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earth. When we consider the possibility of the existence of any life resembling our own, our survey is narrowed.

Mercury is very near the sun, and even if it has an atmosphere it must be intensely hot. Life as we know it could scarcely exist there.

Venus is nearly the same size as the earth.

It has an atmosphere and is not too near the sun. Life could exist on it, but its cloudladen atmosphere would in any case make it impossible for us to perceive on its surface the signs of habitation. PART OF THE MILKY WAY, SHOWING

THOUSANDS OF TELESCOPIC STARS Passing beyond

From In the High Heavens,by Sir Robert the orbit of the earth

Ball. By permission of Messrs Sir Isuac we come to Mars. Pitman and Sons, Ltd. Without discussing the evidence of the so-called 'canals,' life of a kind not too remotely resembling our

resembling our own is possible here.

Crossing the zone of the minor planets we come to Jupiter, the largest of the major planets. Jupiter and the rest have but little resemblance to the earth. They are much lighter, much larger, and probably much hotter. It is not certain that they have a solid surface,

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and they have an atmosphere of clouds that the sun's rays could scarcely penetrate.

Of the materials of which the universe is made we know something through meteorites. These are solid fragments of heavenly bodies, and may be members of the solar system, or enter it from outer space. We see them as meteors or shooting stars when they enter the earth's atmosphere. Till then they are invisible, but as they rush at immense speed through the air friction changes their energy of motion into heat and light. They exist for a moment as luminous vapour before they vanish. Most of them are very small-usually they weigh no more than a grain--but they may be the size of peas, or occasionally much larger. Now and then they enter the atmosphere slowly, escape vaporization, and reach the ground. They have been analysed, and no elements have ever been found in them which do not occur in the composition of the earth. The materials of which the universe is composed would appear, then, to be identical with those which form the substance of the earth.

As far as the earth is concerned, the most important members of the solar system are the sun and the moon.

As has been said before, it is the sun which, by the power of gravitation, holds the earth in its course. By the same power the sun and moon jointly produce those movements of the waters on the earth's surface which we call tides. The sun's heat and light are the sole source of life and energy upon the earth.

The tidal movements due to the gravitational pull exerted on the earth by the sun and moon are not confined to the oceans and seas, though it is on shores bordering shallow seas that they are most easily recognized in the familiar recurrence of high and low water,

spring and neap tides. No solid body is perfectly rigid, and it is probable that the solid earth yields to the pull as well as the water, though to a much slighter extent. Again, a solid body having yielded to a stress regains its shape when the stress is removed in proportion to its elasticity.

According as the earth, then, is perfectly or imperfectly elastic, it will recover from the deformation due to the tide-causing force.

Moreover, the earth is our time-keeper, a day of twenty-four hours being the time it takes to make one complete turn on its axis, and a year the time it takes to revolve once in its orbit round the sun. The frictional action of the tides, however, very probably prevents the earth from being an absolutely perfect time-keeper, and consequently we cannot be sure that an hour has the same duration as it had thousand or a million years ago.

Since the life of this planet depends upon the sun's heat, it is obviously a question for us how that heat is maintained.

Supposing that the nebula out of which the solar system was formed was composed of luminous gas, it is exceedingly difficult to understand how it could retain its heat for long. Its material would be so thin that it would rapidly lose its heat by radiation and cease to be luminous. This is what happens when now and then a new star appears in the sky. Something has caused the flaring up of a formerly dark and invisible bodypossibly a collision between two dead stars, or the passage of a cool star through a nebula. Great quantities of luminous gas are formed and the new star may be as brilliant as any in the sky. But the heat is soon lost, and in a few weeks the star is again invisible.

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On the other hand, supposing the nebula was composed of innumerable solid meteorites, the heat due to

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MOON CRATERS
The earth viewed from the surface of the moon

From In Starry Realms,by Sir Robert Ball

By permission of Messrs Sir Isaac Pitmangand Sons, Ltd. collision would produce a constant supply of hot and luminous gas, some cooling and condensing while fresh was formed. Again, though the sun is radiating heat, yet it maintains it by the contraction due to gravity.

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Energyin one form_namely,motion—is being converted into energy in another form-namely, heat. It has been calculated that the sun's heat could be maintained by a shrinkage of sixteen inches a day, but there may be other yet unexplored sources of heat in the sun, such as the energy which we know exists in atoms of matter and which is usually imprisoned in them, but may be released and produce heat. The maintenance of the sun's heat would be completely accounted for by the presence in it of a very small percentage of materials resembling radium and therefore called radio-active.

The moon is a satellite of the earth—or the earth and the moon may be looked upon as twin planets revolving about the sun. The moon is a dead planet. She possesses no atmosphere, no clouds, and the face she presents to us shows no sign of the action of water. All we can see is a wilderness of sharp-cut volcanoes which have long been cold. Although the mass of the moon is very small compared with that of the sun, she has more to do with the causing of our tides, the smallness of her mass being more than counteracted by her comparative nearness.

It is known that the earth is infinitely small in comparison with the universe. On the other hand, it is large in comparison with its inhabitants, and its problems are countless and great. There is no true connexion between bulk and greatness, and the infinitely small may contain the infinitely great.

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