[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.
[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.
[78]Jordan, Handbuch der Vermessungskunde. Stuttgart, 1897. Bd. II, p. 522.
[79]A New Method of Coast Surveying, Survey Department Paper, No. 21. Cairo, 1911.
[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.
[81]Berichte der Commission für Oceanographische Forschungen, Sechste Reihe. Wien, 1898. pp. 221, 223.
[82]Magnetic Observations in Egypt, 1895-1905, Survey Dept. Paper, No. 6. Cairo, 1907. p. 18.
[83]Red Sea and Gulf of Aden Pilot. 5th edition. 1900. p. 7.