d air of the mountains, but, what is far more
effective as a refrigerating cause, the aerial current is made to flow
upwards, and to ascend to a height of several thousand feet above the
sea. Both the air and the vapor contained in it, being thus relieved of
much atmospheric pressure, expand suddenly, and are cooled by
rarefaction. The vapor is condensed, and about 500 inches of rain are
thrown down annually, nearly twenty times as much as falls in Great
Britain in a year, and almost all of it poured down in six months. The
channel of every torrent and river is swollen at this season, and much
sandstone horizontally stratified, and other rocks are reduced to sand
and gravel by the flooded streams. So great is the superficial waste (or
_denudation_), that what would otherwise be a rich and luxuriantly
wooded region, is converted into a wild and barren moorland.

After the current of warm air has been thus drained of a large portion
of its moisture, it still continues its northerly course to the opposite
flank of the Khasia range, only 20 miles farther north, and here the
fall of rain is reduced to 70 inches in the year. The same wind then
blows northwards across the valley of the Brahmapootra, and at length
arrives so dry and exhausted at the Bhootan Himalaya (lat. 28 degrees
N.), that those mountains, up to the height of 5000 feet, are naked and
sterile, and all their outer valleys arid and dusty. The aerial current
still continuing its northerly course and ascending to a higher region,
becomes further cooled, condensation again ensues, and Bhootan, above
5000 feet, is densely clothed with vegetation.[264]

In another part of India, immediately to the westward, similar phenomena
are repeated. The same warm and humid winds, copiously charged with
aqueous vapor from the Bay of Bengal, hold their course due north for
300 miles across the flat and hot plains of the Ganges, till they
encounter the lofty Sikkim mountains. (See map, Chap. XVIII.) On the
southern flank of these they discharge such a deluge of rain that the
rivers in the rainy season rise twelve feet in as many hours. Numerous
landslips, some of them extending three or four thousand feet along the
face of the mountains, composed of granite, gneiss, and slate, descend
into the beds of streams, and dam them up for a time, causing temporary
lakes, which soon burst their barriers. "Day and night," says Dr.
Hooker, "we heard the crashing of falling trees, and the sou