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 three- quarters 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 hottestparts 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, first carrying 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 only kept 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 an expenditure1 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 chilly1 Expenditure: laying out; spending.
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.