On the other hand, the total amount of rain which falls upon the land of the globe must be enormous. It has been estimated, for example, that about sixty-eight cubic miles of water annually descend as rain even upon the surface of the British Isles, and there are many much more rainy regions. If you inquire about this rain which falls upon the land, you will find that it does not at once disappear, but begins another kind of circulation. Watch what happens during a shower of rain. If the shower is heavy, you will notice little runs of muddy water coursing down the streets or roads, or flowing out of the ridges of the fields. Follow one of the runs. It leads into some drain or brook, that into some larger stream, the stream into a river; and the river, if you follow it far enough, will bring you to the sea. Now think of all the brooks and rivers of the world, where this kind of transport of water is going on, and you will at once see how vast must be the part of the rain which flows off the land into the ocean. But does the whole of the rain flow off at once into the sea in this way? A good deal of the rain which falls upon the land must sink underground and gather there. You may think that surely the water which disappears in that way must be finally withdrawn from the general circulation which we have been tracing. When it sinks below the surface, how can it ever get up to the surface again? Yet, if you consider for a little, you will be convinced that whatever becomes of it underneath, it can not be lost. If all the rain which sinks into the ground be forever removed from the surface circulation, you will at once see that the quantity of water upon the earth’s surface must be constantly and visibly diminishing. But no such changes, so far as can be seen, are really taking place. In spite of the rain which disappears into the ground, the circulation of water between the air, the land, and the sea continues without perceptible diminution. You are driven to conclude, therefore, that there must be some means whereby the water underground is brought back to the surface. This is done by springs, which gush out of the earth, and bring up water to feed the brooks and rivers, whereby it is borne into the sea. Here, then, are two distinct courses which the rainfall takes—one below ground, and one above. It will be most convenient to follow the underground portion first.