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position of this atmosphere by the movement of the moon would produce a gradual extinction of the star, and not the sudden phenomenon usually observed.

The absence of air and of water from the moon, explains the peculiar and weird ruggedness of the lunar scenery. We know that on the earth the action of wind, and of rain, of frost, and of snow, is constantly tending to wear down our mountains and reduce their asperities, but no such agents are at work on the moon. Volcanoes sculptured the moon into its present condition, and, though the volcanoes have been silent for ages, the traces of their handiwork seem nearly as fresh to-day as they were when the mighty fires were extinguished.

"The cloud-capped towers, the gorgeous palaces, the solemn temples" have but a brief career on earth. It is chiefly the incessant action of water and of air that makes them vanish like the "baseless fabric of a vision." On the moon these causes of disintegration and of decay are all absent, though perhaps the changes of temperature in the transition from lunar day to lunar night would be attended with expansions and contractions that might compensate in some degree for the absence of more potent agents of dissolution.

It seems probable that a building on the moon would remain for century after century just as it was left by the builders. There need be no glass in the windows, for there is no wind and no rain to keep out. There need not be fireplaces in the rooms, for fuel cannot burn without air. Dwellers in a city in the moon would find that no dust can rise, no odours be perceived, no sounds be heard.

Man is a creature adapted for life in circumstances which are very narrowly limited. A few degrees of temperature, more or less; a slight variation in the composition of air; the precise suitability of food; make all the difference between health and sickness, between life and death. Looking beyond the moon, into the length and breadth of the universe, we find countless celestial globes with every conceivable variety of temperature and of constitution.

Amid this vast number of worlds with which space

teems, are there any inhabited by living beings? To this great question science can make no response, save this; we cannot tell. Yet it is impossible to resist a conjecture. We find our earth teeming with life in every part. We find life under the most varied conditions that can be conceived. We have life under the burning heat of the tropics; we have life in the everlasting frost at the poles. We have life in caves where not a ray of light ever penetrates. We have life in the depths of the ocean, at the pressure of tons on the square inch. Whatever be the external circumstances, some form of life can generally be found to which those circumstances are congenial.

It is not at all likely that even among the million spheres of the universe there is a single one exactly like our earth; like it in the possession of air and of water, like it in size and in composition. It does not seem probable that a man could live for one hour on any body in the universe except the earth, or that an oak-tree could live in any other sphere for a single season. Men can dwell on the earth, and oak-trees can thrive there, because the constitutions of the man and of the oak are specially adapted to the particular circumstances of the earth.

Could we obtain a closer view of some of the celestial bodies, we should probably find that they, too, teem with life, but with life specially adapted to the environment. Life in forms strange and weird; life far stranger to us than Columbus found it to be in the New World when he first landed there. Life, it may be, stranger than ever Dante described, or Doré drew. Intelligence may yet have a home among those spheres no less than on the earth. There are globes greater and globes less-atmospheres greater and atmospheres less. And the truest philosophy on this subject is crystallised in the exquisite language of Tennyson :

"This truth within thy mind rehearse,

That in a boundless universe

Is boundless better, boundless worse.

Think you this mould of hopes and fears
Could find no statelier than his peers
In yonder hundred million spheres ?"

CHAPTER IV.

THE SOLAR SYSTEM.

Exceptional Importance of the Sun and Moon-The Course to be pursued-The Order of Distance-The Neighbouring Orbs-How are they to be discriminated?-The Planets Venus and Jupiter attract notice by their brilliancySirius not a neighbour-The Planets Saturn and Mercury-Telescopic Planets -The Criterion as to whether a Body is to be ranked as a neighbour-Meaning of the word Planet-Uranus and Neptune-Comets-The Planets are Illuminated by the Sun-The Stars are not-Ths Earth is really a Planet-The four Inner Planets, Mercury, Venus, the Earth, and Mars-Velocity of the EarthThe Outer Planets, Jupiter, Saturn, Uranus, Neptune-Light and Heat received by the Planets from the Sun-Comparative Sizes of the Planets-The Minor Planets-The Planets all Revolve in the same direction-The Solar System-An Island Group in Space.

In the two preceding chapters of this work we have endeavoured to describe the heavenly bodies in the order of their relative importance to mankind. Passing on from the preliminary chapters, which principally discussed the means by which we were enabled to observe the heavenly bodies, we then proceeded to describe those objects themselves. Could we hesitate for a moment as to which of the bodies in the universe should be the first to receive our attention? We do not now allude to the intrinsic significance of the sun when compared with other bodies or groups of bodies scattered through space. It may be that the sun has many orbs rivalling it in real splendour, in bulk, and in mass. We shall, in fact, show later on in this volume that this is the case; and we shall afterwards attempt to indicate the. true rank of the sun amid the countless host of heaven. But whatever may be the importance of the sun, viewed merely as one of the bodies which teem through space, there can be no hesitation for a moment as to how immeasurably his influence on the earth surpasses that of all these bodies in the universe put together. It was therefore natural -indeed inevitable-that our first excursion into the abyss of

G

space should be to explore that mighty body which is the source of our life itself.

Nor could there be very much hesitation as to the second step which ought to be taken. The intrinsic importance of the moon, when compared with other celestial bodies, may be small; it is, indeed, we shall afterwards see, almost infinitesimal. But in the economy of our earth the moon still plays, and has played, a part second only in importance to that of the sun himself. The moon is so close to us that her brilliant rays pale to invisibility countless orbs of a size and an intrinsic splendour incomparably greater. The moon also occupies quite an exceptional position in the history of astronomy; for the greatest discovery that science has yet witnessed was accomplished by means of her motion. It was therefore natural that an early chapter in our Story of the Heavens should be devoted to a body whose interest approximated so closely to that of the sun himself.

But the sun and the moon partly described (we shall afterwards find it necessary to return to both of them), some hesitation is natural in the choice of the next step. The two great luminaries being abstracted from our view, there is no other celestial body of such exceptional interest and significance as to make it quite clear what course to pursue, in the attempt to unfold the story of the heavens in the most natural manner. Did we attempt to describe the celestial bodies in the order of their actual magnitude, our ignorance must at once pronounce the attempt to be impossible. We cannot even make a conjecture as to which body in the heavens is to stand first on the list. Even if that mightiest body be within reach of our telescopes (in itself a highly improbable supposition), we have not the least idea in what part of the heavens it is to be sought. And even if this were possible-if we were able to arrange all the visible bodies rank by rank in the order of their magnitude and their splendour-still the scheme would be impracticable, for of most of them we know little or nothing.

We are therefore compelled to adopt a different method of procedure, and the simplest, as well as the most natural, will be to follow as far as possible the order of distance of the different

bodies. We have already spoken of the moon as the nearest neighbour to the earth, we shall next consider some of the other celestial bodies which are comparatively near to us; then, as the work proceeds, we shall discuss the objects further and further away, until towards the close of the volume we shall be engaged in considering the most distant bodies in the universe which the telescope has yet revealed to us.

Even when we have decided on this principle, our course is still not free from doubt. Many of the bodies in the heavens are moving so that their distances from the earth change, but this is a difficulty which need not really detain us. We shall make no attempt to adhere closely to the principle in all its details. It will be sufficient if we first describe those great bodies-not a very numerous class-which are, comparatively speaking, in our vicinity, though still at vast and varied distances; and then we shall pass on to the almost infinitely numerous bodies which are separated from us by distances so stupendous that the imagination is baffled in the attempt to realise them.

Let us, then, scan the heavens, to choose therefrom those neighbouring orbs which are to form the material for our earliest consideration. The sun has set, the moon has not risen; a cloudless sky discloses a heaven glittering with countless gems of light. Some are grouped together into well-marked constellations; others scattered promiscuously hither and thither, with every degree of lustre, from the very brightest down to the faintest point that the eye can just discover. Amid all this host of objects, how are we to identify those which are nearest to the earth? Look to the west: the sun has but lately set, and over the spot where his departing beams still linger we see the lovely evening star shining forth. This is the planet Venus-a beauteous orb, twin-sister to the earth. The brilliancy of this planet, its rapid changes both in position and in lustre, would suggest at once that it was much nearer to the earth than other star-like objects. This presumption has been amply confirmed by careful measurements, and therefore Venus is to be included in the list of the orbs which constitute our neighbours.