Page:Philosophical Transactions - Volume 145.djvu/125

Rh the rock on which they may rest had not been ascertained, I determined to make an effort to have the experiment made, even to the limited extent within my means, as the results thus obtained might lead the way to other researches on a greater scale.

The ground upon which I hoped to be able to form a chronometric scale by which the total depth of sediment reached might be measured, was the same as that on which the French engineers in 1800 had proceeded, viz. the accumulation of Nile sediment around monuments of a known age. Certain works of art of a very early age exist near the Nile, the approximate dates of whose erection have been established upon reliable evidence; and we know also that the sediment has accumulated to a considerable height above their base. If that depth of sediment be divided by the number of centuries that have elapsed since the date of the erection of the monument, we obtain a scale of the secular increase of which the base of the monument is the zero, assuming, as we are entitled to do, that the average increase from century to century has been uniform within an area of some extent. If the excavation be continued below the base stone, and the sediment passed through exhibits similar characters as to composition with that above the base line of the monument, it would be fair to apply the same graduation below the zero-point of the scale as above it; and, if we reached so far, we should be able to estimate the time that has elapsed since the first layer of sediment was deposited on the rock forming the channel over which the water spread when it first flowed northward from its source in the interior of Africa; subject, however, to correction for causes that might make a difference in the rate of increase between earlier and later periods; an investigation of which causes forms a necessary, but a very difficult part of such an inquiry.

I submitted my project to the President and Council of the Royal Society, stating, that it appeared to me to be a scientific inquiry of sufficient importance to justify my asking for a grant from the Donation Fund under their control, for the purpose of defraying the expense of the proposed excavations. My proposal was favourably received, and the Council were pleased to place a liberal grant of money at my disposal.

I have thought it advisable before entering upon the narrative of the researches carried on towards the accomplishment of the object of this inquiry, to give a brief sketch of the physical geography of Egypt and of its geological structure; and a somewhat more detailed account of the annual inundations of the Nile and of the sediment it deposits.

Egypt is separated from Nubia by a low hilly region, fifty miles in breadth from north to south, which is a part of a range extending from the Red Sea in an east and