previously
held in suspension by the warmer of the two is given out, and if it be
in sufficient abundance is precipitated in the form of rain.

As the temperature of the atmosphere diminishes gradually from the
equator towards the pole, the evaporation of water and the quantity of
rain diminish also. According to Humboldt's computation, the average
annual depth of rain at the equator is 96 inches, while in lat. 45
degrees it is only 29 inches, and in lat. 60 degrees not more than 17
inches. But there are so many disturbing causes, that the actual
discharge, in any given locality, may deviate very widely from this
rule. In England, for example, where the average fall at London is
24-1/2 inches, as ascertained at the Greenwich Observatory, there is
such irregularity in some districts, that while at Whitehaven, in
Cumberland, there fell in 1849, 32 inches, the quantity of rain in
Borrowdale, near Keswick (only 15 miles to the westward), was no less
than 142 inches![262] In like manner, in India, Colonel Sykes found by
observations made in 1847 and 1848, that at places situated between 17
degrees and 18 degrees north lat., on a line drawn across the Western
Ghauts in the Deccan, the fall of rain varied from 21 to 219
inches.[263] The annual average in Bengal is probably below 80 inches,
yet Dr. G. Hooker witnessed at Churrapoonjee, in the year 1850, a fall
of 30 inches in 24 hours, and in the same place during a residence of
six months (from June to November) 530 inches! This occurred on the
south face of the Khasia (or Garrow) mountains in Eastern Bengal (see
map, Chap. XVIII.), where the depth during the whole of the same year
probably exceeded 600 inches. So extraordinary a discharge of water,
which, as we shall presently see, is very local, may be thus accounted
for. Warm, southerly winds, blowing over the Bay of Bengal, and becoming
laden with vapor during their passage, reach the low level delta of the
Ganges and Brahmapootra, where the ordinary heat exceeds that of the
sea, and where evaporation is constantly going on from countless marshes
and the arms of the great rivers. A mingling of two masses of damp air
of different temperatures probably causes the fall of 70 or 80 inches of
rain, which takes place on the plains. The monsoon having crossed the
delta, impinges on the Khasia mountains, which rise abruptly from the
plain to a mean elevation of between 4000 and 5000 feet. Here the wind
not only encounters the col