138 Observations on Sculptured Marks on 



" In general, the velocity may be estimated at from half-a-mile 

 to two miles and three-(][uarters per hour ; but the mean velocity may 

 be reckoned at two miles per hour. In the year 1794, the late Mr 

 Rennie found the velocity of the Thames at Windsor two miles and 

 a half per hour."* 



It will thus be seen that the velocity of the Nile is probably 

 greatly inferior to that of the Thames ; for it appears that, except 

 during the inundation, for more than half the year the depth is incon- 

 siderable. The average fall when greatest, that is, including the pro- 

 vince of Batn-el-Hadjar, where the rapids chiefly occur, is considerably 

 less than that of any part of the above course of the Thames ; so that 

 there must be long intervals between the rapids where the fall must 

 bo far less than 13 inches in a mile. The breadth of the Nile is 

 vastly greater ; but supposing the depth of the water to be the same 

 as that of the Thames, on account of the friction of the bed, the 

 greater breadth would add very little to the velocity. If we assume 

 the average depth of the Thames in the above distance to be 5 feet, 

 and that it flows with an average velocity of 2 miles in an hour, and 

 if we assume the average depth of the Nile in that part of its course 

 where the fall is 13-12 inches to be 10 feet, when not swollen by 

 the rise, the velocity would be 2| miles nearly in an hour,j" if the 

 fall were equal to that of the Thames. We shall probably come near 

 the truth, by assuming the velocity of the Nile on this part at 2 

 miles in an hour. That it must be considerably less in the other 

 divisions of the course I have named, and especially in that part im- 

 mediately below the second cataract, where the average fall is only 

 5*30 inches for a distance of 96 miles, is quite evident. 



The power of a river to abrade the soil over which it flows, so far 

 as water is by itself capable of doing so, must depend upon its 

 volume and velocity, and the degree of hardness of the material 

 acted upon. The power is increased when the water has force enough 

 to transport hard substances. But even transported gravel has little 

 action on the rocks with which it comes in contact, when it is free 

 to move in running water, unless the fall be considerable, and, con- 

 sequently, the velocity and force of the stream great. When stones 

 are firmly set in moving ice, they then acquire a great erosive power, 

 cutting and wearing down the rocks they are forcibly rubbed against ; 

 but this condition never obtains in Lower Nubia, as ice is unknown 

 there. 



Geological Structure of Lower Nubia. 

 One kind only of regularly stratified rock occurs in the 776 miles 



* Rennie, Report on Hydraulics, in the Fourth Report of the British As- 

 sociation for the Advancement of Science, 1834, p. 487. 



t I state this on the authority of my friend, W. Hopkins, Esq., of Cambridge. 



