The average per diem for the three hottest months of the year, according to these observations, is ·0107, as against ·01 deduced from observations of the levels of Lake Qurûn. The figure given for July in the above list is higher than I should have expected, as the evaporation of that month I should estimate to be very slightly in excess of that of June or August; that is, about 1 centimetre a day. [Illustration: PLATE I. THE BAHR YÛSUF, SKIRTING THE LIBYAN DESERT, IN THE NILE VALLEY.] _The Bahr Yûsuf outside the Fayûm._—The Bahr Yûsuf (Canal of Joseph) is the watercourse that carries the Fayûm supply. It is not an artificial canal, but a naturally formed sinuous channel, resulting from the Nile flood water draining off the lands and following the line of least resistance along the low-lying part of the Nile Valley on the side of the Libyan Desert. Plate I. shows where the Bahr Yûsuf touches the desert and forms a line of separation between the fertile land of the Nile Valley with its grateful shade of trees and the barren desert sands under a scorching sun. As is usual in the case of rivers which periodically overflow their banks, the land is highest alongside the Nile, and slopes away from it to the hills on either side. The high margins of the Nile are known in Egypt as the “Sahel.” As the Bahr Yûsuf has for a great many years been given an artificial connection with the Nile, and been used as a channel to carry flood water to inundate the lands along its course on both sides, it has imitated the Nile in its action on its borders, and raised a “Sahel” of its own on a smaller scale. The cross-section of the Nile Valley is thus roughly represented in the following diagram. A former artificial connection of the Bahr Yûsuf with the Nile is plainly visible at Derût, 200 kilometres farther up the Nile Valley than Lahûn, the point at which the Bahr Yûsuf turns westward into the Fayûm. The artificial part is easily distinguished from the natural channel, as the former is straight with high spoil banks, resulting from the earth excavated to form the channel, whereas the latter is sinuous without any signs of spoil banks. The length of the channel from Derût to Lahûn, measured along its windings, is 270 kilometres, as against 200 kilometres measured as the crow flies. [Illustration]