440 
DIFFUSION OF WATER IN THE SOIL. 
beneath it from losing its humidity by evaporation. Hence, 
city-grown trees find moisture enough for their roots, and 
though plagued with smoke and dust, often retain their tresh- 
ness while those planted in the open fields, where sun and 
wind dry up the soil faster than the subterranean fountains 
can water it, are withering from drought. Without the help 
of artificial conduit or of water carrier, the Thames and the 
Seine refresh the ornamental trees that shade the thorough¬ 
fares of London and of Paris, and beneath the hot and reeking 
mould of Egypt, the Hile sends currents to the extremest bor¬ 
der of its valley.* 
* 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, (Memoires 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 kilometre from 
the river, as water is found by digging to the depth of twelve or fifteen 
metres 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 main¬ 
tained 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 
Petrsea 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 
