he
neighbouring sea. Here and there detached mountainous masses produce a
singular local increase in the amount of the rainfall. Thus in the
lake district in northwestern England the rainfall on the seaward side
of mountains, not over four thousand feet high, is very much greater
than it is on the other slope, less than a score of miles away. These
local variations are common all over the world, though they are but
little observed.

In general, the central parts of continents are likely to receive much
less rainfall than their peripheral portions. Thus the central
districts of North America, Asia, and Australia--three out of the five
continental masses--have what we may call interior deserts. Africa has
one such, though it is north of the centre, and extends to the shores
of the Mediterranean and the Atlantic. The only continent without this
central nearly rainless field is South America, where the sole
characteristic arid district is situated on the western slope of the
Cordilleran range. In this case the peculiarity is due to the fact
that the strong westerly setting winds which sweep over the country
encounter no high mountains until they strike the Andean chain. They
journey up a long and rather gradual slope, where the precipitation is
gradually induced, the process being completed when they strike the
mountain wall. Passing over its summit, they appear as dry winds on
the Pacific coast.

Even while the winds frequently blow in from the sea, as along the
western coast of the Americas, they may come over water which is
prevailingly colder than the land. This is characteristically the case
on the western faces of the American continent, where the sea is
cooled by the currents setting toward the equator from high latitudes.
Such cool sea air encountering the warm land has its temperature
raised, and therefore does not tend to lay down its burden of
moisture, but seeks to take up more. On this account the rainfall in
countries placed under such conditions is commonly small.

By no means all the moisture which comes upon the earth from the
atmosphere descends in the form of rain or snow. A variable, large,
though yet undetermined amount falls in the form of dew. Dew is a
precipitation of moisture which has not entered the peculiar state
which we term fog or cloud, but has remained invisible in the air. It
is brought to the earth through the radiation of heat which
continually takes place, but which is most effective