volume of water which must annually disappear from the lakes and marshes by evaporation. A further consideration bearing on this point is that whatever may have been the agency by which the depression of Siwa was formed, that same agency almost certainly operated to produce the other depressions of the northern chain; and it seems most unlikely that a connection should have been opened up with the underground water-bearing beds in Siwa, and not also in the larger and much deeper depression of Qattara. It would be a difficult matter to estimate the relative proportion of the water entering the depressions by underground flow from the south, to that contributed by local rainfall and seepage from the northern slopes. But that is not necessary for our immediate purpose. It is sufficient to show that there must be _some_ influx into the depressions from the same source as that which supplies the wells of the greater oases, to establish the existence of that underground water-connection which is all that we need to justify us in regarding the levels of the lakes and salt-marshes as furnishing us with points on the static water- surface; and from the considerations mentioned above it seems to me certain that some influx of underground water really does take place. I have also thought it justifiable to include the Birket el Qarun in my collection of static water-level data, because although that lake was probably first formed by an overflow of the Nile into the Faiyum, and is even now being fed by Nile water through the Faiyum drains at the rate of some 350 million tons a year, there is a certain amount of evidence suggesting that it has some underground water-connection with the Qattara depression. That evidence, to which attention was first drawn by Professor Schweinfurth,[20] consists in the relatively low salinity (1·3 per cent.) of the lake, notwithstanding the long period through which it has been subject to evaporation and the fact of its having shrunk to dimensions very much smaller than it possessed in ancient times. Unless there has been a large underground efflux of salt water from the lake, it appears impossible to account for its present degree of freshness. In Professor Schweinfurth’s day, of course, the existence of the Qattara depression was unknown, and it was puzzling to suggest where the salt water had gone to.[21] An underground leakage from the Birket el Qarun into the Qattara depression is quite conceivable, for although the two places are separated by some 200 kilometres, there is a very considerable fall between them. Thus the salt in the marshes of the Qattara depression may possibly have come in part from the Birket el Qarun. The present rate of discharge of the Faiyum drains into the lake is, however, just sufficient to make up for an average daily evaporation from the lake-surface of a little over 4 mm., which is about the rate we might expect; and although the level of the lake-surface has fallen some