the enervating
effects of tropical climate at the sea-level.

Altitude likewise affects the amount of rainfall. Most plateaus are
arid. As a rule, they are arid because of their altitude; and because of
their aridity they are deficient in their power to produce food-stuffs.
They are therefore sparsely peopled.

=Effects of Rainfall.=--Regions having considerably more than one hundred
inches of rain annually are very apt to be forest-covered, and
therefore to be deficient in food-producing plants. Such localities have
usually a sparse population, in spite of the profusion of vegetation. In
some parts of India, lands that have been left idle for a few seasons
produce such a dense jungle of wild vegetation that to reclaim them for
cultivation is wellnigh impossible.

A deficiency of rainfall is even a greater factor in restricting the
density of population than too much rain. With less than fifteen or
twenty inches a year few regions produce good crops of grains and
grasses, and as a result they are sparsely peopled. Some of the
exceptions, however, are important. If the rainfall is not quite enough
to produce a normal overflow to the sea, the soil may be very rich,
because the nutrition is not leached out and carried away.

Many small areas of this character produce enormous crops when
artificially watered, and many of them, such as Persia, parts of Asia
Minor, northern Utah, and large areas of Australia and Chile have become
regions of considerable commercial importance. The products of such
regions are apt to be unique in character and of unusual value. Thus,
the wool of Persia and Australia and the fruit of the Iberian peninsula
are important articles of commerce.

In Egypt one may see the results of irrigated lands. The area of
geographical Egypt is somewhat less than half a million square miles;
the habitable part of the country is confined to a narrow strip, which,
one or two places excepted, varies from three to six miles in width. In
other words, almost the whole population of the country is massed in the
flood-plain and delta of the Nile; the remaining part is a desert
producing practically nothing.

The water that makes these lands productive falls, not in Egypt, but in
the highlands of Abyssinia, 2,000 miles away. The September overflow of
the flood-plain is the chief factor in the irrigation of these lands,
but the area has been greatly increased by the construction of barrages
and dams at Assiut and As