— 58 — The mean yearly rainfall for a period of seven years is given as 107 mm. by Captain Lyons in his " Physiography of the Nile Basin."

As a rule the rainfall is associated with severe heat, thunderstorms and high wind ; in fact, the phenomenon has all the appearances of a small cyclone.

Occasionally the rain may continue for some hours, but generally the showers are of short duration.

Exposure.—The exposed position of the farm has accentuated the influence of desiccating winds and driven sands from the adjacent desert. A remedy is being sought by the formation of wind breaks in the shape of hedges and belts of trees. Their influence in mitigating the climatic extremes within the enclosed area is both mechanical and physiological, for not only do they break the force of the wind and check the sand drift, but the relative humidity of the atmosphere is increased and evaporation lessened by the transpiration of water vapour from the foliage.

SECTION II, The Soils of the Central Research Farm.

In the present section a brief account will be given of the main features of the farm soils. Two distinct areas are involved, representing different geological formations, with a corresponding variation in composition and texture.

The experimental plots of the farm are situated on the right bank of the river, and are of ancient alluvial origin. The soil consists of a loam containing a rather high percentage of clay. The texture is close, and the penetration of water slow. Considerable shrinkage occurs on drying, and large cracks are formed which in some cases extend to a considerable depth.

This area is representative of the sakia lands throughout Khartoum and Berber Provinces.

Tuti Island soil is of much more recent origin, and similar formations may be seen in the making every year, whenever for any reason a slackening of the current of the river occurs, with the consequent deposition of silt.

The surface layers are mainly wind-borne from the desert behind, and contain a large percentage of sand, in consequence of which the texture is much more open, and little cracking occurs on drying.

This type constitutes the most fertile soil in the Sudan, for not only is it naturally rich in soluble salts, but the formation of nitrates is vigorous, so that both the quality and the quantity of the crop exceed that obtained on the older soil.