elongated may be their outline. The regular nebulæ ap pear globular to all but the most powerful instruments, because the outer layers are usually faint. From this cause it is generally asserted that the globular form prevails. Annular nebulæ are considered regular, for they are probably hollow spherical shells of stars. Planetary nebulæ and globular clusters also come under this head.

Irregular nebulæ are the most extensive objects in the heavens. Their forms are most capricious, imitating a dumb-bell, a fan, and in one instance a human head and breast. Their true form is not however seen by us, and it may be that many nebulæ lying nearly in our visual line are by our eye blended into one. Probably in some cases they really consist of systems of systems. Indeed double nebulæ, or those lying very near one another, occur so frequently that their proximity cannot be attributed to chance. The nebulous system appears to be distant from the sidereal, though involving it, and perhaps to our eyes intermixed with it. The limits of our sidereal system have not in all directions been defined. The distribution of the nebulæ is not like that of the milky way in a zone or band. One third of

the nebulous contents of the heavens are congregated in a broad irregular patch occupying about one eighth of the whole surface of the sphere, almost entirely situated in the northern hemisphere, and occupying the head and shoulders of the Virgin, and the surrounding constellations. Within this region are several local centres of accumulation where the nebulæ are exceedingly crowded. The lesser nebulous region of the northern hemisphere lies in Pisces, and is much less concentrated.

§ 87. In the southern hemisphere a more uniform distribution prevails. With the exception of the two Nubeculæ, which are full of nebulæ, and the greater of which is even richer than the denser portion of the northern group, this hemisphere contains alternating patches of nebula and vacuities of greater or less extent. In one of the vacuities in which comparatively few nebulæ occur the south pole is situated, having one nebula, however, within half a degree of it, as the north pole also has one within five or six minutes. This barren region extends nearly 15° on all sides

of the pole, and immediately on its borders occurs the smaller Nubecula. One of the most remarkable features in the southern nebulous system is the extraordinary display of fine resolvable and globular clusters which occur in the region occupied by Corona Australis, the body and head of Sagittarius, the tail of Scorpio, with part of Telescopium and Ara. Here in a circular space of 18° in radius are collected no less than thirty of these beautiful objects. Are we to suppose them to be a bunch of general nebulous systems nearer to us than the rest? Or is it merely that on this side we approach nearer the milky way? It cannot be doubted that some of these objects form a part of the milky way.

§ 88. The bright fleecy spots, long known to mariners as the Magellanic clouds, are composed of large patches of unresolvable nebulæ, and of nebulosity in every stage of resolution, up to perfectly resolved stars like the milky way, as 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 denomination of clusters of stars. The Nubecula Minor contains within an area not much exceeding ten square degrees, forty-three nebulæ and clusters; the Nubecula Major, within an area of about fortytwo square degrees, contains 278, without reckoning fifty or sixty outlines, making an average of about six and a half to the square degree, which very far exceeds any thing that is to be met with in any other regions of the heavens. This intermixture of stars and unresolved nebulosity makes it probable that the nubiculæ are systems which resemble none in our hemisphere. Sir John Herschel has ascertained the places of 919 stars, nebulæ and clusters in the Nubecula Major, and of 244 in the Nubecula Minor, as an approximation toward a catalogue of the objects they contain. He has also fixed the places of 4,015 nebulæ or clusters, of which the southern hemisphere contains the larger portion. Each hemisphere contains about as many as the eye sees stars on an average night.

Numerous and vast as these clusters are, the distances

which separate them are yet more astounding. Imagine clusters of suns, each sun lying so far distant from the other that the eye can pass over only six such intervals in one direction, while the cluster contains from end to end hundreds of such suns lying at such intervals. Then imagine these clusters lying so widely separated from one another that they are but as handfuls of dust in space. How wide must be that universe of which man cannot comprehend a corner!

§ 89. It may assist us to realize their vast distances if we consider how long light travelling 192,000 miles a second would be in travelling from them to us. Light is one and one quarter seconds passing from the moon to the earth; eight minutes from the sun; three to twelve years from the nearest fixed stars; 140 years from the most distant stars visible to the naked eye; thousands of years traversing our cluster in its longest direction, from Aquila to Monoceros; and millions of years coming from distant clusters, a period long enough to allow important changes in the cluster from which it emanates. Thus the moon may have been dispersed into atoms for more than a second, and the sun for eight minutes, and we should still see them perfect and entire. The star a Centauri may have changed its color three years ago, and we should still see it of its former hue. The bright star Vega, must have been placed in the heavens nine years before its rays struggled to our little world; and more distant stars may have been shining for centuries yet not so long that their light has reached the earth. The light which now meets our eyes may come from stars long since quenched in darkness. A human being may be born, pass through the seven stages of life and die, while light from the smallest stars visible to the naked eye is reaching us. Nay, our whole historic period is about the length of time which light occupies coming from the nearer or cleft edge of our cluster to the earth. Thus the astronomer who records the aspect and variations of a distant nebulæ gives its history millions of years since. If the solar system and the fixed stars were called into existence at the same moment, from the earth no other body would at first have been seen.

The moon would have appeared in a second and a quarter, the sun in eight minutes, the stars would have peeped out one by one in the course of years; there would have been no field for the telescope under a century. An exact chronicle of their times of appearing would have been a perfect measure of their respective distances from us.

CHAPTER V.

INTERIOR OF OUR SIDERAL SYSTEM.

Absolute and Relative Motion. Motions of the Fixed Stars. Proper Motion of 61 Cygni, and of Arcturus. Motion of the Solar System. Investigations of Herschel, Struve, and Argelander. The Central Sun. Double and Multiple stars. New Stars. Variable Stars. Color of Stars.

§ 90. Having obtained a general idea of the form of our cluster, and of its distance from other clusters, we will now study its interior. We will inquire whether the bodies which compose it move among themselves, and whether they undergo any changes of constitution?

Of absolute motion we can know nothing. Relative motion is all it concerns us to know. But we want some fixed point to which we may refer motions; and the heavens afford us no such fixed point. The cluster of which our sun is a unit may, for all we know, be rushing on with unimaginable speed; but its suns retaining their relative position would still appear to be at rest. Our sun may be changing his place among the stars, but only centuries of observation can make his motion evident. The earth is certainly revolving round the sun and rotating on its axis. These two latter motions we perceive by reference to the stars, which do not partake of them, but how are we to ascertain any motion we have in common with the stars.

We know not whether there be in the universe one star deserving to be called absolutely fixed, and we

must be contented to refer our motion to the bodies nearest us.

§ 91. By observing and reasoning we shall be able to ascertain whether the motions discovered are entirely our own, entirely belonging to the stars, or compounded of both. We have instances of all these kinds of apparent motion in the heavens. The rising and setting of the sun and its yearly motion are entirely apparent and owing to the earth's motion. The moon's motion is made up partly of our real motion, partly of her revolution; and besides this the moon and earth share a third motion round the sun. We do not see in the solar system a motion which is not influenced, either accelerated, delayed, or changed in direction by the earth's own motion. When we inquire into the motion of the fixed stars, the earth's proper motion no longer embarrasses us; it is too small to be of the least account; but we must allow for possible motion of the sun and solar system together. We must inquire whether this motion belongs to them, or to us, or to both. We must observe whether the apparent motion is common to all the stars, whether it is in such a direction that the motion of our sun would account for it, or whether absolute and parallactic motions unite in producing the apparent motions of the stars.

§ 92. The fact that the fixed stars change place among themselves has long been suspected. Maps and observations made at intervals of fifty years differ. So that it may be affirmed with certainty that a map of the heavens, correct this year, will, after a few years, and still more after a few centuries, be found faulty. The immense distance of the stars causes their motions to appear slight, or not to appear at all to the naked eye. Millions of miles of their path subtend to our eyes not even an angle of one second only. The annual motion of sixty-one Cygni is more than a thousand millions of miles, yet we call it a fixed star. To the eye of a common observer the heavens present the same features they did thousands of years ago. But by comparison of catalogues many minute changes may be detected, and modern instruments can measure their changes from year to year.