The theory of a lowering of the bed of the river by wearing, involves two main considerations, viz., the power of the stream, and the degree of hardness of the rocks acted upon. The power depends upon the volume and velocity of the river—the velocity on its depth, and the degree of inclination of the bed: the hardness of the rocks we can form a tolerable estimate of when we know their nature. To judge, therefore, of the probability of the hypothesis of Professor Lepsius, we must inquire into the physical and geological features of the Nile Valley, in Nubia.
In the observations I have now to offer, my information has been derived of course entirely from the works of other travellers, particularly those of Burckhardt, Rüppell, and Russegger,[55] and especially the latter, who travelled in Nubia in 1837; for he not only enters far more into the details of the natural history of the country, but he is the only traveller in Nubia who appears, from previous acquirements, to have been competent to describe its natural history with any degree of accuracy—I refer more particularly to the physical and geological features of the country. Besides full descriptions in his volumes, he has given a geological map of Nubia, and also several sections, or what may more properly be called vertical sketches—a term that would, perhaps, be a more appropriate designation for all sections that are not drawn to a true scale, or at least when the proportion of height to horizontal distance is not stated.
[51] Bericht über die zur Bekantmachung geeigneten Verhandlungen der Königl. Preuss. Akademie der Wissenshaften zu Berlin. Aus dem Jahre 1844.
[52] The breadth of the river itself. See Letter to Hr. Böckh, p. 27.
[53] See Plate I.
[54] Miss Martineau's Eastern Life, vol. i., p. 99.
[55] Reisen in Europa Asien und Afrika, in der Jahren 1835, bis 1841.—Stuttgart 1841-1846.
The Physical Geography of Lower Nubia.[56]
Russegger informs us,[57] that he believes he was the first traveller who had succeeded in making a series of barometrical measurements along the Nile Valley, from the Mediterranean to Sennaar and Kordofan, and thence to the 10th degree of north latitude. He gives the following altitudes, above the sea:—
| Paris Feet. | English Feet. | ||
| The upper part of the Cataract of Assuan, | 342 | = | 364·37 |
| Korusko, on the right bank of the Nile, in Nubia, | 450 | = | 479·43 |
| Wadi-Halfa, | 490 | = | 522·00 |
| New Dongola, | 757 | = | 806·52 |
| Abu Hammed, | 963 | = | 1026·00 |