[Footnote 75: In particular, the adoption of the above three longitudes as of equal accuracy with Suez will lead to a better longitude for Jidda. The longitude of Jidda is at present fixed only by chronometer transport from Suez, a distance of some 650 miles; but by regarding Halaib as a fixed point the distance of comparison can be reduced to 150 miles. The data for this revision of the longitude of Jidda already exist in the Reports of the “Pola” expedition, and the discussion is certainly worth undertaking before any future surveys are conducted in the central portion of the Red Sea.] [Footnote 76: The disturbance of the needle was of course most frequent in the areas where dark igneous and metamorphic rocks prevailed; in these regions disturbances of 10° were very frequent, and in some cases, as for example on the serpentine deposits of Sikait, as much as 40° was noticed. In the sandstone areas no disturbances were marked.] [Footnote 77: I have given a somewhat fuller account of this process in a little paper on “Long-distance Tacheometry.” _Cairo Scientific Journal_, Vol. IV, February 1910.] [Footnote 78: JORDAN, _Handbuch der Vermessungskunde_. Stuttgart, 1897. Bd. II, p. 522.] [Footnote 79: _A New Method of Coast Surveying_, Survey Department Paper, No. 21. Cairo, 1911.] [Footnote 80: The observations at Halaib were made at a point far enough from the fort to avoid any chance of disturbance by the iron rails which form part of the building.] [Footnote 81: _Berichte der Commission für Oceanographische Forschungen_, Sechste Reihe. Wien, 1898. pp. 221, 223.] [Footnote 82: _Magnetic Observations in Egypt_, 1895-1905, Survey Dept. Paper, No. 6. Cairo, 1907. p. 18.] [Footnote 83: _Red Sea and Gulf of Aden Pilot._ 5th edition. 1900. p. 7.] CHAPTER IV. * * * * * =THE WADIS DRAINING WESTWARDS TO THE NILE.= * * * * * It has been already mentioned (p. 21) that all the drainage westward from the main watershed ultimately reaches the Nile by the three great trunk Wadis Shait, Kharit, and Alaqi. In the present chapter a detailed description will be given of those parts of these wadis and their tributaries which are known within the area under discussion. The following scheme shows the relations of the principal tributaries, important feeders of the tributaries themselves being indicated in (brackets):— { Wadi Dweig. { WADI SHAIT { Wadi Muelih (Wadi Gerf, Rod el Ligah, Wadi Sibrit). { { Wadi Hamish.