Why Does Darfur Not Give Rise To Great Rivers,
Like Londa And The Country East Of It?
The prevailing winds in the ocean
opposite the territory pointed out are said to be from the N.E.
And S.E.
during a great part of the year; they extend their currents
on one side at least of the equator quite beyond the middle of the continent,
and even until in Angola they meet the sea-breeze from the Atlantic.
If the reader remembers the explanation given at page 109,*
that the comparative want of rain on the Kalahari Desert is caused
by the mass of air losing its humidity as it passes up and glides over
the subtending ridge, and will turn to the map, he may perceive
that the same cause is in operation in an intense degree
by the mountains of Abyssinia to render the region about Darfur
still more arid, and that the flanking ranges mentioned
lie much nearer the equator than those which rob the Kalahari of humidity.
The Nile, even while running through a part of that region,
receives remarkably few branches. Observing also that there is
no known abrupt lateral mountain-range between 6 Deg. and 12 Deg. S.,
but that there is an elevated partition there, and that
the southing and northing of the southeasters and northeasters
probably cause a confluence of the two great atmospheric currents,
he will perceive an accumulation of humidity on the flanks and crown
of the partition, instead of, as elsewhere, opposite the Kalahari and Darfur,
a deposition of the atmospheric moisture on the eastern slopes
of the subtending ridges. This explanation is offered with all deference
to those who have made meteorology their special study,
and as a hint to travelers who may have opportunity to examine the subject
more fully. I often observed, while on a portion of the partition,
that the air by night was generally quite still, but as soon as the sun's rays
began to shoot across the upper strata of the atmosphere in the early morning,
a copious discharge came suddenly down from the accumulated clouds.
It always reminded me of the experiment of putting a rod
into a saturated solution of a certain salt, causing instant crystallization.
This, too, was the period when I often observed the greatest amount of cold.
-
* Since the explanation in page 109 [Chapter 5 Paragraph 5] was printed,
I have been pleased to see the same explanation given
by the popular astronomer and natural philosopher, M. Babinet,
in reference to the climate of France. It is quoted from
a letter of a correspondent of the `Times' in Paris:
"In the normal meteorological state of France and Europe,
the west wind, which is the counter-current of the trade-winds
that constantly blow from the east under the tropics -
the west wind, I say, after having touched France and Europe
by the western shores, re-descends by Marseilles and the Mediterranean,
Constantinople and the Archipelago, Astrakan and the Caspian Sea,
in order to merge again into the great circuit of the general winds,
and be thus carried again into the equatorial current.
Whenever these masses of air, impregnated with humidity
during their passage over the ocean, meet with an obstacle,
such as a chain of mountains, for example, they slide up the acclivity,
and, when they reach the crest, find themselves relieved
from a portion of the column of air which pressed upon them.
Thus, dilating by reason of their elasticity, they cause
a considerable degree of cold, and a precipitation of humidity
in the form of fogs, clouds, rain, or snow.
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