Out of this fissure the Nile has created a narrow strip of fruitful soil--the valley is not more than three or four hours in breadth on an average--which is secured by the heights on the west from the moving sand and the storms of the great desert, and is separated by the mountain on the east from the rocky crags, the desolate flats, and sandy dunes which fill up the space between the valley of the river and the Red Sea. To this valley the mighty river not only gives a refreshing coolness and moisture by the mass of its waters, it fertilises and manures it from year to year by its overflow. At the summer solstice, when the snow on the peaks of the lofty mountains, in which the two arms of the Nile take their rise, is melting, and the tropical rains fall upon its upper course, the waters of the river slowly and gradually rise. Towards the end of July it passes over the banks and overflows the whole valley as far as the enclosing lines of hills, so that towards the end of September it stands more than twenty feet above the lowest water level. Falling as gradually as it rose after more than four months it sinks back to the ordinary level. Wherever the overflowing waters have covered the land, there is left behind a fertilising mud or slime. This is the soil which the two rivers before uniting have washed from the upper hills. Carried down by the stream, it is deposited by the gentle flow of the waters on the surface of the valley. The refreshment of the earth by the inundation, the fertilisation by this slime, and the cooling of the air by the immense body of water, are the essential advantages which Egypt owes to her river, and hence, even as early as Herodotus, Egypt seemed to be the gift of the Nile. The watering of the soil and the cooling of the air just in the very hottest months of the year, are the more invaluable because the blue and gleaming sky of the upper valley is never darkened by rain clouds, while the heat is severe, and the storms from the south-west occasionally carry the sand and the dust over the Libyan hills into the Nile. In the Delta, the region along the lowest course of the Nile, showers occasionally rise from the neighbouring sea; and through eight months of the year the whole valley opening on the Mediterranean is fanned by refreshing winds from the north, which also facilitate navigation against the stream.