METEOROLOGICAL EFFECTS OF IRRIGATION. 371 
primitive ages, tlie precipitation of winter in these hilly coun¬ 
tries was, in great part, retained for a time in the superficial 
soil, first by the vegetable mould of the forests, and then by 
the artificial arrangements I have described. The water im¬ 
bibed by the earth was partly taken up by direct evaporation, 
partly absorbed by vegetation, and partly carried down by 
infiltration to subjacent strata which gave it out in springs at 
lower levels, and thus a fertility of soil and a condition of the 
atmosphere were maintained sufficient to admit of the dense 
population that once inhabited those now arid wastes. At 
present, the rain water runs immediately off from the surface 
and is carried down to the sea, or is drunk up by the sands of 
the wadis, and the hillsides which once teemed with plenty 
are bare of vegetation, and seared by the scorching winds of 
the desert. 
In Southern Europe, in the Turkish Empire, and in many 
other countries, a very large proportion of the surface is, if not 
absolutely flooded, at least thoroughly moistened by irrigation, 
a great number of times in the course of every season, and this, 
especially, at periods when it would otherwise be cpiite dry, 
and when, too, the power of the sun and the capacity of the 
air for absorbing moisture are greatest. Hence it is obvious 
that the amount of evaporation from the earth in these coun¬ 
tries, and, of course, the humidity and the temperature of both 
the soil and the atmosphere in contact with it, must be much 
affected by the practice of irrigation. The cultivable area of 
Egypt, or the space accessible to cultivation, between desert 
and desert, is more than seven thousand square statute miles. 
Much of the surface, though not out of the reach of irrigation, 
lies too high to be economically watered, and irrigation and 
cultivation are therefore confined to an area of five or six thou¬ 
sand square miles, nearly the whole of which is regularly and 
constantly watered when not covered by the inundation, ex¬ 
cept in the short interval between the harvest and the rise of 
the waters. For nearly half of the year, then, irrigation adds 
five or six thousand square miles, or more than a square equa¬ 
torial degree, to the evaporable surface of the Nile valley, or. 
