— 51 The simplest and most effective plan is to measure the combined effect as expressed by the evaporating power of the air with a standard instrument, which, in spite of many shortcomings, may at least be employed to give comparative results when exposed to different physical conditions.

The instrument used throughout this work has been the Pich6 Evaporimeter, carrying papers with an effective evaporating surface of 11 "2 square millimetres. All the instruments were standardised one with another, but no deductions can be drawn from the readings as to the evaporation from a free water surface. In order that comparison may be made with the Wild Evaporimeter, the figures should be multiplied by the factor 0'7.

At both screens tubes were placed inside, where they were sheltered from the direct heating effect of the sun, and also outside, where full exposure was obtained to sun and wind. Obssrvations were read at 8 a.m.

It is unfortunate that th6re are not more data available from other countries, as the excessive evaporation at Khartoum would thus be more strongly emphasised. In explanation of the following comparative table it may be explained that the low readings recorded at Cairo are largely due to the influence of the cultivated land of the iJelta, while Gambeila and Mongalla lie in the southern portion of the Sudan, and enjoy a much greater rainfall than the regions around Khartoum.

Evaporation as Recorded at certain Agricultural Centres Piehe Evaporimeter.

Millimetres per 24 hours, Station.