At Addis Ababa in the highlands of Abyssinia, the mean monthly temperature ranges between 19° and 15°, with a mean of 17°. The winds are south-east and east through the year. In 1902 the rainfall was 980 millimetres, and in 1903 it was 1340 millimetres. ([Table 75]).
At Wad Medani in the Grezireh south of Khartoum, the mean monthly temperature ranges between 35° and 24°, with a mean of 30°. The winds from October to April are from the north and from May to September from the south. In 1902 there were 350 millimetres of rain and in 1903 there were 310. ([Table 76]).
At Khartoum the mean monthly temperature ranges between 34° and 19°, with a mean of 28°. The winds from October to April are from the north and from May to September from the south. In 1902 there were 120 millimetres of rain and in 1903 there were 70. ([Table 77]).
For Alexandria, Cairo and Assuân, representing the Nile valley in Egypt, I have prepared the following table:—
| Month | ALEXANDRIA | CAIRO | ASSUAN | |||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| THERMOMETER Centigrade. | Rain- fall. | THERMOMETER Centigrade. | Rain- fall. | THERMOMETER Centigrade. | Rain- fall. | |||||||
| Max. | Min. | Mean | Max. | Min. | Mean | Max. | Min. | Mean | ||||
| MILL. | MILL. | MILL. | ||||||||||
| January | 25·0 | 5·4 | 24·1 | 54 | 26·6 | -0·7 | 12·4 | 6 | 32·5 | 4·0 | 14·8 | .. |
| February | 29·7 | 6·2 | 14·8 | 22 | 35·3 | 1·2 | 14·2 | 3 | 37·0 | 5·0 | 22·0 | .. |
| March | 37·0 | 5·5 | 16·1 | 17 | 41·2 | 3·2 | 16·9 | 5 | 42·0 | 8·0 | 24·3 | .. |
| April | 38·9 | 11·0 | 18·5 | 2 | 42·6 | 5·7 | 20·9 | 2 | 46·6 | 11·0 | 27·3 | .. |
| May | 38·9 | 13·3 | 21·3 | 13 | 44·2 | 9·0 | 24·4 | 2 | 46·0 | 17·0 | 29·6 | .. |
| June | 39·4 | 13·8 | 24·0 | .. | 45·2 | 13·7 | 27·3 | .. | 47·0 | 20·0 | 34·2 | .. |
| July | 37·0 | 20·5 | 26·1 | .. | 44·3 | 17·4 | 28·5 | .. | 46·0 | 23·0 | 33·6 | .. |
| August | 35·0 | 20·3 | 26·6 | .. | 41·6 | 16·5 | 27·7 | .. | 46·0 | 21·0 | 35·1 | .. |
| September | 40·0 | 18·7 | 25·6 | .. | 40·6 | 14·0 | 25·3 | .. | 47·0 | 17·0 | 31·0 | .. |
| October | 37·8 | 15·0 | 23·7 | 9 | 42·1 | 12·1 | 23·2 | 1 | 42·0 | 18·0 | 28·7 | .. |
| November | 32·2 | 10·8 | 20·0 | 38 | 33·6 | 3·5 | 18·1 | 6 | 40·0 | 11·0 | 23·1 | .. |
| December | 28·9 | 6·8 | 16·0 | 80 | 29·4 | 1·3 | 14·4 | 6 | 34·0 | 6·0 | 19·0 | .. |
| Year | 40·0 | 5·5 | 20·6 | 235 | 45·2 | -0·7 | 21·1 | 31 | 47·0 | 4·0 | 26·9 | 0 |
From the above table we may conclude that at Alexandria, Cairo and Assuân the absolute maximum thermometers may be taken as 40°, 45° and 47° Centigrade; or 104°, 113°, 116°, Farenheit. The minimum thermometers as 5·5°, -0·7°, 4·0° Centigrade; or 42°, 31°, 39° Farenheit. The rainfall at Alexandria, Cairo and Assuân respectively may be taken as 235, 31, and 0 millimetres; or 9, 11⁄4, and 0 inches. The heaviest rainfall in any individual year at Alexandria and Cairo respectively has been 308 and 55 millimetres; or 12 and 2 inches. The lightest rainfall at Alexandria and Cairo respectively in any individual year has been 108 and 7 millimetres; or 41⁄4 and 1⁄4 inches. Assuân is practically rainless. It does rain sometimes at Assuân, but there has been no rain during the last three years while meteorological observations have been taken.
7. The Geology of the Nile Valley.
—South of Gondokoro along the Victoria and Albert Niles, and at the lakes, the rocks are generally granites, crystalline schists and quartzites. The hills of Uganda are covered with red clay and marl on the higher lands, while the valleys consist of a rich black loam. All the cataracts are granites and granitic rocks or diorites. The Ruenzori range consists of lofty volcanoes. The surface of the ground is covered with a fine Kankar (nodulated limestone) in many places. North of Gondokoro the plains are formed of sandy deposits mixed with coarse peat in places. The hills of the Bahr-el-Gazelle and Arab river are all crystalline. Abyssinia is a volcanic plateau. It is the detritus of this rich volcanic soil swept down by the Blue Nile and Atbara which constitutes the richness of the soil of Egypt and of the water of the Nile. Those parts of the eastern Sudan south of Khartoum and El-Damer and at Kassala, which are the deltas of the Blue Nile, the Dinder, the Rahad, the Atbara, and the Gaäsh, are possessed of a soil in every sense similar to that of Egypt itself. At Khartoum and in the bed of the Blue Nile at Kamlin are extensive deposits of nodular limestone corresponding to the Kankars of India.
The main Nile from Khartoum to Assuân flows between low hills and tables of Nubian sandstone overlying crystalline rocks of gneiss, mica schists, hornblendic granite and red granite. Where the crystalline rocks come to the surface we have cataracts; where the Nubian sandstone is at the surface we have reaches of unbroken water.
From Assuân[4] to near Edfu the Nile flows between hills of Nubian sandstone, the best known of which is Gebel Silsila. From Edfu to near Luxor, the Nubian sandstone which overlies the crystalline rocks dips under the Nile and its place along the Nile Valley is taken by green and grey clays containing nitrate and phosphate deposits. The former are inexhaustible and have constituted the manure of this part of the valley for thousands of years. With these deposits are thick banks of soft white limestone.