32 
PHYSICAL ASPECTS OF ADEN. 
soil formed by the weathering* * of the lava, offers no special feature which 
might not be met with, v.g. on the vast Deccan plateau of India, except 
for the fact that the amount of humus at Aden, owing to the scanty 
vegetation, is unusually small. 
The soluble materials, however, play an important part only in those 
localities of our area, where they are present in excessive amounts. This 
is the case on the isthmus and on the flat sandy portion of the British 
territory generally, including the low sandy shores of both peninsulas, 
where the soil is impregnated with considerable amounts of alkali. The 
term alkali is applied rather loosely to the more readily soluble saline 
matters which accumulate in the soils or in the water of desert 
regions. In spite of the name such salts are mostly neutral in reaction 
consisting chiefly of chlorids, sulphates, and nitrates of the bases sodium, 
potassium and magnesium. Only the carbonates of sodium and potassium, 
constituting the much-dreaded “ black alkali,” are strongly alkaline in 
reaction, and on account of their caustic nature much more deleterious to 
most plants than are the neutral salts or “ white alkali.” The effect of 
alkalis upon water content and absorption is not altogether understood. 
Experiments indicate that they are injurious chiefly indirectly by render- 
ing the soil water too concentrated a solution and thereby unfitted to 
nourish the roots. No analyses have been made which represent accu- 
rately the conditions in the soil water of the localities indicated, but there 
can be no doubt that the soil is highly alkaline. If the origin of the soil 
and its present position are not sufficient to substantiate this statement, 
then the unmistakable character of the plants that grow upon it must 
clearly establish the fact. 
(b) Bain , — The dependence of water content upon rainfall is absolute 
in all localities except those where the supply of water is constant, owing 
to the presence of springs, streams, ponds, or other bodies of water. As 
none of these latter sources of supply are to be found at Aden , 1 the rain- 
fall is the only factor to be considered in this place. 
1 Aden is not devoid of wells and tanks, and even of torrents during a very short time 
immediately after a rainfall, hut they are not of such a character as to furnish a constant 
supply to the needs of the vegetation or to modify in any way the water content of the soil. 
According to Hunter, water of a good quality is found at the head of the valleys within the 
crater and to the west of the town. Wells are sunk in the solid rock to a depth of from 
120 to 190 feet j in the best the water stands at a depth of 70 feet below sea level j it yields a 
daily average of 2,500 gallons. The temperature of the water is 102® F., the specific gravity 
*999, and it contains T16 parts of saline matters in every 2,000 gallons. 
The tanks and resevoirs in which to store rain-water are about 50 in number, and if entirely 
cleared would have an aggregate capacity of nearly 30 million imperial gallons. In 1856 the 
restoration of these magnificent public works was commenced, and 13 have been completed 
(up to 1877), capable of holding 7,718,630 gallons of water. 
A very moderate fall of rain suffices to send a stupendous torrent of water down the valleys 
which, ere it reaches the sea, not unfrequently attains the proportions of a river. (From 
Hunter, p. 10-13.) , 
Since the restoration of the tanks they have only been filled six times, in May 1866, May 
1870, and September 1877, 1889, 1893, 1897. 
