The inundation of the Nile gives renewed life every year to those plains of Egypt which it has slowly formed, and so it is with the Tigris and Euphrates. Lower Mesopotamia is entirely their creation, and if the time were to come when their vivifying streams were no longer to irrigate its surface, it would very soon be changed into a monotonous and melancholy desert. It hardly ever rains in Chaldaea.[18] There are a few showers at the changes of the season, and, in winter, a few days of heavy rain. During the summer, for long months together, the sky remains inexorably blue while the temperature is hot and parching. In winter, clouds are almost as rare; but winds often play violently over the great tracts of unbroken country. When these blow from the south they soon lose their warmth and humidity at the contact of a soil which, but a short while ago, was at the bottom of the sea, and is, therefore, in many places still strongly impregnated with salt which acts as a refrigerant.[19] Again, when the north wind comes down from the snowy summits of Armenia or Kurdistan, it is already cold enough, so that, during the months of December and January, it often happens that the mercury falls below freezing point, even in Babylonia. At daybreak the waters of the marshes are sometimes covered with a thin layer of ice, and the wind increases the effect of the low temperature. Loftus tells us that he has seen the Arabs of his escort fall benumbed from their saddles in the early morning.[20] It is, then, upon the streams, and upon them alone, that the soil has to depend for its fertility; all those lands to which they never reach are doomed to barrenness and death. It is fortunate for the prosperity of the country through which they flow, that the Tigris and Euphrates swell and rise annually from their beds, not indeed like the Nile, almost on a stated day, but ever in the same season, about the commencement of spring. Without these periodical floods many parts of the plain of Mesopotamia would be beyond the reach of irrigation, but their regular occurrence allows water to be stored in sufficient quantities for use during the months of drought. To obtain the full advantage of this precious capital, the inhabitants must, however, take more care and expend more labour than is necessary in Egypt. The rise of the Euphrates and of the Tigris is neither so slow nor so regular as that of the Nile. The waters do not spread so gently over the soil, neither do they stay upon it so long;[21] since they have been abandoned to themselves as they are at present, a great part of them are lost, and, far from rendering a service to agriculture, they turn vast regions into dangerous hot-beds of infection.