Imágenes de páginas
[blocks in formation]

1. THE earth is surrounded, as we all know, by air, and it is to this air, which rests on the world and rises for many miles into the sky, like a great ocean, that the name atmosphere, or "circle of air" is given. We do not see it, indeed, but when the wind blows, that is, when the air moves more or less swiftly, we both feel it and see its effects.

2. The power of the atmosphere when thus in motion is, indeed, sometimes wonderful. It raises the sea into great waves; it roots up or breaks-across trees; it throws down whatever stands in its way, and spreads desolation alike on sea and land. Everyone has heard of a West Indian tornado or hurricane, which is a great wind-storm on the land, and few have not read of a cyclone, which is a terrible wind-storm, most commonly at sea.

3. Air was long thought to be an element, that is a substance which could not be separated into anything more simple. But this was a mistake, for it is a mixture of two

gases-oxygen and nitrogen, with a very small quantity of a third, called carbonic acid gas. There is, besides, a large quantity of water in the air, in the form of vapour, that is, steam. There is about one part of oxygen to four of nitrogen, but carbonic acid gas supplies not more than about one part in 2,500. Oxygen means, that which produces acids; Nitrogen means, that which produces nitre.

4. These gases are so perfectly mixed, by a law which governs all gases, that every part of the air is exactly the same, and the whole is thus suited to our wants, in every part of the world.

There is a difference, however, in one way, in the air near the surface of low-lying parts, and that which we meet as we ascend mountains. The weight of the immense body of air above the earth presses the lower air into much less bulk than the higher, and we are so made as to need this pressed air for our breathing. But as we ascend mountains, or if we go up in a balloon, there is, of course, less weight of air pressing on that which we breathe, and it thus expands and becomes thinner and lighter. The consequence is that when we get very high up, it is not possible to take enough oxygen into our lungs at a time, and the whole body, especially the head, suffers for want of it. At a certain height, indeed, the air becomes too thin to breathe.

5. The weight of the air pressing on us at the level of the sea is no less than fourteen pounds on every square inch, and what this means, you may judge, by the pressure on the body of an average man being about fifteen tons. This pressure, however, does not hurt us, and indeed, we do not feel it, because there is as much from within our bodies, outward, as from outside, inwards, and the two balance each other exactly.


1. THE vapour or watery steam in the atmosphere, is supposed to be not less than a fiftieth part of the whole weight of the ocean of air above us.

2. This vapour rises from the earth by what is called evaporation, or turning into vapour or steam. This is constantly going on from every part of the surface both of land and sea.

If you put water in a saucer and set it in the open air, the saucer will be dry in a day or two. The heat of the sun acting through the air turns it into vapour, and carries it off into the skies, to help to make clouds.

3. It is, indeed, necessary for life that there be a quantity of watery vapour in the air. If there were none, we should be dried up and die. It is because they dry the air so much that close stoves are so hurtful in rooms, and the terrible thirst that torments travellers in hot parched deserts, is because there is not enough vapour in the air to make up for the loss of water from our bodies by perspiration.

4. It is also from the greater quantity of watery yapour in the air in cold climates than in hot, that the nights in cold countries are much warmer, in proportion, than those in hot countries. The vapours prevent the heat received by the earth from the sun through the day from radiating, or raying off, into space, and thus keep it warm.

5. The quantity of water raised from the whole earth in a year, in the shape of vapour, or steam, may be judged by the quantity of rain, dew, snow, hail, mist, &c., which descends from the skies in the same period over the whole world. Whatever comes down had to be first carried up.

The whole surface of land and sea is constantly giving off vapour. This rises and forms clouds. These fall in one form or other, and form brooks, rivulets, and rivers; and these, in their turn, flow back to the sea.

Thus there is a constant circle of rising and falling waters over the whole world, and that you may judge the better how great it is, I may tell you that the vapour which rises from the Atlantic alone, in a single year, would make a mass of water thirty miles high, broad, and deep!

6. You may ask how the vapour rising from land and sea

Let me, in answer,

ask you if


turns again to water. ever noticed how the outside of a cold tumbler gets dim in a heated room? The water bottle very often grows quite moist, as you know, on your dinner table. The reason is that the air of the room is full of vapour, some of which, coming in contact with the glass, is condensed, or thickened, by the cold of the glass, and, as it were, shrinks together, so that you can see it in the shape of very small drops of moisture, which first dim the glass, and, after a time, join and trickle down its sides.

In the same way, the vapour from the earth meets, in the sky, a cold wind, which condenses it, first into thick mist, or cloud, till it becomes too heavy to float in the air: and then it comes down as mist, snow, hail, or rain.


1. SOME parts of the world have little or no rain. Thus, a shower is very rare in Peru or in Upper Egypt. The Andes catch the clouds from the west in the one case, and the extreme heat of the air prevents the formation of clouds in the other. This last cause prevents rain, nearly altogether, in the great deserts of warm regions. Showers are almost unknown, for example, in the Great Sahara.

2. But where winds laden with vapour drift against cold mountain peaks, or meet colder winds, or where the country is covered with forests, which prevent the heat of the earth from escaping and thus heating the air, and also offer countless surfaces of leaves on which the vapours may condense, there is a more or less copious rainfall.

3. The quantity of rain which falls on the Khasia Hills, on the north-east of India, is not less, when up to the usual quantity, than fifty feet a year. An officer complained to me that one year only forty-two feet had fallen.

4. Even at Stye Head Pass, in Lancashire, the common depth of rain-fall in a year is from eighteen to nineteen feet. This is caused by the winds from the Atlantic, laden

with moisture, losing it when they drift against the Cumberland and other hills.

5. Snow is vapour frozen in the air by cold winds. A snow flake, I may say, is a very beautiful object when seen through a microscope; for it is made up of lovely six-sided crystals of ice, of a great many exquisite shapes.

6. Snow is much lighter than rain, and hence a foot deep of snow, when it melts, forms only a little over an inch deep of water. In a country like ours, snow is always moist, by being only slightly frozen; but in cold countries, like Canada, it is a dry powder, which makes a sound under the feet.

7. Can you tell me why snow is white, and not colourless, like water?

It is because it offers so many surfaces to the light, that it breaks it up, and mixes all the rays of different colours, till they shine to us in the pure whiteness of unbroken or perfect beams of light. It is this, also, that makes the whiteness of the foam on the waves of the sea.

8. Snow is of great use, as a mantle to wrap in the heat of the earth, and thus keep it warm. This preserves the life of plants and seeds during winter in cold countries.

It also supplies different kinds of gases to the plants, &c., which they require for food.

9. Hailstones are caused by the freezing together of raindrops in the sky, when a sudden very cold wind strikes a cloud laden with moisture. They are sometimes very large. In 1832 a mass of ice about a yard in length, and two feet thick, fell in Hungary.


WHEN the cold surface of a mountain condenses the moisture of the clouds that come against it into dew, mist, or rain, the drops run down the stalks of grass and soak into the ground. After a time, a crack in the rock beneath offers them a channel, and becomes, as it were, a pipe

« AnteriorContinuar »