which, throwing the superfluous rain off as it falls, collect it in streams or lakes, and guide it into given places, and in given directions; so that men can build their cities in the midst of fields which they know will be always fertile, and establish the lines of their commerce upon streams which will not fail. Sec. 7. Nor is this giving of motion to water to be considered as confined only to the surface of the earth. A no less important function of the hills is in directing the flow of the fountains and springs, from subterranean reservoirs. There is no miraculous springing up of water out of the ground at our feet; but every fountain and well is supplied from a reservoir among the hills, so placed as to involve some slight fall or pressure, enough to secure the constant flowing of the stream. And the incalculable blessing of the power given to us in most valleys, of reaching by excavation some point whence the water will rise to the surface of the ground in perennial flow, is entirely owing to the concave disposition of the beds of clay or rock raised from beneath the bosom of the valley into ranks of enclosing hills. Second use. To give motion to air. Sec. 8. The second great use of mountains is to maintain a constant change in the currents and nature of the _air_. Such change would, of course, have been partly caused by differences in soils and vegetation, even if the earth had been level; but to a far less extent than it is now by the chains of hills, which exposing on one side their masses of rock to the full heat of the sun (increased by the angle at which the rays strike on the slope), and on the other casting a soft shadow for leagues over the plains at their feet, divide the earth not only into districts, but into climates, and cause perpetual currents of air to traverse their passes, and ascend or descend their ravines, altering both the temperature and nature of the air as it passes, in a thousand different ways; moistening it with the spray of their waterfalls, sucking it down and beating it hither and thither in the pools of their torrents, closing it within clefts and caves, where the sunbeams never reach, till it is as cold as November mists, then sending it forth again to breathe softly across the slopes of velvet fields, or to be scorched among sunburnt shales and grassless crags; then drawing it back in moaning swirls through clefts of ice, and up into dewy wreaths above the snow-fields; then piercing it with strange electric darts and flashes of mountain fire, and tossing it high in fantastic storm-cloud, as the dried grass is tossed by the mower, only suffering it to depart at last, when chastened and pure, to refresh the faded air of the far-off plains. Third use. To give change to the ground.