In regard to the wells and springs, it was obvious that only those known to derive their supplies from artesian sources could be utilized as giving points on the static water-surface. This consideration cut out from the discussion the springs of Owenat and Arkenu, which are known to be fed by local rainfall, and also the small water-sources of Kurkur, Dungul, Nakheil, and Ain Amur, which occur in situations where percolation from occasional local rainfall seems to be the only possible source of supply. And for reasons which will appear presently, none of the wells and springs situated to the north of the Siwa-Qattara-Faiyum chain of depressions could be considered as entering into the problem. With these exceptions, every water-source situated within the area of the Libyan Desert covered by the map, and whose level was known, was utilized; but in the greater oases and in the Wadi Natrun the wells and springs are so numerous and so close together that in these localities it was necessary to select one or two wells as representatives of a group. I had no hesitation in including the wells of Siwa and the Wadi Natrun, because the temperature of some of the wells and springs of Siwa, and the quantity of the output of water at both places, seem to me to afford conclusive evidence of the artesian character of their supply.[18] The wells of the little oasis of Lageita, to the east of the Nile near Qena, were included, for although they are not in the Libyan Desert, they most probably derive their supplies from the same underground flow which feeds the western oases. I have included only one well in which the water does not rise nearly to the ground-level. That well is one which was bored by the British Army during the Great War, at a place called B6, some 40 kilometres to the east of Baharia Oasis. The level of the ground at this point is 112 metres above sea-level, and as the water was stationary in the bore at 78 metres below the ground, the static level here is 34 metres above sea.[19] The well is said to have yielded some 800 gallons per hour without the water-level in the bore being sensibly changed. The level of the well at Sarra has recently been determined by Prince Kemal el Din; but I have not included it in my data, because he informs me that the water-level fluctuates by 20 metres or more in different years, while the level of the artesian water of the Egyptian oases and Merga is very nearly constant. The inference I draw from the great fluctuations in the water-level at the Sarra well is that it is dependent on percolation from a more or less local rainfall rather than on the same flow which feeds the Egyptian oases. 5. _Permanence of Lakes._