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 Assuan. In the western highland region of the United States considerable areas already have been made productive by irrigation, and it is estimated that about two million acres of barren land can be reclaimed by impounding the waters of the various streams now running to waste. The distribution of rain with respect to the season in which it falls is quite as important as its distribution with respect to quantity. In tropical regions the ocean winds, and therefore the rainfall, come from the east. The eastern slopes of such regions, therefore, have a season in which rains may be expected daily, and another in which no rain falls for several months. In the temperate zones seasonal rains for a similar reason are on the western coasts. Thus on the Pacific coast of the United States the rainfall varies from about one hundred inches in southern Alaska to about twelve in San Diego, Cal. Practically all the rain falls between October and the following May; very little or none falls in the interval between May and October. As a result, ordinary turf-grass, which will not withstand long droughts, grows in only a few localities of the Pacific slope. It is replaced by hardier grasses whose roots, instead of forming turf, grow very deep in the soil.