The Beauties of Nature, and the Wonders of the World We Live In Part 20

There is scarcely a form which the fancy of Man has not sometimes detected in the clouds,--chains of mountains, splendid cities, storms at sea, flights of birds, groups of animals, monsters of all kinds,--and our superst.i.tious ancestors often terrified themselves by fantastic visions of arms and warriors and battles which they regarded as portents of coming calamities. There is hardly a day on which Clouds do not delight and surprise us by their forms and colours. They belong, however, to our Earth, and I must now pa.s.s on to the heavenly bodies.

[Ill.u.s.tration: THE MOON.

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THE MOON

The Moon is the nearest, and being the nearest, appears to us, with the single exception of the Sun, the largest, although it is in reality one of the smallest, of the heavenly bodies. Just as the Earth goes round the Sun, and the period of revolution const.i.tutes a year, so the Moon goes round the Earth approximately in a period of one month. But while we turn on our axis every twenty-four hours, thus causing the alternation of light and darkness--day and night--the Moon takes a month to revolve on hers, so that she always presents the same, or very nearly the same, surface to us.

Seeing her as we do, not like the Sun and Stars, by light of her own, but by the reflected light of the Sun, her form appears to change, because the side upon which the Sun s.h.i.+nes is not always that which we see. Hence the "phases" of the Moon, which add so much to her beauty and interest.

Who is there who has not watched them with admiration? "We first see her as an exquisite crescent of pale light in the western sky after sunset.

Night after night she moves further and further to the east, until she becomes full, and rises about the same time that the Sun sets. From the time of full moon the disc of light begins to diminish, until the last quarter is reached. Then it is that the Moon is seen high in the heavens in the morning. As the days pa.s.s by, the crescent shape is again a.s.sumed. The crescent wanes thinner and thinner as the Moon draws closer to the Sun. Finally, she becomes lost in the overpowering light of the Sun, again to emerge as the new moon, and again to go through the same cycle of changes."[66]

But although she is so small the Moon is not only, next to the Sun, by far the most beautiful, but also for us the most important, of the heavenly bodies. Her attraction, aided by that of the Sun, causes the tides, which are of such essential service to navigation. They carry our vessels in and out of port, and, indeed, but for them many of our ports would themselves cease to exist, being silted up by the rivers running into them. The Moon is also of invaluable service to sailors by enabling them to determine where they are, and guiding them over the pathless waters.

The geography of the Moon, so far as concerns the side turned towards us, has been carefully mapped and studied, and may almost be said to be as well known as that of our own earth. The scenery is in a high degree weird and rugged; it is a great wilderness of extinct volcanoes, and, seen with even a very moderate telescope, is a most beautiful object.

The mountains are of great size. Our loftiest mountain, Mount Everest, is generally stated as about 29,000 feet in height. The mountains of the Moon reach an alt.i.tude of over 42,000, but this reckons to the lowest depression, and it must be remembered that we reckon the height of mountains to the sea level only. Several of the craters on the Moon have a diameter of 40 or 50--one of them even as much as 78--miles. Many also have central cones, closely resembling those in our own volcanic regions. In some cases the craters are filled nearly to the brim with lava. The volcanoes seem, however, to be all extinct; and there is not a single case in which we have conclusive evidence of any change in a lunar mountain.

[Ill.u.s.tration: Fig. 50.--A group of Lunar Volcanoes.]

The Moon, being so much smaller than the earth, cooled, of course, much more rapidly, and it is probable that these mountains are millions of years old--much older than many of our mountain chains. Yet no one can look at a map of the Moon without being struck with the very rugged character of its mountain scenery. This is mainly due to the absence of air and water. To these two mighty agencies, not merely "the cloud-capped towers, the gorgeous palaces, the solemn temples," but the very mountains themselves, are inevitable victims. Not merely storms and hurricanes, but every gentle shower, every fall of snow, tends to soften our scenery and lower the mountain peaks. These agencies are absent from the Moon, and the mountains stand to-day just as they were formed millions of years ago.

But though we find on our own globe (see, for instance, Fig. 21) volcanic regions closely resembling those of the Moon, there are other phenomena on the Moon's surface for which our earth presents as yet no explanation. From Tycho, for instance, a crater 17,000 feet high and 50 miles across, a number of rays or streaks diverge, which for hundreds, or in some cases two or three thousand, miles pa.s.s straight across plains, craters, and mountains. The true nature of these streaks is not yet understood.

THE SUN

The Sun is more than 400 times as distant as the Moon; a mighty glowing globe, infinitely hotter than any earthly fiery furnace, 300,000 times as heavy, and 1,000,000 times as large as the earth. Its diameter is 865,000 miles, and it revolves on its axis in between 25 and 26 days.

Its distance is 92,500,000 miles. And yet it is only a star, and by no means one of the first magnitude.

The surface of the Sun is the seat of violent storms and tempests. From it gigantic flames, consisting mainly of hydrogen, flicker and leap.

Professor Young describes one as being, when first observed, 40,000 miles high. Suddenly it became very brilliant, and in half an hour sprang up 40,000 more. For another hour it soared higher and higher, reaching finally an elevation of no less than 350,000 miles, after which it slowly faded away, and in a couple of hours had entirely disappeared.

This was no doubt an exceptional case, but a height of 100,000 miles is not unusual, and the velocity frequently reaches 100 miles in a second.

The proverbial spots on the Sun in many respects resemble the appearances which would be presented if a comparatively dark central ma.s.s was here and there exposed by apertures through the more brilliant outer gases, but their true nature is still a matter of discussion.

During total eclipses it is seen that the Sun is surrounded by a "corona," or aureola of light, consisting of radiant filaments, beams, and sheets of light, which radiate in all directions, and the true nature of which is still doubtful.

Another stupendous problem connected with the Sun is the fact that, as geology teaches us, it has given off nearly the same quant.i.ty of light and heat for millions of years. How has this come to pa.s.s? Certainly not by any process of burning such as we are familiar with. Indeed, if the heat of the Sun were due to combustion it would be burnt up in 6000 years. It has been suggested that the meteors, which fall in showers on to the Sun, replace the heat which is emitted. To some slight extent perhaps they do so, but the main cause seems to be the slow condensation of the Sun itself. Mathematicians tell us that a contraction of about 220 feet a year would account for the whole heat emitted, and as the present diameter of the Sun is about 860,000 miles, the potential store of heat is still enormous.

To the Sun we owe our light and heat; it is not only the centre of our planetary system, it is the source and ruler of our lives. It draws up water from the ocean, and pours it down in rain to fill the rivers and refresh the plants; it raises the winds, which purify the air and waft our s.h.i.+ps over the seas; it draws our carriages and drives our steam-engines, for coal is but the heat of former ages stored up for our use; animals live and move by the Sun's warmth; it inspires the song of birds, paints the flowers, and ripens the fruit. Through it the trees grow. For the beauties of nature, for our food and drink, for our clothing, for our light and life, for the very possibility of our existence, we are indebted to the Sun.

What is the Sun made of? Comte mentioned as a problem, which it was impossible that man could ever solve, any attempt to determine the chemical composition of the heavenly bodies. "Nous concevons," he said, "la possibilite de determiner leurs formes, leurs distances, leurs grandeurs, et leurs mouvements, tandis que nous ne saurions jamais etudier par aucun moyen leur composition chimique ou leur structure mineralogique." To do so might well have seemed hopeless, and yet the possibility has been proved, and a beginning has been made. In the early part of this century Wollaston observed that the bright band of colours thrown by a prism, and known as the spectrum, was traversed by dark lines, which were also discovered, and described more in detail, by Fraunhofer, after whom they are generally called "Fraunhofer's lines."

The next step was made by Wheatstone, who showed that the spectrum formed by incandescent vapours was formed of bright lines, which differed for each substance, and might, therefore, be used as a convenient mode of a.n.a.lysis. In fact, by this process several new substances have actually been discovered. These bright lines were found on comparison to coincide with the dark lines in the spectrum, and to Kirchhoff and Bunsen is due the credit of applying this method of research to astronomical science. They arranged their apparatus so that one-half was lighted by the Sun, the other by the incandescent gas they were examining. When the vapour of sodium was treated in this way they found that the bright line in the flame of soda exactly coincided with a line in the Sun's spectrum. The conclusion was obvious; there is sodium in the Sun. It must, indeed, have been a glorious moment when the thought flashed upon them; and the discovery, with its results, is one of the greatest triumphs of human genius.

The Sun has thus been proved to contain hydrogen, sodium, barium, magnesium, calcium, aluminium, chromium, iron, nickle, manganese, t.i.tanium, cobalt, lead, zinc, copper, cadmium, strontium, cerium, uranium, pota.s.sium, etc., in all 36 of our terrestrial elements, while as regards some others the evidence is not conclusive. We cannot as yet say that any of our elements are absent, nor though there are various lines which cannot as yet be certainly referred to any known substance, have we clear proof that the Sun contains any element which does not exist on our earth. On the whole, then, the chemical composition of the Sun appears closely to resemble that of our earth.

THE PLANETS

The Syrian shepherds watching their flocks by night long ago noticed--and they were probably not the first--that there were five stars which did not follow the regular course of the rest, but, apparently at least, moved about irregularly. These they appropriately named Planets, or wanderers.

Further observations have shown that this irregularity of their path is only apparent, and that, like our own Earth, they really revolve round the Sun. To the five first observed--Mercury, Venus, Mars, Jupiter, and Saturn--two large ones, Ura.n.u.s and Neptune, and a group of minor bodies, have since been added.

The following two diagrams give the relative orbits of the Planets.

[Ill.u.s.tration: Fig. 51.--Orbits of the inner Planets.]

MERCURY

It is possible, perhaps probable, that there may be an inner Planet, but, so far as we know for certain, Mercury is the one nearest to the Sun, its average distance being 36,000,000 miles. It is much smaller than the Earth, its weight being only about 1/24th of ours. Mercury is a shy though beautiful object, for being so near the Sun it is not easily visible; it may, however, generally be seen at some time or other during the year as a morning or evening star.

[Ill.u.s.tration: Fig. 52.--Relative distances of the Planets from the Sun.]

VENUS

The true morning or evening star, however, is Venus--the peerless and capricious Venus.

Venus, perhaps, "has not been noticed, not been thought of, for many months. It is a beautifully clear evening; the sun has just set. The lover of nature turns to admire the sunset, as every lover of nature will. In the golden glory of the west a beauteous gem is seen to glisten; it is the evening star, the planet Venus. A week or two later another beautiful sunset is seen, and now the planet is no longer a glistening point low down; it has risen high above the horizon, and continues a brilliant object long after the shades of night have descended. Again a little longer and Venus has gained its full brilliancy and splendour. All the heavenly host--even Sirius and Jupiter--must pale before the splendid l.u.s.tre of Venus, the unrivalled queen of the firmament."[67]

Venus is about as large as our Earth, and when at her brightest outs.h.i.+nes about fifty times the most brilliant star. Yet, like all the other planets, she glows only with the reflected light of the Sun, and consequently pa.s.ses through phases like those of the Moon, though we cannot see them with the naked eye. To Venus also owe we mainly the power of determining the distance, and consequently the magnitude, of the Sun.

THE EARTH

Our own Earth has formed the subject of previous chapters. I will now, therefore, only call attention to her movements, in which, of course, though unconsciously, we partic.i.p.ate. In the first place, the Earth revolves on her axis in 24 hours. Her circ.u.mference at the tropics is 24,000 miles. Hence a person at the tropics is moving in this respect at the rate of 1000 miles an hour, or over 16 miles a minute.

But more than this, astronomers have ascertained that the whole solar system is engaged in a great voyage through s.p.a.ce, moving towards a point on the constellation of Hercules at the rate of at least 20,000 miles an hour, or over 300 miles a minute.[68]

But even more again, we revolve annually round the Sun in a mighty orbit 580,000,000 miles in circ.u.mference. In this respect we are moving at the rate of no less than 60,000 miles an hour, or 1000 miles a minute--a rate far exceeding of course, in fact by some 100 times, that of a cannon ball.

How few of us know, how little we any of us realise, that we are rus.h.i.+ng through s.p.a.ce with such enormous velocity.

MARS

To the naked eye Mars appears like a ruddy star of the first magnitude.

It has two satellites, which have been happily named Phobos and Deimos--Fear and Dismay. It is little more than half as large as the Earth, and, though generally far more distant, it sometimes approaches us within 35,000,000 miles. This has enabled us to study its physical structure. It seems very probable that there is water in Mars, and the two poles are tipped with white, as if capped by ice and snow. It presents also a series of remarkable parallel lines, the true nature of which is not yet understood.

THE MINOR PLANETS

A glance at Figs. 51 and 52 will show that the distances of the Planets from the Sun follow a certain rule.