Page:Philosophical Transactions - Volume 145.djvu/132

116 Egyptian cubits, informs us, that every cubit is divisible into fourteen parts, each of two digits, and the length of the cubit being 20⋅625 inches, we have 0⋅736 inch for each digit. It will thus be seen that the measurements of Sir G. and those of the French engineers very nearly agree, the difference being only between 20⋅75 and 20⋅625.

Upon the island of Rhoda near Cairo a Nilometer was erected more than a thousand years ago, and is the only measure of the rise and fall of the Nile referred to in the present day. It is known by the name of the (instrument of measure). For a reason that will afterwards appear, I give the description of it by the French engineers in 1800. They describe it as an octagonal pillar, having a scale divided into 16 cubits, and each cubit into 24 digits. Each cubit of this scale they found to be equal to 541 millimnetres, or 21⋅3 English inches.

Between the first entrance of the Nile into Egypt and its mouth, the mass of water must be vastly diminished from the following causes: it receives no tributary; spread over so wide a surface under a burning sun and a cloudless sky, the evaporation must be very great; the water is drawn off from the main channel by numberless canals, and there is a further absorption by infiltration through the soil, for several miles inland, on the left bank in Upper Egypt, and on both sides in Lower Egypt. Thus, while the rise of the river at the island of Rhoda on an average of years is 24 feet, near Ramanyeh about sixty-five miles in a direct line north of the apex of the Delta, the difference between highest and lowest water is about 13 feet, and at Rosetta and Damietta not more than 42 inches.

From observations made in 1799 at Siut, about midway between Assouan and Cairo, the French engineers estimated the volume of water in the Nile to be as follows:—that every second of time a mass of water passes a given line across the river equal to 678 cubic metres at low Nile, and 10,247 cubic metres at high Nile. Linant Bey states that the volumes near Cairo are 414 cubic metres at low Nile, and 9440 at high Nile. M. makes on this subject the following observations: "En partant des données qui fouirnit M.  dans son Mémoire sur la Vallée d'Egypte, j'ai cherché à calculer le produit moyen du Nil; il resulte de ce calcul, que la hauteur moyenne du Nil, d'après les quatre années d'observations certaines qui nous ont été fournies, soit par l'Expedition, soit par M., étant de 3m⋅23, correspondrait à un débit moyen de 2860 mètres cubes par seconder on d'environ 90,000 millions de mètres cubes par an." But the Rosetta branch, at its mouth, is not more than about 600 metres (656 yards) wide, and at lowest water the depth is only 1⋅60metre (5 feet 3 inches); in the Damietta branch the width is about 300 metres (328 yards), and the depth, at the same period, 2⋅50 metres (8 feet 2 inches).