Below Mosul, the region begins to change its character. The mountains cease, the plain begins, the soil becomes alluvial and through the regular overflow of the two rivers in the rainy season, develops an astounding fertility. This overflow begins, in the case of the Tigris, early in March, reaches its height in May, and ceases about the middle of June. The overflow of the Euphrates extends from the middle of March till the beginning of June, but September is reached before the river resumes its natural state. Not only does the overflow of the Euphrates thus extend over a longer period, but it oversteps its banks with greater violence than does the Tigris, so that as far north as the juncture with the Khabur, and still more so in the south, the country to both sides is flooded, until it assumes the appearance of a great sea. Through the violence of these overflows, changes constantly occur in the course that the river takes, so that places which in ancient times stood on its banks are to-day removed from the main river-bed. Another important change in Southern Babylonia is the constant accretion of soil, due to the deposits from the Persian Gulf. This increase proceeding on an average of about one mile in fifty years has brought it about that the two rivers to-day, instead of passing separately into the Gulf, unite at Korna--some distance still from the entrance. The contrast of seasons is greater, as may be imagined, in Upper Mesopotamia than in the south. The winters are cold, with snowfalls that may last for several months, but with the beginning of the dry season, in May, a tropical heat sets in which lasts until the beginning of November, when the rain begins. Assyria proper, that is, the eastern side of Mesopotamia, is more affected by the mountain ranges than the west. In the Euphrates Valley, the heat during the dry season, from about May till November, when for weeks, and even months, no cloud is to be seen, beggars description; but strange enough, the Arabs who dwell there at present, while enduring the heat without much discomfort, are severely affected by a winter temperature that for Europeans and Americans is exhilarating in its influence.