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say of the astronomer who discovers a universe of worlds?

Irregular nebulce are those which have no definite form. Many of them present all the irregularities of clouds torn and rent by the tempest. Some of the likenesses which may be traced by the fancy are strangely fantastic: for example, the "dumb-bell nebula" in the constellation Vulpecula, and the "crab nebula" near the southern horn of Taurus. There is also one known as " the great nebula in the sword-handle of Orion," in which may be seen a faint resemblance to the wings of a bird.

Nebulous stars are so called because they are enveloped by a faint nebula, usually of a circular form. The star is generally seen at the centre, although some which are elliptical surround two stars, one in each focus. It is thought that these may be suns possessing immense atmospheres, which are rendered visible somewhat as that of our sun is in the zodiacal light; and that in like manner our sun

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itself to those in space presents the appearance of a nebulous star. The luminous atmosphere of the star in Cygnus, if located at the distance of a Centauri, is of an extent equal to "fifteen times the distance of Neptune from the sun."

Variable nebula.—Certain changes take place among the nebiilse which can be accounted for only under the supposition that they, like some of the stars, are variaMe. Mr. Hind tells us of one in Taurus which was distinctly visible with a good telescope in 1852, but in 1862 it had vanished entirely out of the reach of a much more powerful instrument. It seems to have disappeared altogether. The great nebula in Argo, when observed by Herschel in 1838, had in the centre a vacant space containing a star of the first magnitude completely enshrouded by nebulous matter. In 1863, the nebulous matter had disappeared, and the star was only of the sixth magnitude. These facts as yet defy explanation. They only illustrate the vast and wonderful changes constantly taking place in the heavens.

Double nebulce.—There seems to be a physical connection existing between some df the nebulae, similar to that already noticed in respect to certain stars. In the case of the latter, this inter-relation has been proved, since their movements even at their distances can yet be traced in the lapse of years. "But owing to the almost infinite depths in the abyss of the heavens at which these nebulae exist, thousands of years, perhaps thousands of centuries, would be necessary to reveal any movement." (Guillemin.)

Magellanic Clouds.—Not far from the southern pole of the heavens there are two cloud-like masses, distinctly visible to the naked eye, known to navigators as " Cape Clouds." Sir John Herschel describes them as consisting of swarms of stars, clusters, and nebulae, seemingly grouped together in the wildest confusion. In the larger, he found 582 single stars, 46 clusters, and 291 nebulae.

The Milky Way —Via Lactea or Galaxy, as it is variously termed—is that luminous, cloud-like band that stretches across the heavens in a great circle. It is inclined to the celestial equator about 63°, and intersects it in the constellations Cetus and Virgo. This stream of suns is divided into two branches from a Centauri to Cygnus. To the naked eye it presents merely a diffused light; but with a powerful telescope it is found to consist of myriads of stars densely crowded together. These stars are not uniformly distributed through its entire extent. In some regions, within the space of a single square degree we can discern as many as can be seen with the naked eye in the entire heavens. In other parts there are broad open spaces. A remarkable instance of this occurs near the Southern Cross. There is a dark pear-shaped vacancy with a single bright star at the centre, glittering on the blue background of the sky. In viewing it, one is said to be impressed with the idea that he is looking through an opening into the starless depths beyond the Milky Way.

The number of stars in the galaxy which may be seen by Herschel's great reflector is estimated at twenty-one and a half millions. With the more powerful instruments now being made it is probable the number will be largely increased. The northern galactic pole is situated near Coma Berenices, and the southern in Cetus. Advancing from either pole

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toward the Milky Way, the number of stars increases, at first slowly and then more rapidly, until the proportion at the galaxy itself is thirty-fold.

Herschel's theory.—Sir W. Herschel has conjectured that the stars are not indifferently scattered through space, but are collected in a stratum something like that shown in the cut, and that our sun

Fig. 89.

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occupies a place at S, near where the stream branches. A and E are the galactic poles. It is evident that, to an eye viewing the stratum of stars in the direction SB, SC, or SD, they would seem much denser than in the direction SA or SE. Thus are we to think of our own sun as a star of the second or third magnitude, and our little solar system as plunged far into the midst of this vortex of worlds, a mere atom along that

"Broad and ample road
WhOBe dnst is gold and pavement Btarg." ?•

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