— 54 — be remembered that the latter instrument is very much affected by wind, for the evaporating surface is small, and is surrounded by an atmosphere which is greedy for moisture, whereas the tank is surrounded on all sides by a water surface, the atmosphere above which is highly charged with water vapour, and consequently possesses a much smaller evaporating power.

A point of considerable interest to the agriculturist is that evaporation is at its highest when the river is at its lowest ; consequently the demand for water set up by the crops and the cost of pumping are at a maximum during the same period. This fact will largely determine the nature and the scope of agricultural effort, for the crop must be so chosen, that, ceteris paribus, its period of greatest water requirement coincides with the time when the cost of pumping is as near a minimum as possible.

Influence of Vegetation on Evaporation.—The influence of vegetation on evaporation is well recognised, and little need be said in explanation of the accompanying diagram beyond a description of the general arrangement of the instruments in the experiment it is intended to illustrate.

The position of the evaporimeters at the farm and the desert screens has already been indicated. A third Piche Bvaporimeter was placed on the south side of a patch of wheat, one metre from the ground, exposed to sun and wind.

At this period of the year the wind blows constantly from the north, so that the air which impinged on the evaporating disc of the instrument had first passed over an area covered with transpiring plants. As will be seen the effect was very marked, and gives some idea of the reduction in evaporation which would take place, were any extensive agricultural operations undertaken in this region.

Evaporation and the Movement of Water in the Soil.—The most convincing proof of the difficulties attendant on a study of the evaporation of >vater from the soil is that very few figures exist which can be quoted with any degree of reliability.

Numerous determinations have been made during the past three years of the moisture content of the farm soils, but the figures do not readily lend themselves to an accurate estimation of the losses due to evaporation from the soil alone. In some cases the plots were under cultivation, so that much of the loss of water was due to the transpiring plants of the crop. The figures, however, will serve to indicate some of the changes in water content which occur during the season in cultivated soil similar to that of the farm.

In June, 1912, sixty-three moisture determinations were made over an area of fifty-five feddans, the samples being taken from depths of 16 cm., 45 cm. and 75 cm. The whole of the ground had lain fallow for several months, during the hottest and driest part of the year.