In the world of life we see how animals are sustained by a constant series of chemical changes in their blood, every respiration of air adding, as it were, fresh fuel to the flame of life within. In plants, too, there is an analogous process. The atmospheric air is by them decomposed, part of it being given off again, and part retained to build up the organic structure. Plants withdraw mineral matter from the soil, animals feed upon plants, and thus the earthy substances, after having formed a part, first of rock masses, then of vegetable, and subsequently of animal organizations, are returned again to the soil, whence to be once more withdrawn and undergo new cycles of mutation. But this perpetual interchange is not confined to the vital world. We see it in the action of winds, when heated air rises and moves in one direction, and the colder parts sink and travel the opposite way. The same principle is exhibited by the oceanic currents, the removal of a body of water, from whatever cause, always necessitating the ingress of a corresponding quantity to supply its place. But perhaps one of the most beautiful instances of these interchanges in the whole inorganic world is the ceaseless passage of water from the land to the sea, and from the sea to the land. The countless thousands of rivulets, and streams, and gigantic rivers, that are ever pouring their waters into the great deep, do not in the least raise its level or diminish its saltness. And why? Simply because the sea gives off by evaporation as much water as it receives from rain and rivers. The vapour thus exhaled ascends to the upper regions of the atmosphere, where it forms clouds, and whence it eventually descends as rain. The larger part of the rain probably falls upon the ocean, but a considerable amount is nevertheless driven by winds across the land. This finds its way into the streams, and so back again to the sea, only, however, to be anew evaporated and sent as drizzling rain across the face of land and sea. This interchange is constantly in progress, and seems to have been as unvarying during past ages. But the ceaseless passage of water between land and sea is not a mere isolated and independent phenomenon. Like all the rest of Nature's processes, even the simplest, it produces important and complicated effects. And the reader may, perhaps, think it worth looking at for a little, when he reflects that to this seemingly feeble cause we owe no small part of our solid lands, whether as islands wasted by the sea, or as part of vast and variegated continents, wide rolling prairies covered with verdure and roamed over by herds of cattle, or wintry Alpine hills lifeless and bare.