e rest, covered
with rolling waves or drifting sands, fretted with ice or crested with
fire, is set before us for contemplation in an uninhabitable
magnificence. And that part which we are enabled to inhabit owes its
fitness for human life chiefly to its mountain ranges, 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.

[22] I attach great importance to the remaining contents of this
passage, and have had occasion to insist on them at great length in
recent lectures at Oxford.

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 reservoirs 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 dispositions of the beds of clay or rock raised from
beneath the bosom of the valley into ranks of enclosing hills.

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 difference in soil 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 in a thousand different states; 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 col