nearly eight thousand miles in diameter. But the diameter of Jupiter is about eighty-five thousand miles. Quite enormous compared with the globe on which we live; but not at all enormous compared with the Sun.

DIFFERING LENGTHS OF THE PLANET-YEARS, MERCURY TO MARS

About eleven Earths might be strung on a massive wire and placed within Jupiter, reaching from one side to the other, at his equator. But about the same number of Jupiters might be strung on a yet more massive wire, and placed within the body of the Sun, reaching to either side. Also, something like one

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thousand Earths might be packed inside a hollow globe, the size of Jupiter. But something like one thousand Jupiters might be packed inside a greater hollow globe, the size of the Sun.

In shape this planet is much flattened at the poles,

DIFFERING LENGTHS OF THE PLANET-YEARS, JUPITER TO NEPTUNE

and much bulged out at the equator, and he spins on his axis at a fine rate, whirling completely round in less than ten earthly hours; which means that he has a day of only five hours and a night of the same, inclusive of twilight. His axis is placed very nearly upright to the “Plane of the Ecliptic," instead of sloping like

that of Earth. This would mean little or no seasonchanges. If he had seasons, such as ours, they would be very lengthy, since a single "year" on Jupiter lasts through nearly twelve of our years.

Yet this long year of his is short compared with others. For Saturn's "6 year " lasts through nearly thirty of ours; and Uranus' "year" through eightyfour of ours; and Neptune's year" through one

hundred and sixty-five of ours.

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II. STILL RATHER WARM!

Though it is right to speak of Jupiter, in a sense, as a "world," this must not be taken to mean a planet which is inhabited, or even which is fit to be inhabited by animals or by men. Jupiter is not a sun. He is not the scene of fiercelyheated and intensely-glowing gases like a star. Nevertheless, he cannot be said to have reached the calm and cooled condition of our Earth.

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We saw how it may be that Mars long ago passed through the particular stage of existence in which Earth now is. But without any may be," we can confidently say that Jupiter is far from having come anywhere near the present state of our little world, being rather in the condition which we suppose to have been ours long ages ago.

You have heard how a large heated globe must always take longer to cool than a small heated globe of the same materials. This is not difficult to prove. If you had three iron balls, one the size of a marble, one the size of an orange, one the size of a school-globe; if you

could heat them all to red-heat in a furnace and could then set them aside to cool, you would see for yourself. The small one would grow cold first; then the medium-sized one; and lastly the big one.

And in the slow cooling of stars and worlds the same is found.

Mars, being a very small globe, must have cooled far more quickly than our Earth, though not so fast as Mercury, which is smaller still. Venus is in bulk much

the same as Earth, and if we could penetrate her veil of clouds, we might find her to be at somewhere about the same stage as ourselves.

With Jupiter and the other three outer planets matters are widely different.

So far as size is concerned, Jupiter is equal to some thirteen hundred Earths. But he is so much lighter in make that, so far as weight is concerned, he is equal only to about three hundred and twenty Earths.

And the explanation is not hard to find. A body weighs more or less according to its make according to the number of tiny particles which are packed into a given space. A solid mass is heavier than a liquid mass; and a liquid mass is heavier than a vaporous or gaseous mass; because the particles lie more closely together in a solid than in a liquid, and more closely together in a liquid than in a gas or vapour.

Our Earth, being on the whole cool and solid, weighs more than another body of the same size, but of a looser make. And whatever the inside condition of Jupiter may be, the vast outer cloudy covering is not solid, is not even in the main liquid. As that huge body grows more cool it will shrink into a smaller

size; and since the materials will then be more closely pressed together, it will weigh more in proportion to its bulk.

But though so light in form, weighing hardly, if at all, more than an equal body of water, the planet's enormous size gives him a force of attraction far beyond what we know here.

If you and I could be somehow transported thither, and could stand on his surface-though to talk of standing upon masses of heated cloud and scalding steam has a curious sound!-we should find ourselves to be amazingly heavier than on Earth. In fact, it would be no question of standing, for we should be dragged downward with such force as to be compelled to lie flat, helpless and overpowered—if again one could be supposed to lie down on such a surface and not instantly to sink through to the uttermost depths.

A child, weighing here only six stone, would there weigh about fifteen stone. A man, weighing here twelve stone, would there weigh something like thirty stone. Human muscles could not manage so unwieldy a body.

Jupiter's markings are far more beautiful than any on Venus or Mars. Lovely belts of soft colour, red and salmon, varied by shades of blue and purple, relieved by pure white, and accentuated by dark or tinted or white spots, may be seen through a good telescope. In a general way the broad belts are always there, though lesser changes constantly take place in them.

They are not permanent, in the sense in which certain markings on our Moon are permanent. Instead of