Somewhat The Same Phenomenon Probably Causes
The Periodical Inundations Of The Nile.
The two rivers rise
in the same region; but there is a difference in the period of flood,
possibly
From their being on opposite sides of the equator.
The waters of the Nile are said to become turbid in June;
and the flood attains its greatest height in August, or the period
when we may suppose the supersaturation to occur. The subject is worthy
the investigation of those who may examine the region
between the equator and 10 Deg. S.; for the Nile does not show much increase
when the sun is at its farthest point north, or tropic of Cancer,
but at the time of its returning to the equator, exactly as in the other case
when he is on Capricorn, and the Zambesi is affected.*
-
* The above is from my own observation, together with information
derived from the Portuguese in the interior of Angola; and I may add
that the result of many years' observation by Messrs. Gabriel and Brand
at Loanda, on the west coast, is in accordance therewith.
It rains there between the 1st and 30th of November,
but January and December are usually both warm and dry.
The heavier rains commence about the 1st of February,
and last until the 15th of May. Then no rain falls
between the 20th of May and the 1st of November. The rain averages
from 12 to 15 inches per annum. In 1852 it was 12.034 inches;
in 1853, 15.473 inches. Although I had no means of measuring
the amount of rain which fell in Londa, I feel certain
that the annual quantity exceeds very much that which falls on the coast,
because for a long time we noticed that every dawn was marked
by a deluging shower, which began without warning-drops or thunder.
I observed that the rain ceased suddenly on the 28th of April,
and the lesser rains commenced about a fortnight before
the beginning of November.
-
From information derived from Arabs of Zanzibar, whom I met at Naliele
in the middle of the country, the region to the east of the parts of Londa
over which we have traveled resembles them in its conformation.
They report swampy steppes, some of which have no trees,
where the inhabitants use grass, and stalks of native corn, for fuel.
A large shallow lake is also pointed out in that direction,
named Tanganyenka, which requires three days for crossing in canoes.
It is connected with another named Kalagwe (Garague?), farther north,
and may be the Nyanja of the Maravim. From this lake is derived, by numerous
small streams, the River Loapula, the eastern branch of the Zambesi,
which, coming from the N.E., flows past the town of Cazembe.
The southern end of this lake is ten days northeast of the town of Cazembe;
and as that is probably more than five days from Shinte,
we can not have been nearer to it than 150 miles. Probably this lake
is the watershed between the Zambesi and the Nile, as Lake Dilolo
is that between the Leeba and Kasai. But, however this may be,
the phenomena of the rainy season show that it is not necessary to assume
the existence of high snowy mountains until we get reliable information.
This, it is to be hoped, will be one of the results of the researches
of Captain Burton in his present journey.
The original valley formation of the continent determined
the northern and southern course of the Zambesi in the centre,
and also of the ancient river which once flowed from the Linyanti basin
to the Orange River. It also gave direction to the southern and northern flow
of the Kasai and the Nile. We find that between the latitudes,
say 6 Deg. and 12 Deg. S., from which, in all probability,
the head waters of those rivers diverge, there is a sort of elevated partition
in the great longitudinal valley. Presuming on the correctness
of the native information, which places the humid region to which
the Nile and Zambesi probably owe their origin within the latitudes indicated,
why does so much more rain fall there than in the same latitudes
north of the equator? 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.
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