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. Ineither case, water particles are set free, and our fairy force "cohesion1" 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 in a 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 of the 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: if you dig up a small plot, you will find innumerable tiny roots1 Cohesion: the law of nature by which the particles of a body are held together.
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 fine ether1 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 of oxygen 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 in1 Ether: a medium in all space, through which light and heat pass readily.
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.