[Footnote 18: Lucas (‘Natural Soda Deposits in Egypt’ (Cairo, 1912), p. 15) regarded the water of the Wadi Natrun as being due to infiltration from the Nile, mainly on the ground that he supposed the water visibly entering the wadi (as small streams and trickles) to do so on the north- eastern side. But this, I think, is a mistake, as it neglects consideration of the large springs of warm fresh water which bubble up in and near the salt-lakes themselves, and also the large wells of the monasteries which lie south-west of the lakes.] [Footnote 19: A far better site for this well would have been in the depression some 20 kilometres to the north-west, the ground-level at the lowest point of which is only 61 metres above sea; but at the time when the site of the well was chosen the existence of this depression was unknown.] [Footnote 20: See Appendix III. in the second volume of Willcocks’ ‘Egyptian Irrigation,’ 1913, p. 853.] [Footnote 21: Schweinfurth (_op. cit._, p. 855) even thought it might have percolated to Siwa, in spite of the great distance (480 kilometres) and the adverse direction of the difference of level between the two places.] [Footnote 22: Mr. Beadnell (‘An Egyptian Oasis’ (London, 1909), p. 139) estimates the total discharge of the Kharga wells at 53,000 cubic metres per diem. We should not, I think, err greatly in estimating the total output of all the Egyptian oasis wells and springs at about five times this, or 250,000 cubic metres per diem. As the average daily discharge of the Nile in Egypt is about 180 million tons, a local influx into it several times as great as the total discharge of the oasis wells would represent but an insignificant addition to the discharge of the Nile. I think, however, that the influx of the warm water might possibly be detected by careful thermometric measurements made in the river near its banks at the time of low Nile.] [Footnote 23: This was the lowest level recorded at the time when I drew the static contours on the map. The further exploration of the depression by Mr. Walpole in the present year has shown that at one place the ground-level descends to − 134 metres; the situation of this local deepening is, however, such that the drawing of the static contours would not have been affected by its inclusion in the list of adopted data.] [Footnote 24: ‘An Egyptian Oasis’ (London, 1909), p. 156.] [Footnote 25: “The Mutual Interference of Artesian Wells,” _Geol. Mag._ (London, 1909), pp. 23-26; also ‘An Egyptian Oasis,’ chap. x.] [Footnote 26: ‘An Egyptian Oasis,’ p. 140.] [Footnote 27: ‘Drei Monate in der libyschen Wüste’ (Cassel, 1875), p. 115.] [Footnote 28: See Jordan’s ‘Physische Geographie und Meteorologie der libyschen Wüste’ (Cassel, 1875), p. 204. Jordan states that there were on an average not more than two to four plants per acre in this locality.] [Footnote 29: See Rohlfs’ ‘Drei Monate in der libyschen Wüste’ (chapter ix. of which was written by Ascherson), p. 250.]