watered, others having long seasonal periods of drought followed
by periodical rains. It would appear, too, the uncertainty of rainfall
seemed to increase rather than diminish, for in the valley of the
Euphrates, as well as in the valley of the Nile, the inhabitants were
forced to resort to artificial irrigation for the cultivation of their
crops.

It is not known at what time the Chaldeans began to build their
artificial systems of irrigation, but it must have been brought about
by the gain of the population on the food supply, or perhaps an
increased uncertainty of rainfall. At any rate, the irrigation works
became a systematic part of their industry, and were of great size and
variety. It took a great deal of engineering skill to construct
immense ditches necessary to control the violent floods of the
Euphrates and the Tigris. So far as evidence goes, the irrigation was
carried on by the gravity system, by which canals were built from
intakes from the river and extended throughout the cultivated district.
In Egypt for a long time the periodical overflow of the Nile brought in
the silt for fertilizer and water for moisture. When the flood
subsided, seed was planted and the crop raised and harvested. As the
population spread, the use of water for irrigation became more general,
and attempts were made to distribute its use not only over a wider
range of territory but more regularly throughout the seasons, thus
making it possible to harvest more than one crop a year, or to develop
diversified agriculture. The Egyptians used nearly all the modern
methods of procuring, storing, and distributing water. Hence, in these
centres of warm climate, fertile land, and plenty of moisture, the
earth was made to yield an immense harvest, which made it possible to
support a large population. {154} The food supply having been
established, the inhabitants could devote themselves to other things,
and slowly developed the arts and industries.

_Civilization in Mesopotamia_.--The Tigris and Euphrates, two great
rivers having their sources in mountain regions, pouring their floods
for centuries into the Persian Gulf, made a broad, fertile valley along
their lower courses. The soil was of inexhaustible fertility and easy
of cultivation. The climate was almost rainless, and agriculture was
dependent upon artificial irrigation. The upper portion of this great
river valley was formed of undulating plains stretching away to the
nort