The most effective way of testing any working hypothesis in natural science being the prediction of hitherto unobserved facts, I venture to forecast that if, as is most likely, the water-sources of Selima and Lagia are artesian, their levels when eventually determined will not be found to differ very much from 270 and 390 metres above sea respectively. These are the approximate levels deduced by prolonging the static water-contours of my maps into the localities of these wells, assuming the contours to continue as smooth curves. [Illustration: _Outline map of the Libyan Desert, showing the points where the static water-levels are known, and the deduced contours of the underground static water-surface, on the hypothesis of a continuous hydraulic connection between the points_] Another interesting prognostication which I think may fairly be deduced from the map is that if ever the well at Sarra is considerably deepened, a much more abundant water-supply will probably be obtainable. The ground-level at Sarra, according to observations made by Prince Kemal el Din, is 461 metres above sea, and the water-level in the well varies in different years from about 390 to about 410 metres above sea. But an examination of the static contours of the map shows that the static level of the true artesian water in the neighbourhood of the well is probably somewhere about 500 metres above sea, though an exact estimation of the static level at that spot is not possible because of the lack of data farther west. As already remarked on p. 110, I think the present supply at Sarra is derived from more or less local rainfall, conveyed by higher-lying permeable strata than those which convey the main artesian supplies of Kufra and the Egyptian oases; by deepening the well considerably, lower-lying beds might be reached so as to tap the main supply, and the water might even be expected to overflow at the surface. Interesting as are the static water-level contours in themselves, they become vastly more so when superposed on the ground-contours, as is done in the larger map (_G.J._, July, following p. 96). From the two sets of contours on that map we can estimate at any point the approximate depth of the static water-level below the ground; and this information affords new light on some of the most interesting, but hitherto the most difficult, of the problems connected with the Libyan Desert. [Illustration: _Pottery Hill, possible site of “Zerzura,” from the south-west_] [Illustration: _Bir Kassaba, a watering-place on the Darb el Arba’in_] [Illustration: _The waterless stretch of the Darb el Arba’in between Bir Murr and Kharga_] [Illustration: _Bir Sheb, a well on the Darb el Ar ba’in_] 6. _Can the Present Water-supplies of the Mediterranean Littoral be supplemented by Artesian Borings?_