sing 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.

Sec. 9. The third great use of mountains is to cause perpetual change in
the _soils_ of the earth. Without such provisions the ground under
cultivation would in a series of years become exhausted and require to
be upturned laboriously by the hand of man. But the elevations of the
earth's surface provide for it a perpetual renovation. The higher
mountains suffer their summits to be broken into fragments and to be
cast down in sheets of massy rock, full, as we shall see presently, of
every substance necessary for the nourishment of plants: these fallen
fragments are again broken by frost, and ground by torrents, into
various conditions of sand and clay--materials which are distributed
perpetually by the streams farther and farther from the mountain's base.
Every shower which swells the rivulets enabl