— 57 — The surface of the soil is, moreover, covered with a thick mulch of wind-borne sand, which, being air dry, serves as an effective barrier against the loss of subsoil water by evaporation. Hence the relatively high water content in the intermediate layers.

The level of the water in the wells around the farm naturally varies with the state of the river, but the water table has not been known to rise higher than 5 metres from the surface of the ground during the last three years.

The rate and amount of the rise varies with the distance of the well from the river, and the permeability of the intervening soil, but the maximum height is always attained some time after the Nile flood has reached its highest point.

Indeed the water level in unused wells is often higher than the river, when the latter has begun to subside.

Humidity.—As the influences of the humidity factor is most clearly demonstrated by the character of crop growth, little comment is called for at this point. Graphs showing the daily variations of the relative humidity throughout the year will be given when considering the physiology of the cotton plant.

It will be sufficient to point oixt that for nine months in the year the relative humidity is very low, and that only during the months of July, August and September does the percentage of moisture in the atmosphere attain to that degree usually associated with agriculture in the tropics.

The great variability from day to day is also remarkable for, although this phenomenon also exists in Egypt, it is not found in the cotton-growing districts of the United States.

Wind and Rainfall.—Owing to the extreme flatness of the vast expanse of the Sahara Desert, and its slight elevation, very little disturbing influence is imposed on the general atmospheric movements. The result is that the prevailing winds are very regular in direction and strength.

A definite monsoon effect is experienced over the Central and Northern Sudan during the months of June, July, August, September and part of October, during which time the southerly trade winds, deflected into a southwesterly direction by the rotation of the earth, bring in their train a greatly increased humidity, and, in these latitudes, a light rainfall. For the rest of the year the north-easterly trade winds blow without interruption, and are remarkably constant in strength.

Khartoum lies on the extreme edge of a rainbelt whose precipitation decreases as one proceeds north from the equator, and as there are no elevated areas to influence the regularity of this decrease, the rainfall becomes a negligible factor so far as agriculture is concerned.