[394] Physikalische Geographie, p. 286. It does not appear whether this inference is Mariotte's or Wittwer's. I suppose it is a conclusion of the latter.
[395] Physical Geography of the Sea. Tenth edition. London, 1861, § 274.
[396] Paramelle, Quellenkunde, mit einem Vorwort von B. Cotta, 1856.
[397] Études et Lectures, vi, p. 118.
[398] "The area of soil dried by draining is constantly increasing, and the water received by the surface from atmospheric precipitation is thereby partly conducted into new channels, and, in general, carried off more rapidly than before. Will not this fact exert an influence on the condition of many springs, whose basin of supply thus undergoes a partial or complete transformation? I am convinced that it will, and it is important to collect data for solving the question." Bernhard Cotta, Preface to Paramelle, Quellenkunde (German translation), pp. vii, viii. See Appendix, [No. 54].
[399] See the interesting observations of Kriegk on this subject, Schriften zur allgemeinen Erdkunde, cap. iii, § 6, and especially the passages in Ritter's Erdkunde, vol. i, there referred to.
Laurent, (Mémoires sur le Sahara Oriental, pp. 8, 9), in speaking of a river at El-Faid, "which, like all those of the desert, is, most of the time, without water," observes, that many wells are dug in the bed of the river in the dry season, and that the subterranean current thus reached appears to extend itself laterally, at about the same level, at least a kilomètre from the river, as water is found by digging to the depth of twelve or fifteen mètres at a village situated at that distance from the bank.
The most remarkable case of infiltration known to me by personal observation is the occurrence of fresh water in the beach sand on the eastern side of the Gulf of Akaba, the eastern arm of the Red Sea. If you dig a cavity in the beach near the sea level, it soon fills with water so fresh as not to be undrinkable, though the sea water two or three yards from it contains even more than the average quantity of salt. It cannot be maintained that this is sea water freshened by filtration through a few feet or inches of sand, for salt water cannot be deprived of its salt by that process. It can only come from the highlands of Arabia, and it would seem that there must exist some large reservoir in the interior to furnish a supply which, in spite of evaporation, holds out for months after the last rains of winter, and perhaps even through the year. I observed the fact in the month of June.
The precipitation in the mountains that border the Red Sea is not known by pluviometric measurement, but the mass of debris brought down the ravines by the torrents proves that their volume must be large. The proportion of surface covered by sand and absorbent earth, in Arabia Petræa and the neighboring countries, is small, and the mountains drain themselves rapidly into the wadies or ravines where the torrents are formed; but the beds of earth and disintegrated rock at the bottom of the valleys are of so loose and porous texture, that a great quantity of water is absorbed in saturating them before a visible current is formed on their surface. In a heavy thunder storm, accompanied by a deluging rain, which I witnessed at Mount Sinai in the month of May, a large stream of water poured, in an almost continuous cascade, down the steep ravine north of the convent, by which travellers sometimes descend from the plateau between the two peaks, but after reaching the foot of the mountain, it flowed but a few yards before it was swallowed up in the sands.
[400] It is conceivable that in large and shallow subterranean basins the superincumbent earth may rest upon the water and be partly supported by it. In such case the weight of the earth would be an additional, if not the sole, cause of the ascent of the water through the tubes of artesian wells. The elasticity of gases in the cavities may also aid in forcing up water.