Now, although we never see any water travelling 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, as in the case of the kettle, 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-vapour.  They dart down through the top layers of the water, and shake the water-particles forcibly apart; and in this case the drops fly asunder more easily and before they are so hot, because they are not kept down by a great weight of water above, as in the kettle, but find plenty of room to spread themselves out in the gaps between the air-atoms of the atmosphere.

Can you imagine these water-particles, just above any pond or lake, rising up and getting entangled among the air-atoms?  They are very light, much lighter than the atmosphere; and so, when a great many of them are spread about in the air which lies just over the pond, they make it much lighter than the layer of air above, and so help it to rise, while the heavier layer of air comes down ready to take up more vapour.

In this way the sun-waves and the air carry off water everyday, and all day long, from the top of lakes, rivers, pools, springs, and seas, and even from the surface of ice and snow.  Without any fuss or noise or sign of any kind, the water of our earth is being drawn up invisibly into the sky.

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 22 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 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 upwards 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.  But now, call to mind what happens to the air as it recedes from the earth.  Do you not remember that the air-atoms are always trying to fly apart, and are only kept pressed together by the weight of air above them?  Well, so this water-laden air rises up, its particles, no longer so much pressed together, begin to separate, and as all work requires an expenditure of heat, the air becomes colder, and then you know at once what must happen to the invisible vapour, —­ it will form into tiny water-drops, like the steam from the kettle.  And so, as the air rises and becomes colder, the vapour gathers into the 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.