84 
PHYSICAL ASPECTS OF ADEN. 
As the hills on the western side of the Crater are precipitous, the rain- 
water descending from them is carried rapidly to the sea by means of a 
number of long narrow valleys unconnected with each other. On the 
eastern side the hills are quite as abrupt, but the descent is broken by a 
large table-land occurring midway between the summit and the sea-level, 
which occupies about one-fourth of the entire superficies of Aden. The 
plateau is intersected with numerous ravines, nearly all of which converge 
into one valley, which thus receives a large proportion of the drainage of 
the peninsula. As slope ( i.e the inclination of the surface of a locality 
with respect to the horizon) controls the run-off and drainage, and 
through them the water content of the soil, we can easily understand 
how the steepness of the hills, the hardness of the rocks, and the scarce- 
ness of soil upon them combine to prevent any great amount of absorp- 
tion. 
Almost perpetual drought is, of necessity, the consequence of such 
physical and meteorological conditions. It is, indeed, only after a rainfall 
that the soil of the valleys is uniformly moistened throughout, so that the 
plants are able to draw the necessary supply of water even from the 
upper layers of the ground. After a very short time the rainwater 
partly evaporates, partly sinks deeper, and either flows off on the hard 
inclined rock, or is collected in subterranean reservoirs. 
No observations regarding the effect of rainfall upon water content 
have been made at Aden. It is best ascertained by taking samples, of 
soil in different localities immediately after a rain, and then determining 
the increase in water content. Irregularities in the surface of a locality 
which retard the movement of run-off and cause more of the rain to soak 
into the soil, ought to be specially noted. Their influence is generally 
not very great, but it is always appreciable. In some cases it is of 
considerable importance, as may be judged v.g. from the vegetation in 
the lower part of the broad valley between Steamer Point and Maala. 
(c) Humidity and temperature . — Humidity is an important factor 
determining to a great degree the w T ater content of the soil. It acts 
upon the plant and upon soil water in the same way, viz., by controlling 
evaporation. Water content is affected directly through water loss from 
the soil, and indirectly inasmuch as the water lost by the plant is first 
drawn from the substratum. Since humidity is a direct factor, it ought tc 
be more fully considered in this place, but, unfortunately no statistical 
observations cn the subject are at our disposal. All we know is, that 
Aden is considered to be exceedingly dry ; and it we are allowed to 
conclude from the character of its flora, there can be no doubt on this 
point. 
