In the diagram, FEKHG represents a water-conveying stratum, folded throughout its course and faulted at HK, but having a general downward slope from F to G. A and B represent points at which the water just rises to the ground-level, either through natural fissures or in artificial borings. The straight line drawn through A and B represents very approximately the static level at any point between A and B, that is, the level to which the water would rise in bores carried down to the water-bearing bed. A boring at C, for instance, would have to go down to E to tap the water, but once the bed was tapped the water would rise in the bore as far as D. At the fault HK, the pressure of the water at K will cause it to rise through the crushed rock at the fault-plane and re-enter the porous stratum at H. If there is a considerable outflow at B, and the fault-plane is a very narrow fissure, we may expect some drop in the line AB over the fault, by reason of the extra frictional absorption of head at this place. But unless the thickness and degree of permeability of the fault-rock are markedly different from those of the sandstone bed itself, the drop of pressure will not greatly disturb the general slope AB. In any case it is apparent that the level of the static water-surface at any place between A and B is capable of being estimated with a far closer degree of approximation than is the level of the water-sheet itself. We may therefore justifiably assume a uniform gradient for the static level between points at which that level is known, disregarding folds in the strata; and though we cannot entirely allow in detail for unknown faults and variations in permeability, it must be borne in mind that the _total_ effect of all the unknown factors between any two known points is already automatically allowed for in our data. It is only the variations from uniformity, due to the unknown distribution of the faults and of the departures from the average permeability, which can affect us; and these variations and departures are probably but small in most of the great unexplored areas of the south-west of Egypt, where the geological structure, from all we know of it, appears generally to be remarkably uniform. The first requisite for the construction of a map showing the contours of the static water-surface was, of course, a sufficiency of well- determined positions of points where the static water-level of the artesian supply was fairly exactly known. Such points are the springs and wells of the various oases, the surfaces of lakes occupying depressions and presumably fed by underground supplies, and any places on the Nile where the river taps artesian beds.