a short time after the sun has set, at another time of year in the east for a short time before the sun rises, and it is therefore sometimes called the evening star, and at other times the morning star. Astronomers call this planet

Venus.

The sun with the earth and other planets and their moons, and a vast number of smaller bodies of like character, all of which circle round the same central body, is called the solar system; and if viewed, as I supposed just now, from some star, the whole would be seen hurrying through space, with the enormous speed of four miles in each second of time, or more than three hundred thousand miles in a day. But the real stars are so far off, that if we could watch this motion for many years, the sun and its planets would seem scarcely nearer to them than they are now.

My readers will now understand how small and insignificant a body our earth really is, when we think of it in comparison with the sun and the stars. The real stars are

all suns like our own, and some of them very much larger; but at so great a distance, that they look only like bright points; and indeed by far the greater part of them cannot be seen at all without the help of a telescope.

Let us now sum up what we have learned.

Our earth is a globe or ball that turns round or revolves on its own axis (which is only an imaginary line passing through its centre), while it travels round the sun. It turns completely round its axis once a day; and as it revolves, that half which is turned towards the sun is lighted up by it, and has daylight, while the side that is turned away from it is dark, and has night. The earth also travels round the sun once in a year, and the moon travels round the earth in the same way once in a month, while she revolves on her own axis also once a month. The same face of the moon is therefore always presented to the earth, whereas all sides are successively presented to the sun. That side of the moon which is turned towards the sun is bright, and that which is away from it is dark; so that if people could live on the moon, they would have day and night as we have, but their day

20

SUMMARY.

[CHAP. 1. would last nearly half a month, and their night another half month. There are other bodies called planets that travel round the sun like our earth, and some of them have several moons accompanying them. Were we writing about astronomy, we should have a great deal to say about these planets, but we must return to earth in the next chapter, and we shall find that there is plenty to interest us on our own earth, although it is so small in comparison with the sun and the stars. Lastly we have learned that this system of sun and planets is travelling rapidly through infinite space, and that although at great distances from each other, they are kept together by that mutual attraction, or pulling at each other which is called gravitation, and which is the same cause that keeps us standing on the earth, and causes a ball that has been thrown up into the air to fall back to the ground.

CHAPTER II.

THE ATMOSPHERE.

OUR globe is completely surrounded by a covering of air, which is called its atmosphere. That air is something, although we cannot see it, we must acknowledge, when we call to mind that we feel its pressure with every puff of wind. And when the wind is very violent, as in those great storms that pass over Bengal sometimes at the change of the monsoons, we see the effects of its power in uprooted trees, wrecked boats, houses levelled and scattered, and sometimes, most terrible of all, in destructive floods of sea-water, which the wind first piles up on the sea and then drives forward over the land, destroying and submerging everything in their path. Clearly then air is something, and at times a very powerful something.

Although air is met with everywhere on the surface of the earth, even on the tops of the highest mountains, we have no reason to believe that it extends very far from the earth; or that, for example, it fills all the space between us and the moon. Even on the lofty ridges of the Himalaya, the air is, so to speak, of thinner consistency than here on the plains; that is to say, every cubic foot of it contains a less weight of air than down here in Bengal: and one reason why it is so difficult to climb very high mountains, and that no one has ever yet reached the top of the most lofty mountains, is that the air there is so attenuated that people cannot breathe well, and soon become exhausted. We do not

22

DECREASING DENSITY WITH ELEVATION.

[CHAP.

know exactly how high we should have to mount from the earth, to be beyond the atmosphere altogether, but at a height of 50 miles or about ten times as high as the highest peak of the Himálaya, there must be so little, that if it were to blow with the speed of our great storms, we should not feel it. The atmosphere is, then, a very thin layer compared with the size of the earth. If we suppose our earth to be represented by a ball one foot across, two sheets of ordinary country paper laid on the surface, one over the other, would about represent the thickness of the atmosphere.

The reason why the lower layers of the atmosphere are more dense than the upper layers, is that, in the first place, air has weight, and in the second place that it is very compressible; by enclosing a quantity of air in a vessel, from which it cannot escape, but in which it can be squeezed or compressed, it can be made to occupy a smaller and smaller space by continually increasing the pressure upon it. The means of making the experiments to prove these facts are explained in books on physics: the facts so established are all we need attend to at present. Bearing these two facts in mind, it is easy to see why the lower layers of air are more dense than those above them. The former are pressed upon by all those above them, viz., a thickness of at least 40 miles; but as we ascend we leave the more compressed, and therefore heavier layers below us, so that at a height of about 18,000 ft. that which still remains above us has only about half the weight of the whole at the sea-level. The pressure at this height being therefore only half as great, the air there is only half as much compressed; in other words, a cubic foot of air at 18,000 feet weighs only half as much as a cubic foot of the layer on the surface of the sea. The weight of the atmosphere, or, to speak more accurately, its pressure, is measured by an instrument called the barometer.1

For a short description of the barometer, see the Glossary. The pressure of the atmosphere is not always due to its weight only. By heating air, its pressure will be increased, but not its weight.

To understand what the air does, we must first know what it consists of. It is a mixture of gases, chiefly two, which are called by chemists oxygen and nitrogen. Neither of them can be seen, because they have no colour; nor have they any smell or taste; but as I have already mentioned, we can feel them when they move in a wind, and by proper means we can separate and weigh them. These two gases are called permanent gases, because no one has ever been able to convert them into a liquid like water, still less into a solid like ice, or stone. But besides these, air always contains the vapour of water, that is to say, water in an invisible gas-like condition; but unlike oxygen and nitrogen in thisthat if cooled down sufficiently, it is reconverted into common liquid water. Water in this gaseous state is commonly spoken of as water-vapour, or simply vapour. In the introduction we spoke of the evaporation of water which takes place when a dish of it is exposed to the air. A wet cloth, for instance, if hung up in a wind, is soon dried; because the water that soaks it escapes from it in the form of vapour; and if the sun is shining upon it, or if it be hung near a fire, it dries all the faster, showing us that it is heat or warmth that changes water into vapour, while another experiment will show us that cold brings back the vapour to the state of water, and sometimes even into the solid state of snow or ice. Procure a large piece of ice; crush it, and fill a brass or tin-pot with the fragments, nearly up to the top, taking care that none of it rests on the edge. Dry the outside thoroughly, and then let it stand for a short time in a shady place. The vessel soon becomes cold, but for a minute or less, according to the state of the air at the time, it remains dry on the outside. After a time, however, the polished surface will become dulled, and if you then draw your finger over it you will find it to be wet. The water has not come through the vessel as might be supposed at first sight, but is water which existed as invisible vapour in the air around, and which has been turned into water again or condensed by the coldness of the vessel. If this experiment be repeated, some salt having previously been mixed