GEOGEAPHICAL ASPECTS OF THE NILE LYONS. 489 



more water to the Sobat and Blue Nile, and these two variable factors 

 in the low-stage supply supplement efficiently the volume of the 

 White Nile; if the rains have been weak or have ceased earlier than 

 usual, the springs will diminish early, and the Sobat and Blue Nile 

 will give but little help in the early months of the year. It is under 

 these conditions, when the flood of the previous summer has been 

 deficient and the low-stage levels are abnormally low, that the rain- 

 storms, which occasionally break on the Abyssinian plateau from 

 November to March, are of inestimable value. Their importance has 

 hardly been generally recognized as yet by those interested in Egyp- 

 tian agriculture, but the investigation of the meteorological conditions 

 on which these storms depend is being actively prosecuted, and ob- 

 servations from the Nile basin and the surrounding countries are 

 being studied. The winter of 1906-7 furnishes an instance of the im- 

 portance of these winter rains, for after a flood which was 30 per 

 cent below the average, rain in February and March increased the 

 volume of the Blue Nile so as to raise the river level sufficiently to 

 convert what would otherwise have been a very deficient summer 

 supply into one which was sufficient for all needs. 



Having indicated the geographical phenomena which characterize 

 the Nile Basin as a whole, the peculiarities of certain portions of 

 the river system may be examined. 



On the equatorial plateau, where rain falls during the greater part 

 of the year, vegetation grows everywhere luxuriantly, and man can, 

 with little exertion, cultivate as much as is necessary for subsistence ; 

 streams flow in every valley, and habitations are not necessarily 

 restricted to certain areas. Under such conditions of a moderately 

 high temperature and plentiful precipitation, the river system does 

 not influence the distribution of animal and vegetable life, nor the 

 habits of the inhabitants, to the same extent that it does in more arid 

 regions. 



On leaving the plateau, however, and following the Nile north- 

 ward, we soon leave behind us the protracted rainy season, and on 

 the plains which stretch away from the foothills rain falls from 

 May to September, Avhile the winter half of the year is almost dry. 

 The annual amount of precipitation, which near the foothills reaches 

 50 inches, decreases rapidly as we go northward, and at the mouth 

 of the Sobat (lat. 9° 30' N.) does not exceed 30 inches, and we find 

 that near the Bahr el Jebel the three conditions of a warm climate, 

 an almost level country, and a moderate rainfall, followed by a dry 

 season lasting for six months, have caused the peculiar cliaracter of 

 this portion of the basin. A ridge of granite and gneiss of no great 

 height extends in a northwesterly direction from about Wadelai, and 

 down its northern slope flow the streams which empty themselves 

 into marshes occupying much of the low ground ; to the eastward the 



