GEOGRAPHICAL ASPECTS OF THE NILE LYONS. 485 



which reduce that from the equatorial plateau to an almost constant 

 supf)ly of about 12,000 or 14,000 cubic feet per second, while the 

 Abyssinian rivers, on the other hand, must furnish about half a 

 million cubic feet per second in a high flood. 



In the majority of rivers the volume of water which they discharge 

 increases from a very small amount near their sources to a maximum 

 near the point where they empty themselves into the sea or an inland 

 lake. With the Nile it is quite otherwise ; not only has its volume a 

 very marked seasonal change of volume, but from point to point of 

 its course it varies greatly, now increasing and now decreasing as local 

 conditions affect it. The maximum is reached Avhen the Atbara joins 

 the Nile, and from this point onward the volume diminishes by evapo- 

 ration, seejDage, and by the utilization of the water by the agricultural 

 poj^ulation of F^gypt. The whole of this sui:)ply is furnished by a 

 seasonal rainfall, which oscillates during the year from about 15° 

 south of the equator to 15° north of it; in January British Central 

 Africa receives its heaviest rainfall; in Uganda, the spring maximum 

 occurs in April and May, while during July and iVugust CO per cent 

 of the year's rain falls on the Abyssinian Plateau ; in the autumn the 

 rain belt travels southward, again passing the equator about Novem- 

 ber and reaching its southern limit in January. Thus the equatorial 

 plateau has two rainy seasons and two dry seasons, while the Abys- 

 sinian Plateau and the Sudan plains have a single rainy season in the 

 summer months. North of Berber practically no rain falls, and that 

 which the northern part of the delta, receives in winter does not reach 

 the river. 



The supply effectively furnished to the river at different points by 

 this rainfall is shown in figure 1, where each division of the vertical 

 scale represents a discharge of 35,300 cubic feet (1,000 cubic meters) 

 per second. Each of the discharge curves south of Dueim depends 

 on a few measurements only, but the diagrams for these stations have 

 been corrected as far as possible with the aid of the daily gauge read- 

 ings; those for Dueim, Khartoum, Atbara, and Wadi Haifa are from 

 numerous actual measurements. The flood of the year 1903, which 

 these diagrams represent, was in volume 11 per cent below an average 

 flood at Aswan. 



The diagram is exceedingly instructive, for we see that, while the 

 change of level of the Victoria Lake makes but little difference to the 

 discharge, a very marked seasonal variation occurs at the Murchison 

 Falls, where the low-stage volume is about 21,000 cubic feet per second, 

 compared with 56,000 in flood, and this increase is due to the July- 

 November rainfall on the northern edge of the plateau, since up- 

 stream of Foweira the water level of the Victoria Nile varies but little. 

 At Wadelai the level varies with that of the Albert Lake, and the dis- 

 charge shows a maximum toward the end of the year. At Gondo- 



