ten feet in

The body Illustration of
Let the

the appearance of a ring like the Galaxy. We there-
fore reject the hypothesis of sphericity and try again.
Suppose that an aquarium is a circle
diameter, in which the water is a foot deep.
of water has the shape of a thin cheese.
aquarium be well stocked with minnows, and let a
single fish somewhere near the center look about him.
When he looks horizontally, no matter toward what
point of the compass, he sees a goodly number of his
companions. If he looks straight up or down he sees
comparatively few. If he looks obliquely upward or
downward he sees more fish than when he looked
straight up, and fewer than when he looked horizon-
tally. If he had an agile brain and pondered over the
matter, would he not conclude that the reason why he
saw the most fish when looking horizontally, was that
the aquarium extended farthest in that direction? The
more he studied the case the more confident would he
be that the aquarium was cheese-shaped.

Does not this illustration represent what an astrono-
mer sees when he looks about? If he looks toward the
Milky Way, which appears to surround him, he sees a
large number of stars. It has been stated that the
further he looks from the Milky Way the fewer stars
he sees.
Is it not reasonable then to suppose that the
sidereal universe occupies a space shaped somewhat like
a thin cheese or a silver dollar?

an aquarium.

The solar

But more persistent inquiry will bring out some interesting facts. Those stars whose distances from us have cluster. been measured are mostly bright, and are scattered pretty evenly in all directions from us, showing no tendency to crowd together near the Milky Way; their spectra are chiefly like the sun's spectrum. The sun therefore is a member of a cluster of stars similar to

Milky Way.

itself in composition and probably globular in form. The faint stars in and near the Milky Way are, almost

The form of the without exception, at distances which defy our powers of measurement. Of faint stars of any particular order of

Saturn on a huge scale.

brightness those
near the Milky Way
are in general
further from us than
those in other parts
of the heavens.
Shall we not say
then that most of
the stars in the
Milky Way consti-
tute a ring sur-
rounding us? Stars
whose spectra are
like that of Sirius
are very abundant
in and near the Gal-
axy, and scattered
sparsely in other
regions; this fact
has led Professor
Pickering to say
that the Milky Way
may well be regard-
ed as
"a distinct
cluster of stars, to

which, from its com

position or its age, the sun does not seem to belong." The mental picture of the stellar universe which springs from the preceding considerations rudely resembles the planet Saturn. Within is a ball of stars, of

which the sun is one. Surrounding the ball is an irregular ring composed of faint stars in and adjacent to the Milky Way. Such a theory as this cannot be considered final, but it commends itself as the best that can be devised in the light of present knowledge.

Our next inquiry is about the motion of this stupendous system; the only available light comes from a study of the movements of a great many stars scattered in all parts of the heavens. Many stars are moving slowly across the face of the sky, despite their designation of fixed stars. Star No. 1830 in Groombridge's catalogue moves a degree in five hundred years. Arcturus, which also has a large proper motion, has shifted its position by an equal amount during the Christian era. Such rapid motions are quite exceptional. If a star is moving toward us or from us, its velocity of approach or recession is obtained by spectroscopic observations; not velocity yet measured exceeds fifty miles a second. A star which is moving directly toward us, or away from us, has no proper motion," because it does not alter its position on the face of the sky.

Proper motions of stars.

Some groups

motion.

Many groups of stars have a common proper motion. Only a few out of four hundred stars in the Pleiades, whose have a common proper motions have been measured, refuse to drift along in the same direction as the others. It may almost be laid down as a principle that most of the stars in any group drift together, as though they were really connected.

A prevailing

The stars are going in all directions, so that it seems impossible to deduce any general results about their drift. movements. But patient study of large numbers of proper motions has clearly brought out a prevailing drift. Stars in Hercules and Lyra are spreading apart very slowly; those on the opposite side of the celestial sphere are coming together.

Is there a central sun?

Various systems.

Is there evidence of design?

A passenger on a ferry-boat plying between two cities at night sees lights along the wharves of each city. The lights in one set are spreading apart; in the other they are coming closer together. He knows at once that he is going toward the spreading lights. In like manner the astronomer concludes that the sun, carrying along its family of planets, is moving toward that region of the heavens which Hercules and Lyra grace. Whether the sun is moving in a straight line, or in the majestic sweep of some grand orbit, cannot yet be decided.

There is a persistent idea that there exists a central sun, about which all the starry hosts move obediently in vast cycles of time. But the motions of the stars are so complex that no one can hope to locate a point about which all bodies in the universe revolve.

There are hosts of subsidiary systems, which are orderly in their ongoings. The solar system is ruled despotically by the sun. Binary systems move in proper fashion, bound by a common tie. The stars composing a group like the Pleiades seem to be impelled toward a common goal. Thus the entire sidereal universe is composed of groups which are practically independent of one another. There is, in the present state of astronomical knowledge, no inkling of a general plan in accordance with which all the stars move.

But the design of the Creator may not involve any particular form of orderly movement which the mind of man has yet conceived. The fact that the molecules which compose a marble statue do not revolve about a common center, or move in curves whose sinuosities can be embraced in a formula, does not detract from its beauty, or argue the absence of design. The entrancing beauty shines forth, and speaks eloquently of the cunning hand of the sculptor.

CHAPTER XVIII.

THE NEBULÆ.

Regions of lucid matter taking forms,
Brushes of fire, hazy gleams."

- Tennyson.

Different

nebulæ.

NEBULA are cloud-like masses, of a great variety of form. Planetary nebulæ are small and round; they are classes of usually somewhat brighter in the center than at the edge. If there is a very marked central condensation, the object may be called a nebulous star. Annular nebulæ are ring-shaped, brighter at the edge than near the center. Spiral nebulæ exhibit coils, like those of a watch-spring, or a corkscrew. The largest nebulæ are irregular in form and enormous in extent, being the largest visible objects in the universe; they dwarf everything else into insignificance. Photographs of Orion show that a large part of the constellation is involved in a great nebula. Many clusters contain nebulous matter within their boundaries; large nebulæ often appear to shelter stars within their ample folds.

distribution.

About 8,500 are now known; new ones are being continually discovered. Photography offers a distinct Discovery and advantage for the work of discovery, since the sensitive film captures objects too faint to impress the eye. They are not scattered uniformly over the sky; near the Milky Way few are to be found. Where stars are few nebulæ abound, being most numerous near the galactic poles, as previously stated.

No one has succeeded in measuring the distance of a