from the sun, but that cannot be the case with the stars. These doubtless then are themselves suns, and may perhaps, each in its sphere, be the presiding centre round which other planets or bodies, of which we can form no conception from any analogy offered by our own system, may be circulating.”

Another circumstance shows how probable it is that dark bodies are revolving among the stars. The proper motion of Sirius is very irregular—sometimes it is rapid, and at other times slow; the cause is ascribed by MM. Bessel and Peters to a dark companion which revolves with Sirius about their common centre of gravity, and by its attraction disturbs the equable motion of the star.

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 one 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 on the 11th of November, 1572, a star all at once shone forth in Cassiopeia, which rapidly increased in brightness till it surpassed that of Jupiter so much as to be visible at midday. It began to decrease in December of the same year, and, in March, 1574, it had entirely disappeared, having exhibited a variety of tints. It is suspected, however, that this star is periodically variable and identical with stars which appeared in the years 945 and 1264. 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. On the 28th of April, 1848, Mr. Hind discovered a star of the 5th magnitude in the constellation Ophiuchus, which was very conspicuous to the naked eye, and where he was certain no star even so bright as the 9th magnitude had ever existed, nor was there any record of such a star. From the time of its discovery it continued to diminish till it became extinct. Its colour was ruddy, and was thought to undergo remarkable changes, probably an effect of its low position, as its polar distance was 102° 39' 14.

Sir John Herschel discovered very singular variations in the star ? of the constellation Argo. It is surrounded by a wonderful nebula, and between the years 1677 and 1826 it varied twice from the 4th to the 2nd magnitude; but in the beginning of 1838 it suddenly increased in lustre, so as to be nearly as bright as a Centauri. Thence it diminished, but not below the first magnitude till April 1843, when it had again increased, so as to surpass Canopus, and nearly equal Sirius in splendour. With regard to this singular phenomenon, Sir John Herschel observes, that “Temporary stars heretofore recorded have all become totally extinct. Variable stars, as far as they have been carefully attended to, have exhibited periodical and regular alternations (in some degree at least) of splendour and comparative obscurity; but here we have a star fitfully variable to an astonishing extent, and whose fluctuations are spread over centuries, apparently in no settled period, and in no regular progression. What origin can we ascribe to these sudden flashes and relapses? What conclusions are we to draw as to the comfort or habitability of a system depending for its supply of light and heat on so variable a source? Its future career will be a subject of high physical interest. To this account I will only add, that in the beginning of 1838 the brightness of this star was so great as materially to interfere with the observations of that part of the nebula surrounding it.” Sir John has also discovered that a Orionis is variable, 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 first put forth by my father. If there be really a community of nature between the sun and the 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 globe would of course be proportionally greater. I speak now not of periodical, but secular changes. But the argument is complicated with the consideration of the possible imperfect transparency of space, which may be due to material non-luminous particles, diffused irregularly in patches analogous to nebulÆ, but of great extent—to cosmical clouds, in short, of whose existence we have, I think, some indication in the singular and apparently capricious phenomena of temporary stars, and perhaps in the recent extraordinary increase, and hardly less sudden diminution, of ? ArgÛs.” Mr. Hind has come to the same conclusion with Goodricke and Sir John Herschel, that the changes in the variable stars are owing to opaque bodies revolving round them; indeed there are strong reasons to believe that there are solar systems analogous to our own in the remote regions of space. Our sun requires nine times the period of Algol to perform a revolution on its axis, while, on the other hand, the periodic time of an opaque revolving body, sufficiently large to produce a similar temporary obscuration of the sun seen from a fixed star, would be less than fourteen hours.

It is possible that the decrease of light in some of the variable stars may arise from large spots on their surface, like those occasionally seen in the radiant fluid masses on the surface of the sun. One of these spots which was measured by Sir John Herschel on the 20th of March, 1836, with its penumbra, occupied an area of 3780 millions of square miles; and the black central part of a spot that appeared on the 25th of May following would have allowed the globe of the earth to drop through it, leaving a thousand miles clear of contact all around this tremendous abyss.

All the variable stars on record of which the places are distinctly indicated have occurred without exception in, or close upon, the borders of the Milky Way, and that only within the following semicircle, the preceding having offered no example of the kind.

Many stars have actually disappeared from the heavens. 42 Virginis seems to be of the 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 sky. Mr. Cooper, of the Markree Observatory, has given a list of fifty stars that are missing since the publication of his list of stars in 1847. Comparing the present state of the heavens with more ancient catalogues, a much greater number have disappeared.

Thousands of stars that seem to be only brilliant points of light, when carefully examined are found to be in reality systems of two or more suns, many of which are known to revolve about one another. These binary and multiple systems are very remote, requiring powerful telescopes to show the stars separately. They are divided into eight classes, according to the proximity of the two stars. The first class comprises only such as are less than 1 of space apart; those of the second class are more apart than 1 and less than 2, &c. &c. Sometimes the two stars are of equal magnitude, but more frequently a conspicuous star is accompanied by a smaller companion. In some cases the conspicuous star itself is double, as in ? Cancri, ? Scorpio, 11 Monocerotis, and 12 Lyncis, which are triple stars. Each of the two stars of e LyrÆ is a beautiful and close double star; so that which in a common telescope appears merely to be a double star, is found to be quadruple with a very excellent instrument. The multiple system of ? Orionis is one of the most remarkable objects in our hemisphere. To the naked eye and with an ordinary telescope it seems to be a single star, but it really consists of four brilliant stars forming a trapezium, and accompanied by two excessively minute and very close companions, to perceive both of which is the severest test of a telescope.

The first catalogue of double stars in which their places and relative positions are given was accomplished by the talent and industry of Sir William Herschel, who made so many great discoveries, and with whom the idea of their combination in binary and multiple systems originated; and that important fact he established by the discovery of a revolving motion in 50 or 60, and by the determination of the revolution of one star about the other of Castor or a Geminorum, the largest and finest double star in the northern hemisphere. He even assigned the approximate periodic times of this and of several other binary systems. More than 100 stars are now known to be stellar systems. The positions of many hundreds were measured by Sir John Herschel and Sir James South; and the catalogue of the double stars in the northern hemisphere, which have been micrometrically measured, has been increased to more than 6000 by MM. Bessel, Struve, and British astronomers.

Extensive catalogues of double stars in the southern hemisphere have been published by the astronomers in our colonial establishments. To these Sir John Herschel added 1081 during his residence at the Cape of Good Hope: the angles of position and distances of the stars from one another he measured, and found that many of them have very rapid orbital motions. The elliptical elements of the orbits and periodic times of fifteen have been determined by the most eminent astronomers with wonderful accuracy, considering the enormous distances and the extreme delicacy and difficulty of the subject. M. Savary has the merit of having first determined the elements of the orbit of a double star from observation. The difficulty of doing so is great, because the nearest fixed star is 211,000 times farther from the sun than the earth is, and the orbit itself is only visible with the best telescopes; consequently a very small error in observation occasions an enormous error in the determination of quantities at that distance.

In observing the relative position of the stars of a binary system, the distance between them, and also the angle of position, that is, the angle which the meridian, or a parallel to the equator, makes with the line joining the two stars, are measured. The different values of the angle of position show whether the revolving star moves from east to west, or the contrary; whether the motion be uniform or variable, and at what points it is greatest or least. The measures of the distances show whether the two stars approach or recede from one another. From these the form and nature of the orbit are determined. Were observations perfectly accurate, four values of the angle of position, and of the corresponding distances at given epochs, would be sufficient to assign the form and position of the curve described by the revolving star; this, however, scarcely ever happens. The accuracy of each result depends upon taking the mean of a great number of the best observations, and eliminating error by mutual comparison. The distances between the stars are so minute that they cannot be measured with the same accuracy as the angles of position; therefore, in order to determine the orbit of a star independently of the distance, it is necessary to assume, as the most probable hypothesis, that the stars are subject to the law of gravitation, and consequently that one of the two stars revolves in an ellipse about the other, supposed to be at rest, though not necessarily in the focus. A curve is thus constructed graphically by means of the angles of position and the corresponding times of observation. The angular velocities of the stars are obtained by drawing tangents to this curve at stated intervals, whence the apparent distances, or radii vectores of the revolving star, become known for each angle of position, because, by the laws of elliptical motion, they are equal to the square roots of the apparent angular velocities. Now that the angles of position estimated from a given line, and the corresponding distances of the two stars, are known, another curve may be drawn, which will represent on paper the actual orbit of the star projected on the visible surface of the heavens; so that the elliptical elements of the true orbit, and its position in space, may be determined by a combined system of measurements and computation. But, as this orbit has been obtained on the hypothesis that gravitation prevails in these distant regions, which could not be known À priori, it must be compared with as many observations as can be obtained, to ascertain how far the computed ellipse agrees with the curve actually described by the star.

? Virginis consists of two stars of nearly the same magnitude; they were so far apart in the beginning and middle of last century, that they were mentioned by Bradley, and marked in Mayer’s catalogue, as two distinct stars. Since that time they have been continually approaching each other, till in January, 1836, one star was seen to eclipse the other, by Admiral Smyth at his Observatory at Bedford, and by Sir John Herschel at the Cape of Good Hope. A series of observations since the beginning of the present century has enabled Sir John to determine the form and position of the elliptical orbit of the revolving star with extraordinary truth by the preceding method. According to his calculation, it came to its perihelion on the 18th of August of the year 1834. Its previous velocity was so great that the revolving star described an angle of 68° in one year. By the laws of elliptical motion its angular velocity must diminish till it arrives at its aphelion. The accuracy with which the motions of the binary systems are measured, and the skill employed in the deduction of the elliptical elements, are now so great, that the periodic time of ? Virginis, determined by Sir John Herschel and Admiral Smyth from their respective observatories, combined with those of Sir William Herschel, only differ by two years, Sir John having obtained a period of 182 years, Admiral Smyth that of 180. By the aid of more numerous observations Mr. Fletcher has found that the true period is 184·53 years, and that the revolving star passed its perihelion in 1837. It is by such successive steps that astronomy is brought to perfection (N.232).

Some of the double stars have very long periods, such as ? CoronÆ, where the revolving star takes 737 years nearly to accomplish a circuit. Others again have very short periods, as ? CoronÆ, ? Cancri, and ? UrsÆ Majoris, whose periodic times are 42·500, 58·91, and 58·26 years respectively: therefore each of these has performed more than one entire revolution since their motions were observed. ? Herculis, whose periodic time is only about 30¹/₄ years, has accomplished two complete circuits, the lesser star having been eclipsed by the greater each time. The first of these two truly wonderful events, of one sun eclipsing another sun, was seen by Sir William Herschel in 1782.

The orbits and periodic times of so many of these binary systems having been determined proves beyond a doubt that sun revolves about sun in the starry firmament by the same law of gravitation that makes the earth and planets revolve about the sun (N.232).

Since the parallax of 61 Cygni and that of a Centauri have been determined, Sir John Herschel has made the following approximation to the dimensions of their orbits and masses. The distance between the two stars of 61 Cygni, that is the radius vector of the revolving star, has hardly varied from 15·5 ever since the earliest observations; while in that time the star has moved through 50°; it is evident therefore that the orbit must be nearly circular. It is at right angles to the visual ray, and the periodic time is 514 years. The parallax or radius of the earth’s orbit as seen from the star is 0·348, while the radius of the star’s orbit as seen from the earth is 15·5; hence the radius of the star’s orbit is to that of the earth’s orbit as 15·5 to 0·348, or nearly as 45 to 1. So the orbit described by the two stars of 61 Cygni about one another greatly exceeds that which Neptune describes about the sun. Since the mean distance of the stars and their periodic time are given, the sum of the masses of the two stars is computed to be 0·3529, that of the sun being 1. Thus our sun is not vastly greater nor vastly less than the stars composing 61 Cygni, which is a small inconspicuous star to the naked eye, not exceeding the 6th magnitude.

Of all the double stars a Centauri is the most beautiful: it is the brightest star in the southern hemisphere, equal, if not superior, to Arcturus in lustre. The distance between the two stars has been decreasing at the rate of half a second annually since the year 1822, while the angular motion has undergone very little change, which shows that the plane of the orbit passes through the earth like the orbits of 44 BoÖtes, and p Serpentarii; that is to say, the edge of the orbit in these three stellar systems is presented to the earth, so that the revolving star seems to move in a straight line, and to oscillate on each side of its primary. Were this libration owing to parallax, it would be annual from the revolution of the earth about the sun; but as years elapse before it amounts to a sensible quantity, it can only arise from a real orbital motion seen obliquely. In this case five observations are sufficient for the determination of the orbit, provided they be exact; but the quantities to be measured are so minute, that it is only by a very long series of observations that accuracy can be attained. In 1834 Captain Jacob determined the periodic time of the revolving star of a Centauri to be 77 years, and the distance between the two to be 17·5; and since the decrease is half a second annually, the distance or radius vector of the revolving star was 12·5 in the year 1822; and as Mr. Henderson had determined the parallax or radius of the earth’s orbit as seen from the star to be ·913, it follows that the real semi-axis of the revolving star’s orbit is 13¹/₂ times greater than the semi-axis of the earth’s orbit as a minimum. The real dimensions of the ellipse therefore cannot be so small as the orbit of Saturn, and may possibly exceed that of Uranus. It is very probable that an occultation of one of the suns by the other will take place in 1867, or a very close appulse of the two stars.

Singular anomalies have appeared in the motions of 70 Ophiuchi, which was discovered to be a binary system by Sir William Herschel in 1779, and which has since nearly accomplished a revolution. Various orbits have been computed: those which best represent the angles of position fail with regard to the distances of the stars from one another, and vice versÂ. But it is a very remarkable fact that the errors are periodical, being for considerable periods of time alternately in excess and defect. Captain W. S. Jacob, who determined the periodic time of the revolving star to be 93 years, attributes this anomaly to the disturbing action of an opaque body revolving round the lesser star. Assuming that to be the case, and computing, he found that the errors were considerably diminished both in the angle of position and distance. It is a subject of the highest interest, and well worthy of the attention of such astronomers as have the means of making the necessary observations. Among the triple systems, as ? Cancri, two of the stars revolve about one another in 58·9 years; but the motion of the third and most distant is so slow, that it has only accomplished a tenth part of its revolution about the other two since the system was discovered.

It appears from the calculations of Mr. Dunlop that ? Eridani accomplishes a revolution in little more than 30 years. The motion of Mercury is more rapid than that of any of the planets, being at the rate of 107,000 miles an hour. The perihelion velocity of the comet of 1680 was 880,000 miles an hour; but, if the two stars of ? Eridani, or of ? UrsÆ Majoris, be as remote from one another as the nearest fixed star is from the sun, the velocity of the revolving star must exceed the power of imagination to conceive. The elliptical motion of the double stars shows that gravitation is not confined to the planetary motions, but that systems of suns in the far distant regions of the universe are also obedient to its laws. The stellar systems present a kind of sidereal chronometer, by which the chronology of the heavens will be marked out to future ages by epochs of their own, liable to no fluctuations from such disturbances as take place in our system. Some stars are apparently double, though altogether unconnected, one being far behind the other in space, as a LyrÆ, which apparently consists of two stars, one of the first, the other of the eleventh magnitude. Aldebaran, a AquilÆ, and Pollux are remarkable instances of these optically double stars. It has been shown how favourable that circumstance is for ascertaining the parallax of the nearest of the two. (N.232.)

The double stars are of various hues: sometimes both stars are of the same colour, as in a Centauri and 61 Cygni, where the larger stars are of a bright orange and the smaller ones a deeper tint of the same, but they most frequently exhibit the contrasted colours. The large star is generally yellow, orange, or red; and the small star blue, purple, or green. Sometimes a white star is combined with a blue or a purple, and more rarely a red and white are united. In many cases these appearances are due to the influence of contrast on our judgment of colours. For example, in observing a double star, where the large one is a full ruby red, or almost blood colour, and the small one a fine green, the latter loses its colour when the former is hid by the cross wires of the telescope. That is the case with ? AndromedÆ, which is a triple star, the small one, which appears green, being closely double. ? Cancri is an instance of a large yellow star and a small one which appears blue by contrast. Still there are a vast number where the colours are decidedly different, and suggest the curious idea of two suns, a red and a green, or a yellow and a blue, so that a planet circulating round one of them may have the variety of a red day and a green day, a yellow day and a blue day. Sir John Herschel observes, in one of his papers in the Philosophical Transactions, as a very remarkable fact, that, although red stars are common enough, no example of a solitary blue, green, or purple star has yet been produced.

Sirius is the only star on record whose colour has changed. In the time of Ptolemy it was red; now it is one of the whitest stars in the heavens.

M. Struve has found that, out of 596 bright double stars, 375 pairs have the same intensity of light and colour; 101 pairs have different intensity, but the same colour; and 120 pairs have the colours of the two stars decidedly different.

Certain rays, which exist in the sun’s light, are wanting in the spectra of every coloured star, and probably never existed in the light of these stars, as there is no reason to believe that they are absorbed by the stars’ atmosphere, though they may be by the earth’s. There are no defective rays in the white light of Sirius, Procyon, and others; but Sir David Brewster found in the spectrum of the orange-coloured light of ? Herculis a defective band in the red space, and two or more in the blue; consequently, the orange colour of the star is owing to a want of blue rays; for flames in which certain rays are wanting take the colour of the predominating rays. If the black rays in the solar spectrum were owing to the absorption of the sun’s atmosphere, the light from the margin of his disc, having to pass through a greater thickness of it, would exhibit deeper lines than that which comes from his centre; but, as no difference is perceptible, it may be inferred that the analogous bands in the light of the coloured stars are not due to the absorption of their atmospheres, but that they arise from the different kinds of combustion by which these bodies are lighted up.

All the ordinary methods fail for finding the parallax when the distances of the stars are very great. An angle even of one or two seconds, viewed in the focus of our largest telescopes, does not equal the thickness of a spider’s thread, which makes it impossible to measure such minute quantities with any degree of accuracy. In some cases, however, the binary systems of stars furnish a method of estimating an angle of even the tenth of a second, which is thirty times more accurate than by any other means. From them the actual distances of some of the more remote stars will ultimately be known.

Suppose that one star revolves round another in an orbit which is so obliquely seen from the earth as to look like an ellipse in a horizontal position, then it is clear that one-half of the orbit will be nearer to us than the other half. Now, in consequence of the time which light takes to travel, we always see the satellite star in a place which it has already left. Hence, when that star sets out from the point of its orbit which is nearest to us, its light will take more and more time to come to us in proportion as the star moves round to the most distant point in its orbit. On that account the star will appear to us to take more time in moving through that half of its orbit than it really does. Exactly the contrary takes place on the other half; for the light will take less and less time to arrive at the earth in proportion as the star approaches nearer to us; and therefore it will seem to move through this half of its orbit in less time than it really does. This circumstance furnishes the means of finding the absolute breadth of the orbit in miles, and from that the true distance of the star from the earth. For, since the greatest and least distances of the satellite star from the earth differ by the breadth of its orbit, the time which the star takes to move from the nearest to the remotest point of its orbit is greater than it ought to be by the whole time its light takes to cross the orbit, and the period of moving through the other half is exactly as much less. Hence the difference between the observed times of these two semi-revolutions of the star is equal to twice the time that its light employs to cross its orbit; and, as we know the velocity of light, the diameter of the orbit may be found in miles, and from that its whole dimensions; for the position of the orbit with regard to us is known by observation, as well as the place, inclination, and apparent magnitude of its major axis, or, which is the same thing, the angle under which it is seen from the earth. Since, then, three things are known in this great triangle, namely, the base or major axis of the orbit in miles, the angle opposite to it at the earth, and the angle it makes with the visual ray, the distance of the satellite star from the earth may be found by the most simple of calculations. The merit of having first proposed this very ingenious method of finding the distance of the stars is due to M. Savary; but, unfortunately, it is not of general application, as it depends upon the position of the orbit, and a long time must elapse before observation can furnish data, since the shortest period of any revolving star that we know of is 30 years. Still the distances of a vast number of stars may ultimately be made out in this way; and, as one important discovery almost always leads to another, their masses may thus be weighed against that of the earth or sun.

The only data employed for finding the mass of the earth, as compared with that of the sun, are, the angular motion of our globe round the sun in a second of time, and the distance of the earth from the sun in miles (N.233). Now, by observations of the binary systems, we know the angular velocity of the small star round the great one; and, when we know the distance between the two stars in miles, it will be easy to compute how many miles the small star would fall through by the attraction of the great one in a second of time. A comparison of this space with the space through which the earth would descend towards the sun in a second will give the ratio of the mass of the great star to that of the sun or earth. According to M. Bessel, the weight of the two stars of 61 Cygni is equal to half the weight of the sun. Little as we know of the absolute magnitude of the fixed stars, the quantity of light emitted by many of them shows that they must be much larger than the sun. 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 a LyrÆ is five and a half times greater than that of the sun. Sir John Herschel reflected the moon’s light totally by a prism, which, concentrated by a lens, was compared directly with that of a Centauri. After making allowance for the quantity of the moon’s light lost in passing through the lens and prism, he found that the mean quantity of light sent to the earth by a full moon exceeds that sent by a Centauri in the proportion of 27,408 to 1. Now, Dr. Wollaston found the proportion of the sun’s light to that of the full moon to be that of 801,072 to 1. Hence, the light sent to us by the sun is to that sent by a Centauri as about twenty-two thousand millions to one. But, as the parallax of a Centauri is 1, it really is two and a half times brighter than the sun. The light of Sirius is four times that of a Centauri, but its parallax is only 0·230: hence it has an intrinsic splendour 63·02 times that of our luminary. It is therefore estimated to be a hundred times as large; so that, were Sirius in the earth’s place, its surface would extend 150 times as far as the orbit of the moon. 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.

So many of the stars have proper motions altogether independent of the annual rotation of the earth in its orbit, that it may be doubted whether there be such a thing as a fixed star. Groombridge is the most rapid known: it has a proper motion of 7 of arc annually; a Centauri moves at the rate of 3·58 annually, and 61 Cygni describes a line in space of 5·12 in the same time. These motions are probably in curves, but at the distance of the earth they will appear to be rectilineal for ages to come. The motion of little more than five seconds of space, which 61 Cygni describes annually, seems to us to be extremely small; but at the distance of that star an angle of one second corresponds to twenty-four millions of millions of miles; consequently the annual motion of 61 Cygni is 120 millions of millions of miles, and yet, as M. Arago observes, we call it a fixed star. From the same cause it is evident that the crowding of the stars in the Milky Way may be apparent only, and that the stars may be at vast distances from one another, and no doubt are.

Were the solar system and the whole of the stars visible to us carried forward in space by a motion common to all, like ships drifting in a current, it would be impossible for us, moving with the rest, to ascertain its direction. Sir William Herschel perceived that a great part of the motions of the stars is only apparent, arising from a real motion of the sun in a contrary direction. Among many discrepancies he found that the stars in the northern hemisphere have a general tendency to move towards a point diametrically opposite to ? Herculis, which he attributed to a motion of the solar system in a contrary direction. For it was evident to him, that the stars, from the effects of perspective alone, would seem to diverge in the direction to which the solar system was going, and would converge towards the space it had left, and that there would be a regularity in these apparent motions which would hereafter be detected. Since Sir William Herschel’s time the proper motions of the stars have been determined with much greater accuracy, and many have been added to the list by comparing the ancient and modern tables of their places; his views have been established by four of the greatest astronomers of the age, MM. Lundahles, Argelander, Otto Struve, and Peters, who have clearly proved the motion of the sun from that of the stars in the northern hemisphere, and Mr. Galloway has come to the same conclusion from the motions of the stars in the southern hemisphere (N.234). The result is, that the sun, accompanied by all his attendant planets, is moving at the rate of 422,424 miles—or over a space nearly equal to his own diameter—in the course of a day, and that the motion is directed towards a point in a line joining the two stars and p Herculis at a quarter of the apparent distance of these two stars, reckoning from p Herculis. This investigation was founded upon no law assumed or observed, such as the circulation of all the stars of our firmament about a common centre, though philosophers have speculated as to the probability of such a motion in the sun and stars in the plane of the Milky Way. Should the sun and his stellar companions be moving in a nearly circular orbit, the centre of motion would be in the plane passing through the sun perpendicular to the direction of his motion. The constellations through which that great circle would pass are Pisces, Australis, Pegasus, Andromeda, Perseus, &c. M. Argelander is of opinion that the sun’s orbit is nearly in the plane of the Milky Way, and, therefore, that its centre must probably be in Perseus, while M. MÄdler places it in the Pleiades, which seems to be inadmissible; but the data are too uncertain at present to admit of any absolute conclusion as to the sun’s orbit and the general motion of the stellar firmament: for though the stars in every region of the sky tend towards a point in Hercules, it is not yet known whether their motions are uniform or variable, whether the sun’s motion be gradually changing, and whether the stars form different independent systems, each having its own centre of attraction, or if all obey one powerful controlling force which pervades the whole universe. Accurate observations of the places of a select number of stars of all dimensions in the Milky Way continued for a series of years would no doubt decide this point.

The proper motion of a star combined with the progressive velocity of light alters the apparent periodic time of the revolving star of a binary system. If the orbit of a double star be at right angles to the visual ray, and both the sun and the star at rest, the periodic time of the revolving star, say of 10,000 days, would always be the same. But if the centre of gravity of the star were to recede in a direct line from the sun with the velocity of one tenth of the radius of the earth’s orbit in a day, then at the end of 10,000 days it would be more remote from us by 1000 of such radii—a space light would take 57 days to traverse: hence, although the periodic time of the star would really be the same, the completion of its period would only be known to us 57 days after it had taken place, so that the periodic time would appear to us to be 10,057 days instead of 10,000. Were the star to approach to the sun by the same quantity instead of receding, the apparent periodic time would be diminished by 57 days.

As the sun is only a unit in the stellar system, so the Milky Way, and all the stars that adorn the firmament of both hemispheres, constitute a group which is but a unit among the infinite numbers of starry clusters and nebulÆ that are profusely scattered throughout the universe.

By the aid of a good telescope there may be seen on the clear vault of heaven, between the stars of our own stellar system, and far in the depths of space, an immense multitude of objects like comets or clouds of white vapour of all forms and sizes. Some are mixed with stars, others are entirely formed of them. Many appear as if they were stellar, but required a telescope of higher power to resolve them, and vast numbers appear to be matter rarefied in the highest possible degree, giving no indication of a stellar nature; and these are in every state of condensation, from a vague film hardly to be discerned to such as have actually arrived at a solid nucleus of stars. The cloudy appearance is merely the blending of the rays of innumerable stars which are themselves invisible from their extreme distance, like parts of the Milky Way. Sir William Herschel was at first of that opinion, and the nebulÆ that have been resolved by Lord Rosse’s telescope have led astronomers to believe that such is the case. Yet the tails of comets, the zodiacal light, and the extensive luminous atmospheres which encompass many of the stars, show that, in all probability, a self-luminous phosphorescent material substance in a highly diluted or gaseous form exists in vast abundance.

The number of the nebulÆ, like that of the stars, is only limited by the imperfection of our instruments, for each improvement in the telescope only enables us to penetrate a little farther into the infinity of space—to see a few more of these shadowy existences in the far distance, and to resolve a few more of those that are comparatively near. Sir William Herschel examined the nature and determined the position of 2500 nebulÆ in the northern hemisphere whose places were computed from his observations, reduced to a common epoch, and arranged into a catalogue, in order of right ascension, by his sister, Miss Caroline Herschel, who added lustre to the name she bore by her eminence in astronomical knowledge and discovery. Sir John Herschel revised his father’s observations, and added 800 nebulÆ to the catalogue before he went to the Cape of Good Hope, in order to complete the survey of the heavens. On his return he published a catalogue of 2049 nebulÆ of the southern hemisphere, of which 500 were previously unknown, with their position in the heavens. In a work unparalleled for elegance of style, depth of knowledge, and originality of views, he has given engravings from his drawings of the most remarkable objects, so that whatever changes may take place in their form, place, or condensation, will be known by astronomers of future ages.

Though infinite in variety, the nebulÆ are of two distinct classes; one consists of patches of great dimensions, capriciously irregular, assuming all the fantastic forms of clouds, now bright, now obscure; sometimes like vapour flying before the wind; sometimes stretching long arms into space. Many present an ill-defined surface, in which it is difficult to say where the centre of the greatest brightness is. Large portions are resolvable into stars; many have a granulated appearance, as if they were resolvable; and others probably are not so merely from the smallness and closeness of the stars, and possibly from their remoteness, indicating the complex and irregular form the Milky Way would present if seen from a distance. A wonderful nebula of this kind is visible to the naked eye in the constellation of Orion; it is of vast extent, sending branches even into the southern hemisphere; and, although Lord Rosse’s telescope has resolved much that had hitherto resisted others, there are parts that still maintain their nebulous appearance from extreme remoteness, presenting a kind of mottled aspect, like flocks or wisps of wool, or mackerel sky. There can be no doubt of its being an unfathomable congeries of stars, which there is reason to believe has changed its form in some parts within the last fifty years. Vast multitudes of nebulÆ of this kind are so faint as to be with difficulty discerned at all till they have been for some time in the field of the telescope, or are just about to quit it. Occasionally they are so vague, that the eye is conscious of something being present, without being able to define what it is; but the unchangeableness of its position convinces the mind that it is a real object—“an image was before mine eyes, but I could not discern the form thereof.”

No drawing can give an idea of the boundaries of such nebulÆ as that of Orion; even with Lord Rosse’s telescope the edges either fade into a luminous mist, which becomes more rare till it is imperceptible, or end in a tissue of faintish flocculi, or in filaments which become finer and more scattered till they cease to be visible, showing that the real boundaries have not been seen.

The other class of nebulÆ, vastly inferior in size, of definite forms and great variety of character, are scattered through the remote heavens, or congregated in a great nebulous district far from the Milky Way. Many cling to stars like wisps of clouds, others are exactly like comets with comÆ and tails; but the most definite forms are annular and lenticular nebulÆ, nebulous stars, planetary and elliptical nebulÆ, and starry clusters. However, there are two in the northern hemisphere differing from all of these, which are described by Sir John Herschel as amazing objects. One in Vulpecula is like an hourglass or dumb bell of bright matter, surrounded by a thin hazy atmosphere so as to give the whole an oval form, or the appearance of an oblate spheroid; with a higher optical power its form is much the same, but the brighter part is resolved into stars, and the hazy part, though still nebulous, assumes that mottled appearance which shows that the whole is a stellar system of the most peculiar structure: it is a phenomenon that bears no resemblance to any known object. (Fig. 3, plate 8, and fig. 3, plate 9). The other is indeed most wonderful, and its history shows the gradual increase in the space-penetrating power of telescopes. To Messier it appeared merely to be a double nebula with stars; with Sir William Herschel’s telescope it presented the appearance of a bright round nebula encompassed at a little distance by a halo or glory, and accompanied by a companion; while in Sir John Herschel’s 20 feet reflector it appeared to “consist of a bright round nucleus, surrounded at a distance by a nebulous ring split through half its circumference, and having the split portions separated at an angle of 45 degrees each to the plane of the other.” (Fig. 1, plate 10.) This nebula appeared to Sir John to “bear a strong similitude to the Milky Way, suggesting the idea of a brother system bearing a real physical resemblance and strong analogy of structure to our own.”

This object, which disclosed to Lord Rosse the astonishing phenomenon of spiral nebulÆ seen in his telescope, presents the appearance of the fig. 1 in plate 10, in which the partial division of the limb of the ring into two branches is at once recognised in the bright convolutions of the spiral. The outlying nebula is connected by a narrow curved band of light with the ring; the whole is either resolved into stars, or evidently might be with a still higher optical power. With regard to the marvellous nebula in question Lord Rosse observes, that “with each increase of optical power the structure has become more complicated, and more unlike anything that could result from any form of a dynamical law of which we find a counterpart in our system. The connection of the companion with this great nebula, of which there is not the least doubt, adds to the difficulty of forming any hypothesis. It is impossible that such a system could exist without internal movement, to which may be added a resisting medium; but it cannot be regarded as a case of mere static equilibrium.” This is by no means the only instance of a spiral nebula; Lord Rosse has discovered several others: some are easily seen—others require the highest powers of his telescope. From the numerous offsets that branch from the Milky Way and run far into space, it may possibly partake also of the spiral form.

There are seven annular nebulÆ in the northern hemisphere, since Lord Rosse has discovered that five of the planetary nebulÆ belong to this class. One of the finest examples of an annular nebula is to be seen midway between and ? LyrÆ (fig. 2, plate 9). According to Sir John Herschel, it is elliptical in the ratio of 4 to 5, and is sharply defined—the internal opening occupying about half the diameter. This opening is not entirely dark, but filled with a faint hazy light like fine gauze stretched over a hoop. Its diameter, if it is as far from us as 61 Cygni, must be 1300 times greater than the diameter of the earth’s orbit—dimensions that are most astounding. Lord Rosse’s telescope resolves this object into stars of extreme minuteness, with filaments of stars adhering to its edges and a pretty bright star in its interior. These rings are like hollow shells whose borders seem brighter because the nebulous substance, whatever it may be, is more condensed to appearance than the central part. The other annular nebula in the northern hemisphere described by Sir John Herschel is a small faint object, and more easily resolvable into stars. One of the annular nebulÆ seen by Lord Rosse is surrounded by a faint external flat ring; another has ansÆ, as if an annular nebulous ring encompassed it and was foreshortened. Two annular nebulÆ have perforations as if the black sky was seen through openings in the interior haze, for in no instance is the central opening quite dark.

Some nebulÆ are like very elliptical annular systems seen obliquely. If they be elliptical flat rings, the dark centre may be a real opening; but should the systems be a series of very long elliptical concentric shells surrounding a hollow, the dark axis may be merely a line of comparative darkness.

The connection of the elliptical nebulÆ with double stars is mentioned as very remarkable. In one elliptical nebula whose longer axis is 50 there are two individuals of a double star each of the 10th magnitude symmetrically placed rather nearer the vertex of the ellipse than the foci; in another the stars are unequal, but placed exactly at the extremities of the major axis, as in plate 8: besides these there are several other instances.

Double nebulÆ are not unfrequent in both hemispheres, exhibiting all the varieties of distance, position, and relative brightness, with their counterparts the double stars. The rarity of single nebulÆ as large, faint, and as little condensed in the centre as these, makes it extremely improbable that two such bodies should be accidentally so near as to touch, and often in part to overlap each other, as these do. It is much more likely that they constitute systems; and, if so, it will form an interesting object of future inquiry to discover whether they possess orbital motion.

Nebulous stars are beautiful objects, quite different from all the preceding. They are round or oval, increasing in density towards the centre. Sometimes the central matter is so vividly and sharply condensed and defined that the nebula might be taken for a bright star surrounded by a thin atmosphere. One is a star of the 8th magnitude exactly in the centre of a round bright atmosphere 25 in diameter; the star is quite stellar, and not a nucleus: it has not the smallest appearance of being resolvable. Another nebulous star is ? Orionis, which has a broad atmosphere in which is a dark cavity not symmetrical with the star, and a small double star with a similar opening on the edge of the atmosphere. Lord Rosse observes that these openings appear to be of the same nature with that within the bright stars in the trapezium of Orion, the stars being at its edge; and he is convinced that the stars are not only connected with the nebula, but that they are equidistant with it; hence, if their parallax can be found, the distance of this nebula would be determined. The zodiacal light or lenticular shaped luminous haze surrounding the sun which may be seen extending beyond the orbits of Mercury and Venus soon after sunset in the months of April and May, or before dawn in November and December, seems to place our luminary in the class of nebulous stars. The extensive and delicate atmosphere of these nebulous stars assumes all degrees of ellipticity, from the circular to the spindle-shaped ray, or almost the right line.

Planetary nebulÆ have exactly the appearance of planets with round or oval discs, sometimes sharply terminated, at other times hazy and ill-defined. Their surface, which is blue or blueish white, is equable or slightly mottled, and their light occasionally rivals that of the planets in vividness. They are generally attended by minute stars, which give the idea of accompanying satellites. These nebulÆ are of enormous dimensions. One near ? Aquarii has a sensible diameter of about twenty seconds, and another presents a diameter of twelve. Sir John Herschel has computed that, if these objects be as far from us as the stars, their real magnitude, on the lowest estimation, must 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 far inferior to that of the sun, because a circular portion of the sun’s disc subtending an angle of twenty seconds 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 these planetary nebulÆ, and their apparent want of condensation, he presumes that they may be hollow shells emitting light from their surface only. The southern hemisphere is very rich in them, where twenty-eight or twenty-nine have been discovered, some in the midst of a cluster of stars, with which they form a beautiful contrast. Three are of a decided blue colour, one Prussian blue, or verditer green, the other two of a bright sky blue, of great beauty and delicacy. One seems to belong to the class of double nebulÆ or double stellar nebulÆ of the utmost remoteness. Since Lord Rosse’s telescope has shown that five of the planetary nebulÆ are annular, some of those in the southern hemisphere may ultimately be found to belong to the same class.

Probably nine tenths at least of the nebulous contents of the heavens consist of spherical or elliptical forms presenting every variety of elongation and central condensation. Of these a great number have been resolved into stars, and a great many present that mottled appearance which renders it certain that an increase of optical power would decompose them. Those which resist do so on account of the smallness and closeness of the stars of which they consist.

Elliptical nebulÆ are very common; by much the finest may be seen near the star ? in the girdle of Andromeda. It is visible to the naked eye, and has frequently been taken for a comet. With a low optical power it has the spindle-shaped form of fig. 6, plate 5, the brightness being at first gradually and then rapidly condensed towards the centre, so that it has been compared to a star shining through horn, but had never appeared resolvable even with high optical powers till Mr. Bond examined it at the observatory of Cambridge in the United States. He found that its brightness extends over 2¹/₂ degrees in length, and more than a degree in breadth, including two small adjacent nebulÆ, so that it is oval. It is strongly and rapidly condensed into a nucleus on its northern side; and although it was not all resolved, it was seen to be strewed over with star dust, or extremely minute visible stars, which leaves not a doubt of its being a starry system. The most remarkable part of Mr. Bond’s discovery are two very narrow dark lines which extend along one side of the oval parallel to its major axis. These black streaks, difficult to distinguish, indicate a stratified structure, and are not the only instance of that arrangement in nebulÆ. Fig. 1, in plate 9, is from Mr. Bond’s drawing of this nebula.

Multitudes of nebulÆ appear to the unassisted eye, or are seen with ordinary telescopes, like round comets without tails; but when viewed with powerful instruments they convey the idea of a globular space, insulated in the heavens and full of stars, constituting a family or society apart from the rest, subject only to its own internal laws. To attempt to count the stars in one of these globular clusters, Sir John Herschel says, would be a vain task; they are not to be reckoned by hundreds. On a rough computation, it appears that many clusters of this description must contain ten or twenty thousand stars compacted and wedged together in a round space, whose apparent area is not more than a tenth part of that covered by the moon; so that its centre, where the stars are seen projected on each other, is one blaze of light. If each of these stars be a sun, and if they be separated by intervals equal to that which separates our sun from the nearest fixed star, the distance which renders the whole cluster barely visible to the naked eye must be so great, that the existence of such a splendid assemblage can only be known to us by light which must have left it at least a thousand years ago. These magnificent globular or spheroidal aggregates of stars are so arranged that the interior strata are more crowded and become more nearly spherical as they approach the centre. A most splendid object of this nature may be seen in the constellation Hercules (N.235).

Of 131 of these magnificent objects in the southern hemisphere, two of them are pre-eminently splendid. The globular cluster of a Centauri is beyond comparison the finest of its kind: it is perfectly spherical, and occupies a quarter of a square degree; the stars in it are literally innumerable, crowding and densely aggregated towards the centre; and, as its light is not more to the naked eye than that of a star of the 4th or 5th magnitude, their minuteness is extreme. It has a dark hole in its centre, with a bridge of stars across,—a circumstance peculiar to this cluster.

Lacaille’s globular cluster, or 47 Toucani, is completely insulated in a very dark part of the sky not far from the lesser of the Magellanic clouds. The stars, which are of the 14th magnitude, immensely numerous, compressed and white, form three distinct stages round a centre, where they suddenly change in hue, and form a blaze of rose-coloured light. One cluster consists of large ruddy stars and small white ones; another of greater beauty consists of shells or coats of stars of the 11th and 15th magnitude. There are thirty globular clusters of extreme beauty collected within a circular space of not more than eighteen degrees radius, which lies in the part of the sky occupied by the constellations Corona Australis, the body and head of Sagittarius, the tail of Scorpio, part of Telescopium and Ara. The Milky Way passes diametrically across the circular area in question, which gives prodigious brilliancy to this part of the sky. For besides these globular clusters, which all lie in the starry part, and not in the dark spaces, there are the only two annular nebulÆ known to exist in the southern hemisphere. No part of the heavens is fuller of objects beautiful and remarkable in themselves, and rendered still more so by their mode of association, and by the peculiar features assumed by the Milky Way, which are without a parallel for richness and magnificence in any other part of the sky. Some of the globular clusters are so remote that the stars are scarcely discernible—mere star dust. There is a double globular cluster in the southern hemisphere of very small dimensions, separated by a minute interval,—a combination which suggests the idea of a globular cluster revolving about a very oblate spheroidal one in the plane of the equator, and in an orbit which is circular, and seen obliquely like the central nebula itself, with a diameter somewhat more than four times the latter,—a stupendous system doubtless, but of which the reality can hardly be supposed improbable.

There appears to be some connexion between ellipticity of form and difficulty of resolution, for spherical clusters are in general easily resolved into their component stars, while there is scarcely an instance of an elliptical cluster yielding except to a very high optical power. Vast masses of the nebulÆ have never been resolved. Lord Rosse’s great telescope has resolved parts of the nebula of Orion, and various others which had not yielded to instruments of less power; it enables the astronomer to penetrate farther into space, and shows objects with greater clearness, than any other. But, excellent as this instrument is, thousands of nebulÆ are not to be resolved even by it. Those who imagine that any work of man can resolve all the nebulous matter in the heavens must have a very limited idea of the extent and sublimity of creation.

Innumerable nebulÆ in both hemispheres take the form of clusters of stars, but are totally different from the globular clusters, inasmuch as they are of irregular form and follow no uniform law of condensation. The Pleiades is an instance in our own stellar system; for although only 7 or 8 stars are visible to the naked eye, telescopes show that more than 200 belong to the group. In the constellation Cancer there is a luminous spot called the PrÆsepe or Beehive, which a very low power resolves into stars; and the constellation Coma Berenices is another stellar group. Many are of exquisite beauty, as that round a Crucis, which, though consisting of only a hundred and ten stars, is like a piece of fancy jewellery, from the colours of the stars, which are greenish white, green, blueish green, and red. Many of these clusters contain thousands of stars, and are frequently in the poorer parts of the sky, as if in the course of ages the stars had been attracted towards a centre.

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 constitutions of those nebulÆ 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 number 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. They are in general only to be seen with the best telescopes, and are most abundant in a zone whose general direction is not far from the hour circles 0^h and 12^h, and which crosses the Milky Way nearly at right angles. Where that nebulous zone passes over the constellations Virgo, Coma Berenices, and the Great Bear, they are to be found in multitudes.

The nebulous system is nearly divided into two parts by the Milky Way. One-third of the whole visible nebulous contents of the heavens forms a broad irregular mass, interspersed with vacant intervals, which fills about an eighth of the surface of the northern hemisphere. It occupies the constellations Leo, Leo Minor, the body, tail, and hind legs of Ursa Major, the nose of Camelopard, the point of the tail of Draco, Canis Venatica, Coma Berenices, the preceding leg of BoÖtes, and the head, wings, and shoulder of Virgo, which is the richest part. There is a lesser nebulous region in this hemisphere, but entirely separated from the preceding, which occupies the chest and wing of Pegasus, the constellations Pisces and Andromeda. If we could imagine the ring or zone of the Milky Way to encircle or coincide with the horizon, the great nebulous mass would form a canopy over head, descending down to a considerable distance on all sides, chiefly towards the north pole; and the richest part, which is in Virgo, would then be directly over head in the north pole of the Milky Way, that is in 12^h 47^m right ascension, and 64° north polar distance.

With the exception of the Magellanic clouds, there is a much greater uniformity in the distribution of the nebulÆ in the southern hemisphere than in the northern. They are separated by spaces of vacuity of greater or less dimensions. One of these barren regions extends for nearly fifteen degrees all around the south pole, and close on its border; the lesser of the Magellanic clouds occurs completely insulated; while the greater Magellanic cloud is in connexion with something approaching to a zone of connected nebulous patches which extends along the back of Doradus, through a portion of Horologium and Eridanus, part of Fornix, and over the paws of Cetus to the equator, where it unites with the nebulous regions of Pisces.

The Magellanic clouds form two of the most striking features in the southern hemisphere; both of these nebulÆ are visible to the unassisted eye, being nearly of the same intensity as the brighter portions of the Milky Way; but the smaller is entirely effaced by moonlight, and the larger nearly so. They are altogether unconnected with the Milky Way and with one another. The Nubecula Major is far superior to the Nubecula Minor in every respect, though they are similar in internal structure. The former consists of large tracts and ill-defined patches of irresolvable nebulÆ, and nebulosity in every stage of resolution, up to perfectly resolved stars like the Milky Way; and also of regular and irregular nebulÆ, properly so called; of globular clusters in every stage of resolvability; and of clustering groups sufficiently insulated and condensed to come under the designation of clusters of stars. Of these the nebula known as Lacaille’s 30 Doradus is too remarkable to be passed over. It is very large, situate within the Nubecula Major, and consists of an assemblage of nearly circular loops uniting in a centre, in or near which there is a circular black hole. In short, for the number and variety of the objects, there is nothing like this cloud. Within an area of only forty-two square degrees, Sir John Herschel has determined the places, and registered 278 nebulÆ and clusters of stars, with fifty or sixty in outlying members immediately adjacent. Even the most crowded parts of the stratum of Virgo, in the wing of that constellation, or in Coma Berenices, offer nothing approaching to it. It is evident, from the intermixture of stars and unresolved nebulosity, which probably might be resolved with a higher optical power, that the nubeculÆ are to be regarded as systems sui generis, to which there is nothing analogous in our hemisphere.

Next to the Magellanic clouds the great nebula round ? ArgÛs is one of the most wonderful objects of the southern sky. It is situate in that part of the Milky Way which lies between the Centaur and the body of Argus, in the midst of one of those rich and brilliant masses, a succession of which is so curiously contrasted with the profoundly dark adjacent spaces, and surrounded by one of the most beautiful parts of the southern heavens. Sir John Herschel says: “It would be impossible, by verbal description, to give any just idea of the capricious forms and irregular gradations of light affected by the different branches and appendages of this nebula. Nor is it easy for language to convey a full impression of the beauty and sublimity of the spectacle it offers when viewed in a sweep, ushered in as it is by so glorious and innumerable a procession of stars, to which it forms a sort of climax, justifying expressions which, though I find them written in my journal in the excitement of the moment, would be thought extravagant if transferred to these pages. In fact, it is impossible for any one, with the least spark of astronomical enthusiasm about him, to pass soberly in review with a powerful telescope, and on a fine night, that portion of the southern sky which is comprised between the 6th and 13th degrees of right ascension, and from 146° to 149° of north polar distance; such are the variety and interest of the objects he will encounter, and such the dazzling richness of the starry ground on which they are represented to his gaze.” In that portion of the sky there are many fine double stars—rich starry clusters; the elegant cluster of variously coloured stars round ? Crucis; a large planetary nebula with a satellite star; another of a bright blue colour, exquisitely beautiful and unique; and, lastly, ? ArgÛs itself, the most extraordinary instance of a variable star in astronomical history.

It frequently occurred, during Sir John Herschel’s survey of the southern heavens, that some parts of the sky were noted for deeper blackness than others, and no stars could be seen; it frequently happened that far from the Milky Way, or any large nebula or cluster of stars, there were some indications of very remote branches of the Milky Way, or of an independent sidereal system or systems, bearing a resemblance to such branches. These were indicated by an exceedingly delicate and uniform dotting or stippling of the sky by points of light too small to admit of any one of them being steadily and fixedly viewed, and too numerous to be counted even if possible to view them. The truth of this existence was felt at the moment of observation; but the conviction, though often renewed, was not permanent. The places where these appearances occurred are given, in order that those who wish to verify them may have it in their power.

Such is a brief account of a very few of the discoveries contained in Sir John Herschel’s great work on the NebulÆ and other Phenomena of the Southern Hemisphere,—a work which will rise in estimation with the lapse of years. No doubt the form and internal structure of many of these nebulÆ will be changed by telescopes of higher power; but as the places of the leading phenomena have been determined, the date of that great work may be regarded as the epoch of nebular time whence the relative changes that take place in the most distant regions of the universe will be estimated for ages to come; and in the inimitable writings of the highly gifted father and son the reader will find these subjects treated of in a style worthy of it and of them. Of late years the excellence of the instruments, and still more of the astronomers, in the foreign observatories, have aided the progress of sidereal astronomy immensely. Nor has it been cultivated with less success in our home and colonial establishments: certainly one of the most remarkable features of the times is the number of private observatories, built and furnished with the best instruments by private gentlemen, whose zeal has been rewarded by eminent success in all departments of the science. (N.236.)

So numerous are the objects which meet our view in the heavens, that we cannot imagine a point 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 limit of our senses, even these thin and airy phantoms vanish in the distance. If such remote bodies shone 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. Even in our own system, or at its farthest limits, minute bodies may be revolving like the telescopic planets, which are so small that their masses have hitherto been inappreciable, and there may be many still smaller. 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, but leave no residuum. These, which are known as falling stars and meteors, are periodical; but that is by no means the case with aËrolites, which are also ignited by the sudden condensation of the air on entering our atmosphere, and are precipitated in solid masses with such violence on the earth’s surface that they are often deeply buried in the ground.

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 600,000 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; but whence derived is still a mystery.

Shooting stars and meteors burst from the clear azure sky, and, darting along the heavens, are extinguished without leaving any residuum except a vapour-like smoke, and generally without noise. Their parallax shows them to be very high in the atmosphere, sometimes even beyond its supposed limit, and the direction of their motion is for the most part diametrically opposite to the motion of the earth in its orbit. The astonishing multitudes of shooting stars and fire-balls that have appeared at stated periods over different parts of the globe, warrant the conclusion that there is either a nebula or that there are myriads of bodies revolving in groups round the sun which only become visible when inflamed by entering our atmosphere.

One of these nebulÆ or groups seems to meet the earth in its annual revolution on the 12th and 13th of November.

On the morning of the 12th of November, 1799, thousands of shooting stars, mixed with large meteors, illuminated the heavens for many hours over the whole continent of America, from Brazil to Labrador: it extended to Greenland, and even Germany. Meteoric showers were seen off the coast of Spain, and in the Ohio country, on the morning of the 13th of November, 1831; and during many hours on the morning of the 13th November, 1832, prodigious multitudes of shooting stars and meteors fell at Mocha on the Red Sea, in the Atlantic, in Switzerland, and at many places in England. But by much the most splendid meteoric shower on record began at nine o’clock in the evening of the 12th of November, 1833, and lasted till sunrise next morning. It extended from Niagara and the northern lakes of America to the south of Jamaica, and from 61° of longitude in the Atlantic to 100° of longitude in central Mexico. Shooting stars and meteors, of the apparent size of Jupiter, Venus, and even the full moon, darted in myriads towards the horizon, as if every star in the heavens had started from their spheres. They are described as having been frequent as flakes of snow in a snow-storm, and to have been seen with equal brilliancy over the greater part of the continent of North America.

Those who witnessed this grand spectacle were surprised to see that every one of the luminous bodies, without exception, moved in lines which converged in one point in the heavens: none of them started from that point; but their paths, when traced backwards, met in it like rays in a focus, and the manner of their fall showed that they descended from it in nearly parallel straight lines towards the earth.

By far the most extraordinary part of the whole phenomenon is, that this radiant point was observed to remain stationary near the star ? Leonis for more than two hours and a half, which proved the source of the meteoric shower to be altogether independent of the earth’s rotation, and its parallax showed it to be far above the atmosphere.

As a body could not be actually at rest in that position, the group or nebula must either have been moving round the earth or the sun. Had it been moving about the earth, the course of the meteors would have been tangential to its surface; whereas they fell almost perpendicularly, so that the earth in its annual revolution must have met with the group. The bodies or the parts of the nebula that were nearest must have been attracted towards the earth by its gravity, and, as they were estimated to move at the rate of fourteen miles in a second, they must have taken fire on entering our atmosphere, and been consumed in their passage through it.

As all the circumstances of the phenomena were similar on the same day and during the same hours in 1832, and as extraordinary flights of shooting stars were seen at many places both in Europe and America on the 13th of November, 1834, 1835, and 1836, tending also from a fixed point in the constellation Leo, it has been conjectured, with much apparent probability, that this nebula or group of bodies performs its revolution round the sun in a period of about 182 days, in an elliptical orbit, whose major axis is 119 millions of miles; and that its aphelion distance, where it comes in contact with the earth’s atmosphere, is about 95 millions of miles, or nearly the same with the mean distance of the earth from the sun. This body must have met with disturbances after 1799, which prevented it from encountering the earth for 32 years, and it may again deviate from its path from the same cause.

It is now well ascertained that great showers of shooting stars occur also on the 12th of August, whose point of divergence is Camelopardali, so that the earth’s atmosphere comes into contact with a zone of these small bodies twice in the year. By a systematic series of observations, MM. Benzenberg and Brand have clearly made out that the heights at which the falling stars appear and vanish vary from 16 miles to 140, and their velocities from 18 to 36 miles in a second, velocities so great as certainly to indicate a planetary revolution round the sun. As shooting stars are seen almost every night when the sky is clear, Sir John Lubbock has thought it probable that some of these bodies may have come so near, that the attraction of the earth has overcome that of the sun, and caused them to revolve as satellites round it. Should that be the case, they might shine by the reflected light of the sun, and suddenly cease to be visible on entering the earth’s shadow. The splitting of the falling stars like a rocket, and the trains of light, may be accounted for by supposing the stars to graze the surface of the shadow before being eclipsed; and the disappearance would be more or less rapid according to the breadth of the penumbra traversed. The calculations of M. Petit, Director of the Observatory of Toulouse, not only render probable the existence of small satellites, but tend to establish the identity of a body revolving round the earth in three hours and twenty minutes, at a distance of 5000 miles above its surface. It is evident that in this case the same satellite would be seen very often, and a very few would be sufficient to account for their nightly appearance. It is possible, however, that some shooting stars may belong to one class, and some to the other, since one group may be revolving about the sun, and another round the earth. In the case of a satellite shooting star, geometry furnishes the means of ascertaining its exact distance from the spectator, or from the centre of the earth, if the time and place of its disappearance be known with regard to the neighbouring stars. Since the falling stars are consumed in the atmosphere, their masses must be small, but it is possible that occasionally one may be large enough to arrive at the surface of the earth as an aËrolite.