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BY ARABELLA BUCKLEY
Arabella Burton Buckley (1840–1929 ): An English author and naturalist. She
has written several books on scientific subjects for young people, — among them are
“Winners in Life’s Race,” “Life and her Children,” and “The Fairy-Land of Science,” from which this selection is taken.
Although we never see any water traveling from our earth up into the skies, we know that it goes there, for it comes down again in rain, and so it must go up invisibly. But where does the heat come from which makes this water invisible? Not from below, but from above, pouring down from the sun. Wherever the sun-waves touch the rivers, ponds, lakes, seas, or fields of ice and snow upon our earth, they carry off invisible water vapor. They dart down through the top layers of the water, and shake the water particles forcibly apart, and the drops spread themselves out in the gaps between the air atoms of the atmosphere.
It has been calculated that in the Indian Ocean threequarters of an inch of water is carried off from the surface of the sea in one day and night; so that as much as twenty-two feet, or a depth of water about twice the height of an ordinary room, is silently and invisibly lifted up from the whole surface of the ocean in one year. It is true that this is one of the hottest parts of the earth, where the sun-waves are most active; but even in our own country many feet of water are drawn up in the summer time.
What, then, becomes of all this water? Let us follow it as it struggles upward to the sky. We see it in our imagination, firstcarrying layer after layer of air up with it from the sea, till it rises far above our heads, and above the highest mountains. Now the air atoms are always trying to fly apart, and are onlykept pressed together by the weight of the air above them, and so, as this water-laden air rises, its particles, no longer so much pressed together, begin to separate; as all work requires anexpenditure
of heat, the air becomes colder, and then you know at once what must happen to the invisible vapor—it will form into tiny waterdrops, like the steam from the kettle.
And so, as the air rises and becomes colder, the vapor gathers into visible masses, and we can see it hanging in the sky and call it clouds. When these clouds are highest, they are about ten miles from the earth; but when they are made of heavy drops, and hang low down, they sometimes come within a mile of the ground.
Look up at the clouds as you go home, and think that the water of which they are made has all been drawn up invisibly through the air. Not, however, necessarily here where we live, for air travels as wind all over the world, and so these clouds may be made of vapor collected in the Atlantic Ocean, or in the Gulf of Mexico, or even, if the wind is from the north, of chilly particles gathered from the surface of Greenland ice and snow and brought here by the moving currents of air. Only, of one thing we may be sure, that they come from the water of our earth.
Sometimes, if the air is warm, these water particles may travel a long way without ever forming into clouds; and on a hot, cloudless day the air is often very full of invisible vapor. Then, if a cold wind comes sweeping along, high up in the sky, and chills this vapor, it forms into great bodies of water-dust clouds, and the sky is overcast.
At other times, clouds hang lazily in a bright sky, and these show us that just where they are the air is cold, and turns the invisible vapor rising from the ground into visible water-dust, so that exactly in those spaces we see it as clouds. Such clouds form often on a warm, still, summer’s day, and they are shaped like masses of wool, ending in a straight line below. They are not merely hanging in the sky, they are really resting upon a tall column of invisible vapor which stretches right up from the earth; and that straight line under the clouds marks the place where the air becomes cold enough to turn this invisible vapor into visible drops of water.
And now, suppose that, while these or any other kinds of clouds are overhead, there comes along either a very cold wind or a wind full of vapor. As it passes through the clouds it makes them very full of water, for, if it chills them, it makes the water-dust draw more closely together; or, if it bring a new load of water-dust, the air is fuller than it can hold. In either case, water particles are set free, and our fairy force “cohesion
” seizes upon them at once and forms them into large waterdrops. Then they are much heavier than the air, and so they can float no longer, but down they come to the earth ina shower of rain.
There are other ways in which the air may be chilled, and rain made to fall, as, for example, when a wind laden with moisture strikes against the cold tops of mountains. Thus the Khasia Hills in India, which face the Bay of Bengal, chill the air which crosses them on its way from the Indian Ocean. The wet winds are driven up the sides of the hills, the air expands, and the vapor is chilled, and, forming into drops, falls in torrents of rain. The country on the other side of these hills gets hardly any rain, for all the water has been taken out of the air before it comes there.
In this way, from different causes, the water of which the sun has robbed our rivers and seas comes back to us, after it has traveled to various parts of the world, floating on the bosom ofthe air. But it does not always fall straight back into the rivers and seas again; a large part of it falls on the land, and has to trickle down slopes and into the earth, in order to get back to its natural home, and it is often caught on its way before it can reach the great waters.
Go to any piece of ground which is left wild and untouched, you will find it covered with grass, weeds, and other plants: ifyou dig up a small plot, you will find innumerable tiny roots creeping through the ground in every direction. Each of these roots has a spongelike mouth, by which the plant takes up water. Now, imagine raindrops falling on this plot of ground and sinking into the earth. On every side they will find rootlets thirsting to drink them in, and they will be sucked up as if by tiny sponges, and drawn into the plants and up the stems to the leaves. Here they are worked up into food for the plants, and only if the leaf has more water than it needs, some drops may escape at the tiny openings under the leaf, and be drawn up again by the sun-waves as invisible vapor into the air.
Again, much of the rain falls on hard rock and stone, where it cannot sink in, and then it lies in pools till it is shaken apart again into vapor and carried off in the air. Nor is it idle here even before it is carried up to make clouds. We have to thank this invisible vapor in the air for protecting us from the burning heat of the sun by day, and intolerable frost by night.
Let us for a moment imagine that we can see all that we know exists between us and the sun. First, we have the fineether
across which the sunbeams travel, beating down upon our earth with immense force, so that in the sandy desert they are like a burning fire. Then we have the coarser atmosphere ofoxygen and nitrogen atoms hanging in this ether and bending the minute sun-waves out of their direct path. But they do very little to hinder them on their way, and this is why in very dry countries the sun’s heat is so intense. The rays beat down mercilessly, and nothing opposes them. Lastly, in damp countries, we have the larger but still invisible particles of vapor hanging about among the air atoms. Now, these watery particles, although they are very few— only about one twenty-fifth part of the whole atmosphere—do hinder the sun-waves.For they are very greedy of heat, and, though the light-waves pass easily through them, they catch the heat-waves and use them to help themselves to expand. And so, when there is invisible vapor in the air, the sunbeams come to us deprived of some of their heat-waves, and we can remain in the sunshine without suffering from the heat.
This is how the water vapor shields us by day, but by night it is still more useful. During the day our earth and the air near it have been storing up the heat which has been poured down on them, and at night when the sun goes down all this heat begins to escape again. Now, if there were no vapor in the air, this heat would rush back into space so rapidly that the ground would become cold and frozen, even on a summer’s night, and all but the most hardy plants would die. But the vapor, which formed a veil against the sun in the day, now forms a still more powerful veil against the escape of the heat by night. It shuts in the heat-waves, and only allows them to make their way slowly upwards from the earth—thus producing for us the soft, balmy nights of summer and preventing all life being destroyed in the winter.