once more when at their nearest, in about eight hundred

You see how much nearer we are to the Sun than to outlying members of his family. And Jupiter is the closest of all the four great outer worlds.

These are the lesser gaps, dividing our Earth from her brother and sister planets. When she is on one side of the Sun, and a planet is on the further side, the gaps widen enormously.

The planet-pathways, thus described, and all the other planet-orbits in our Solar System, lie in very much the same plane. That is to say, they are placed in the heavens much as the hoops lie on the floor, tilted, indeed, a little, this way or that way, but keeping in the main nearly to one level, not slanting about in all sorts of directions.

PART IV

WHAT THE MOON IS REALLY LIKE

I.—AIR AND WATER

THE poet Wordsworth, speaking of the sky, puts one or two questions, in his serious fashion

"Is nothing of that radiant pomp so good as we have here?
Or gives a thing but small delight that never can be dear?
The silver Moon, with all her vales and hills of mightiest fame-
Doth she betray us when they're seen? or are they but a

name?"

No, she does not "betray us; " for they really are “hills and vales," or rather, mountains and ravines, of a sort; but different in kind from those of Earth.

And it is not impossible that a great deal of “that radiant pomp "--if by the term we may understand the Universe as a whole-may be not only as good as, but a great deal better than aught that "we have here." But for the Moon we can have no hesitation in answering with a decisive "No." Fair and bright though she looks, nobody need wish to exchange life on Earth for life on the Moon; at all events while his spirit inhabits his present body.

So long as he must breathe earthly air to live, he could not exist there for ten minutes. Even if, by any mechanical means, that difficulty could at some future

day be overcome, our sister-world would still be a very uncomfortable habitation.

Though the distance is a mere nothing compared with other celestial distances, very little could be known of her surface before the invention of telescopes. Just the brightness and the markings—the round, complacent face, or the man with his bundle of sticks-and a few guesses whether these might represent lands and seas; and that was nearly all. But telescopes soon made a difference.

Not only could grey plains be clearly seen, but lofty mountains also; and great circular mountain ramparts and rings; and countless volcanic craters, some huge, some small, scattered lavishly about; and curious lighter streaks, amid wide, dusky spaces.

As telescopes have been made with greater and greater powers, so our knowledge of the Moon has grown, especially in late years with the added help of photography.

The question is sometimes asked-how near are we practically brought to the Moon by our largest telescopes? How much can we really see?

This is not merely a question of the largest telescopes. It is seldom that the highest powers can be used with advantage. If they could, we might speak of" bringing the Moon" to a distance of a hundred, or even eighty miles. Actually, it can hardly be said that she can be clearly seen nearer than perhaps a hundred and twenty miles.

Even then the distance, though a mere nothing from one point of view, means a good deal with regard to eyesight. If we think of one hundred and twenty miles

side by side with-let us say, ten thousand miles— the space sinks into a mere speck; yet when we begin to consider how much and how little can be distinguished by the naked eye on Earth, at a distance of a hundred and twenty miles, matters wear another aspect.

However, photographs, taken with powerful telescopes and enlarged, do show us "structures" on the Moon not more than two or three miles in diameter. Craters of that size have been detected in numbers; which is not bad for a world two hundred and forty thousand miles away.

During many years it was looked upon as certain that no air existed on the Moon. If any atmosphere in the least like our own were there, the Moon's outline could not be so sharp and clear. It would, and more especially when seen through a telescope, have a softened and blurred look. But this never happens.

So, practically, it is correct to say that our companion has not an atmosphere, though this does not forbid the possibility of a very, very thin and slight amount of air, so thin and slight that it would not support any animal-life such as we know on Earth.

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Once upon a time our sister-globe may have had a more substantial" atmosphere; but if so it has all but vanished. She is so small in size that her power of attraction has probably been too feeble to hold her atmosphere captive. So gradually all or most of its particles have wandered far away, never to return.

Our Earth holds fast her ocean of air simply by the force of her attractive power; and here, seemingly, the Moon has failed, because she is so much smaller and lighter in make.