of the Sahara, as well as its southern limits towards the fertile
Soudan, are still but little known. If we take on a mean estimation the
parallels of 16-1/2° and 32-1/2° as the outside limits, we obtain for
the Desert, including its Oases, an area of more than 118500 square
German geographical miles; or between nine and ten times the area of
Germany, and almost three times that of the Mediterranean exclusive of
the Black Sea. From the best and most recent intelligence, for which
we are indebted to the French Colonel Daumas and MM. Fournel, Renou,
and Carette, we learn that the desert of Sahara is composed of several
detached basins, and that the number and the population of the fertile
Oases is very much greater than had been imagined from the awfully
desert character of the route between Insalah and Timbuctoo, and that
from Mourzouk in Fezzan, to Bilma, Tirtuma, and Lake Tschad. It is now
generally affirmed that the sand covers only the smaller portion of the
great lowland. A similar opinion had been previously propounded by the
acutely observant Ehrenberg, my Siberian travelling companion, from
what he had himself seen (Exploration Scientifique de l’Algérie, Hist.
et Geogr. T. ii. p. 332). Of larger wild animals, only gazelles, wild
asses, and ostriches are to be met with. “Le lion du désert,” says M.
Carette, (Explor. de l’Alg. T. ii. p. 126-129; T. vii. p. 94 and 97),
“est un mythe popularisé par les artistes et les poètes. Il n’existe
que dans leur imagination. Cet animal ne sort pas de sa montagne où il
trouve de quoi se loger, s’abreuver et se nourrir. Quand on parle aux
habitans du désert de ces bêtes féroces que les Européens leur donnent
pour compagnons, ils repondent avec un imperturbable sang froid, il
y a donc chez vous des lions qui boivent de l’air et broutent des
feuilles? Chez nous il faut aux lions de l’eau courante et de la chair
vive. Aussi des lions ne paraissent dans le Zahara que là où il y a des
collines boisées et de l’eau. Nous ne craignons que la vipère (lefa) et
d’innombrables essaims de moustiques, ces derniers là où il y a quelque
humidité.”
Whereas Dr. Oudney, in the course of the long journey from Tripoli to
Lake Tschad, estimated the elevation of the southern Sahara at 1637
English feet, to which German geographers have even ventured to add
an additional thousand feet, the Ingenieur Fournel has, by careful
barometric measurements based on corresponding observations, made
it tolerably probable that a part of the northern desert is below
the level of the sea. That portion of the desert which is now called
“le Zahara d’Algérie” advances to the chains of hills of Metlili and
el-Gaous, where the northernmost of all the Oases,--that of el-Kantara,
fruitful in dates,--is situated. This low basin, which touches the
parallel of 34° lat., receives the radiant heat of a stratum of chalk,
(full of the shells of Inoceramus), inclined at an angle of 65° towards
the south (Fournel sur les Gisemens de Muriate de Soude en Algérie, p.
6 in the Annales des Mines, 4me Série, T. ix., 1846, p. 546). “Arrivés
a Biscara,” (Biskra), says Fournel, “un horizon indéfini comme celui
de la mer se déroulait devant nous.” Between Biscara and Sidi Ocba
the ground is only 228 (243 Eng.) feet above the level of the sea.
The inclination increases considerably towards the south. In another
work, (Asie Centrale, T. ii. p. 320), where I have brought together
everything relating to the depression of some portions of continents
below the level of the sea, I have already noticed that according to
Le Père the “bitter lakes” on the isthmus of Suez, when they have a
little water,--and, according to General Andréossy, the Natron lakes of
Fayoum,--are also lower than the level of the Mediterranean.
Among other manuscript notices of M. Fournel, I possess a vertical
geological profile, which gives all the inflexions and inclinations
of the strata, representing a section of the surface the whole way
from Philippeville on the coast to the Desert of Sahara, at a spot
not far from the Oasis of Biscara. The direction of the line on which
the barometric measurements were taken is south 20° west; but the
elevations determined are projected, as in my Mexican profiles, on a
different plane,--a north-south one. Ascending uninterruptedly from
Constantine, at an elevation of 332 toises (2122 Eng. feet), the
culminating point is found between Batnah and Tizur, at an elevation of
only 560 toises (3580 Eng. feet). In the part of the desert situated
between Biscara and Tuggurt, Fournel has had a series of Artesian
wells dug with success (Comptes Rendus de l’Acad. des Sciences, t. xx.
1845, p. 170, 882, and 1305). We learn from the old accounts of Shaw,
that the inhabitants of the country knew of a subterranean supply
of water, and relate fabulous tales of a “sea under the earth (bahr
tôht el-erd).” Fresh waters flowing between clay and marl strata of
the old cretaceous and other sedimentary deposits, under the action
of hydrostatic pressure form gushing fountains when the strata are
pierced (Shaw, Voyages dans plusieurs parties de la Berbérie, t. i. p.
169; Rennell, Africa, Append. p. lxxxv). That fresh water in this part
of the world should often be found near beds of rock salt, need not
surprise geologists acquainted with mines, since Europe offers many
analogous phenomena.
The riches of the desert in rock-salt, and the fact of rock-salt having
been used in building, have been known since the time of Herodotus. The
salt zone of the Sahara (zone salifère du désert), is the southernmost
of three zones, stretching across Northern Africa from south-west to
north-east, and believed to be connected with the beds or deposits
of rock-salt of Sicily and Palestine, described by Friedrich Hoffman
and by Robinson. (Fournel, sur les Gisements de Muriate de Soude en
Algérie, p. 28-41; Karsten über das Vorkommen des Kochsalzes auf der
Oberfläche der Erde, 1846, S. 497, 648, and 741.) The trade in salt
with Soudan, and the possibility of cultivating dates in the Oases,
formed by depressions caused probably by falls or subsidences of
the earth in the gypsum beds of the tertiary cretaceous or keuper
promotions, have alike contributed to enliven the Desert, at least to
some extent, by human intercourse. The high temperature of the air,
which makes the day’s march so oppressive, renders the coldness of the
nights, (of which Denham complained so often in the African Desert,
and Sir Alexander Burnes in the Asiatic), so much the more striking.
Melloni, (Memoria sull’ abassamento di temperatura durante le notti
placide e serene, 1847, p. 55), ascribes this cold, produced doubtless
by the radiation from the ground, less to the great purity and serenity
of the sky, (irrigiamento calorifico per la grande serenità di cielo
nell’ immensa e deserta pianura dell’ Africa centrale), than to the
profound calm, the nightly absence of all movement in the atmosphere.
(Consult also, respecting African meteorology, Aimé in the Exploration
de l’Algérie, Physique génerale, T. ii., 1846, p. 147.)
The southern declivity of the Atlas of Morocco sends to the Sahara, in
lat. 32°, a river, the Quad-Dra (Wady-Dra), which for the greater part
of the year is nearly dry, and which Renou (Explor. de l’Alg. Hist.
et Geogr., T. viii. p. 65-78) considers to be a sixth longer than the
Rhine. It flows at first from north to south, until, in lat. 29° N. and
long. 5° W., it turns almost at right angles to its former course, runs
to the west, and, after passing through the great fresh water Lake of
Debaid, enters the sea at Cape Nun, in lat. 28° 46′ N. and long. 11°
08′ W. This region, which was so celebrated formerly in the history
of the Portuguese discoveries of the 15th century, and was afterwards
wrapped in profound geographical obscurity, is now called on the coast
“the country of the Sheikh Beirouk,” (a chief independent of the
Emperor of Morocco.) It was explored in the months of July and August
1840, by Captain Count Bouet-Villaumez of the French Navy, by order of
his government. From the official Reports and Surveys which have been
communicated to me in manuscript, it appears evident that the mouth
of the Quad-Dra is at present very much stopped up with sand, having
an open channel of only about 190 English feet wide. A somewhat more
easterly channel in the same mouth is that of the still very little
known Saguiel el-Hamra, which comes from the south, and is supposed to
have a course of at least 600 geographical miles. One is astonished at
the length of these deep, but commonly dry river beds. They are ancient
furrows, such as I have seen in the Peruvian desert at the foot of the
Cordilleras, between those mountains and the coast of the Pacific.
In Bouet’s manuscript “Relation de l’Expédition de la Malouine,” the
mountains which rise to the north of Cape Nun are estimated at the
great elevation of 2800 metres (9185 English feet).
Cape Nun is usually supposed to have been discovered in 1433, by the
Knight Gilianez, acting under the command of the celebrated Infante
Henry Duke of Viseo, and founder of the Academy of Sagres, which was
presided over by the pilot and cosmographer Mestre Jacomè of Majorca;
but the Portulano Mediceo, the work of a Genoese Navigator in 1351,
already contains the name of Cavo di Non. The passage round this Cape
was then as much dreaded as that of Cape Horn has since been, although
it is 23′ north of the parallel of Teneriffe, and could be reached in
a few days’ voyage from Cadiz. The Portuguese proverb, “quem passa o
Cabo di Num, ou tornarà ou não,” could not deter the Infante, whose
heraldic French motto, “talent de bien faire,” expressed his noble,
enterprising, and vigorous character. The name of the Cape, in which
a play of words on the negative particle has long been supposed, does
not appear to me to have had a Portuguese origin. Ptolemy placed on the
north-west coast of Africa a river Nuius, in the Latin version Nunii
Ostia. Edrisi speaks of a town, Nul, or Wadi Nun, somewhat more to the
south, and three days’ journey in the interior: Leo Africanus calls it
Belad de Non. Long before the Portuguese squadron of Gilianez, other
European navigators had advanced much beyond, or to the southward of,
this Cape. The Catalan, Don Jayme Ferrer, in 1346, as we learn from
the Atlas Catalan published by Buchon at Paris, had advanced as far as
the Gold River, (Rio do Ouro), in lat. 23° 56′; and Normans, at the
end of the 14th century, as far as Sierra Leone in lat. 8° 30′. The
merit of having been the first to cross the equator on the western
coast of Africa belongs, however, like that of so many other memorable
achievements, to the Portuguese.
[17] p. 8.--“_As a grassy plain, resembling many of the Steppes of
Central Asia._”
The Llanos of Caraccas and of the Rio Apure and the Meta, over which
roam large herds of cattle, are, in the strictest sense of the term,
“grassy plains.” Their prevalent vegetation, belonging to the two
families of Cyperaceæ and Gramineæ, consists of various species of
Paspalum, P. leptostachyum and P. lenticulare; of Kyllingia, K.
monocephala (Rottb.), K. odorata; of Panicum, P. granuliferum, P.
micranthum; of Antephora; Aristida; Vilfa; and Anthistiria, A. reflexa,
and A. foliosa. Only here and there are found, interspersed among
the Gramineæ, a few herbaceous dicotyledonous plants, consisting of
two very low-growing species of Mimosa, (Sensitive Plant), Mimosa
intermedia, and Mimosa dormiens, which are great favourites with the
wild horses and cattle. The natives give to this group of plants, which
close their delicate feathery leaves on being touched, the expressive
name of Dormideras--sleepy plants. For many square miles not a tree
is seen; but where solitary trees are found, they are, in moist
places, the Mauritia Palm; in arid districts, a Proteacea, described
by Bonpland and myself, the Rhopala complicata (Chaparro bobo), which
Wildenow regarded as an Embothrium; also the highly useful Palma de
Covija, or de Sombrero; and our Corypha inermis, an umbrella palm
allied to Chamærops, which is used to cover the roofs of huts. How far
more varied is the aspect of the Asiatic plains! Throughout a large
portion of the Kirghis and Calmuck Steppes, which I have traversed
from the Don, the Caspian, and the Orenburg Ural river the Jaik, to
the Obi and the Upper Irtysh near Lake Dsaisang, through a space of 40
degrees of longitude, I have never seen, as in the Llanos, the Pampas,
and the Prairies, an horizon like that of the ocean, where the vault
of heaven appears to rest on the unbroken plain. At the utmost this
appearance presented itself in one direction, or towards one quarter of
the heavens. The Asiatic Steppes are often crossed by ranges of hills,
or clothed with coniferous woods or forests. Even in the most fruitful
pastures the vegetation is by no means limited to grasses; there is a
great variety of herbaceous plants and shrubs. In spring-time small
snow-white and red-flowering rosaceæ and amygdaleæ (Spiræa, Cratægus,
Prunus spinosa, and Amygdalus nana) present a smiling aspect. I have
already mentioned the tall and luxuriant Synantheræ (Saussurea amara,
S. salsa, Artemisias, and Centaureas), and of leguminous plants,
species of Astragalus, Cytisus, and Caragana. Crown Imperials,
(Fritillaria ruthenica, and F. meleagroides), Cypripedias, and tulips,
rejoice the eye by the bright variety of their colours.
A contrast to the pleasing vegetation of these Asiatic plains is
presented by the desolate salt Steppes, particularly by the part of the
Barabinski Steppe which is at the foot of the Altai mountains, and by
the Steppes between Barnaul and the Serpent Mountain and the country
on the east of the Caspian. Here Chenopodias, some species of Salsola
and Atriplex, Salicornias and Halimocnemis crassifolia, (each species
growing “socially”), form patches of vegetation on the muddy ground.
See Göbel’s Journey in the Steppes of the South of Russia (Reise in
die Steppe des südlichen Russlands, 1838, Th. ii. S. 244 and 301). Of
the 500 phanerogamous species which Claus and Göbel collected in the
Steppes, the Syrantheræ, the Chenopodeæ, and the Cruciferæ, were more
numerous than the grasses; the latter being only 1/11 of the whole, and
the former 1/7th and 1/9th. In Germany, from the mixture of hill and
plain districts, the Glumaceæ (_i. e._ the Gramineæ, Cyperaceæ, and
Juncaceæ collectively), form 1/7th; the Synantheræ or Compositæ 1/8th;
and the Cruciferæ 1/18th of all our German phanerogamia. In the most
northern parts of the flat Siberian lowlands, the fine map of Admiral
Wrangell shews that the extreme northern limit of tree and shrub
vegetation (Coniferæ and Amentaceæ) is, in the portion towards the
Behring’s Straits side, in 67-1/4° lat.; and more to the west, towards
the banks of the Lena, in 71°, which is the parallel of the north cape
of Lapland. The plains which border the Icy Sea are the domain of
cryptogamous plants. They are called Tundras (Tuntur in Finnish): they
are swampy districts extending farther than the eye can reach, partly
covered with a thick carpet of Sphagnum palustre and other mosses,
and partly with a dry snow-white covering of Cenomyce rangiferina
(Rein-deer moss), Stereocaulon paschale, and other lichens. Admiral
Wrangell, in describing his perilous expedition to the new Siberian
islands so rich in fossil wood, says: “These Tundras accompanied me to
the extreme arctic coast. Their soil has been frozen for thousands of
years. In the dreary uniformity of landscape, the eye of the traveller,
surrounded by rein-deer moss, dwells with pleasure on the smallest
patch of green turf showing itself now and then on a moist spot.”
[18] p. 8.--“_The causes which lessen both heat and dryness in the New
World._”
I have tried to bring together in a brief and compendious manner the
various causes which produce greater moisture and a less degree of
heat in America; it will of course be understood that the question
respects the _general_ hygrometric state of the atmosphere, and the
temperature of the New Continent as a _whole_. Single districts, such
as the island of Margarita, the Coasts of Cumana and Coro, are as hot
and as dry as any part of Africa. It must also be remarked that the
maximum of heat at certain hours of a summer’s day has been found,
on a series of years, to be almost equal at very different parts of
the earth’s surface, on the Neva, the Senegal, the Ganges, and the
Orinoco; being approximately between 27° and 32° Reaumur (93° and 104°
Fahrenheit), and generally not higher,--providing the observation
be made in the shade, at a distance from all solid bodies which
could radiate heat to the thermometer, not in an air filled with hot
particles of dust or sand, and not with spirit thermometers, which
absorb the light. It is probably to fine grains of sand floating in
the air, and forming centres of radiant heat, that we must ascribe
the dreadful temperature of 40° to 44°.8 Reaumur (122° to 133° Fah.)
in the shade, to which my unhappy friend Ritchie, who perished there,
and Captain Lyon, were exposed for weeks in the Oasis of Mourzouk. The
most remarkable instance of very high temperature, in an air probably
free from dust, has been recorded by an observer who knew well how to
place and to correct all his instruments with the greatest degree of
accuracy. Rüppell found 37°.6 Reaumur, (110°.6 Fahrenheit,) at Ambukol
in Abyssinia, with a clouded sky, strong south-west wind, and an
approaching thunderstorm. The _mean_ annual temperature of the tropics,
or of the proper climate of palms, is, on land, between 20°.5 and
23°.8 Reaumur (or 78°.2 and 85°.5 Fahrenheit) without any considerable
difference between the observations collected in Senegal, Pondichery,
and Surinam. (Humboldt, Mémoire sur les lignes isothermes, 1817, p. 54.
Asie Centrale, T. iii. Mahlmann, Table iv.)
The great coolness, I might almost say cold, which prevails for a
considerable part of the year within the tropics on the coast of Peru,
causing the thermometer to sink to 12° Reaumur (59° Fahrenheit), is, as
I have noticed elsewhere, by no means to be ascribed to the vicinity
of the snow-covered Andes, but rather to the fogs (garua) which veil
the solar disk, and to a _cold sea current_ which, commencing in the
antarctic regions and coming from the south-west, strikes the coast of
Chili near Valdivia and Conçeption, and thence streams rapidly along
the coast to the northward, as far as Cape Pariña. On the coast, near
Lima, the temperature of the Pacific is 12°.5 Reaumur (60°.2 Fahr.),
whilst in the same latitude out of the current it is 21° R. (79°.2
Fahr.) It is singular that so striking a fact should have remained
unnoticed until my visit to the shores of the Pacific, in October 1802.
The variations of temperature of different regions depend in a great
degree on the character of the bottom of the “aerial ocean,” or on
the nature of the floor or base, whether land or sea, continental
or oceanic, on which the atmosphere rests. Seas, often traversed by
currents of warmer or colder water, (oceanic rivers), have an effect
very different from that of continental masses, whether unbroken or
articulated, or of islands, which latter may be regarded as shallows in
the aerial ocean, and which, notwithstanding their small dimensions,
exert, often to a great distance, a notable influence on the climate
of the sea. In continental masses we must distinguish between sandy
deserts devoid of vegetation, savannahs or grassy plains, and
forest-covered districts. In Upper Egypt and in South America, Nouet
in the former, and myself in the latter, found respectively at noon
the temperature of the ground composed of granitic sand 54°.2 and
48°.4 Reaumur (154° and 141° Fahr.) Many careful observations in Paris
have given, according to Arago, 40° and 42° Reaumur, 122° and 126°.5
Fahrenheit. (Asie Centrale, T. iii. p. 176.) The Savannahs, which
between the Missouri and the Mississipi are called Prairies, and which
appear in South America as the Llanos of Venezuela and the Pampas of
Buenos Ayres, are covered with small monocotyledonous plants of the
family of Cyperaceæ, and with grasses of which the thin pointed stalks
or ears, and the delicate lanceolate leaves or blades, radiate towards
the unclouded sky, and possess an extraordinary power of “emission.”
Wells and Daniell (Meteor. Essays, 1827, p. 230 and 278) have even seen
in our latitude, where the atmosphere has so much less transparency,
the thermometer sink 6°.5 or 8° of Reaumur (14°.5 or 18° Fahrenheit),
on being placed on the grass. Melloni, in a memoir, “Sull abassamento
di temperatura durante le notti placide e serene,” 1847, p. 47 and 53,
has shewn how in a calm state of the atmosphere, which is a necessary
condition of strong radiation and of the formation of dew, the cooling
of the grassy surface is also promoted by the particles of air which
are already cooled sinking to the ground as being the heaviest. In the
vicinity of the equator, under the clouded sky of the Upper Orinoco,
the Rio Negro, and the Amazons River, the plains are clothed with dense
primeval forests; but to the north and south of this wooded region
there extend from the zone of palms and lofty dicotyledonous trees,
in the northern hemisphere, the Llanos of the Lower Orinoco the Meta
and the Guaviare, and in the southern hemisphere the Pampas of the Rio
de la Plata and of Patagonia. The space thus occupied by Savannahs or
grassy plains in South America is at least nine times as great as the
area of France.
The wooded region acts in a threefold manner in diminishing the
temperature by cooling shade, by evaporation, and by radiation.
Forests,--which in our temperate zone consist of trees living together
in “society,” _i. e._, many individuals of one, or of a few kinds, of
the families of Coniferæ or Amentaceæ, oaks, beeches, and birches,
but in the tropics, of an immense variety of trees living separately
or “unsocially,”--protect the ground from the direct rays of the sun,
evaporate fluids elaborated by the trees themselves, and cool the
strata of air in immediate contact with them by the radiation of heat
from their appendicular organs or leaves. The latter are far from being
all parallel with each other; they are, on the contrary, variously
inclined to the horizon, and, according to the law developed by Leslie
and Fourier, the influence of this inclination upon the quantity of
heat emitted by radiation is such, that the power of radiation (pouvoir
rayonnant) of a measured surface _a_, having a given oblique direction,
is equal to the “pouvoir rayonnant” which would belong to a surface of
the size of _a_, projected on a horizontal plane. Now in the initial
condition of radiation, of all the leaves which form the summit of a
tree and partly cover each other, those are first cooled which are
directed without any intervening screen towards the unclouded sky. The
cooling result (or the exhaustion of heat by emission) will be the
more considerable the greater the thinness of the leaves. A second
stratum of leaves has its upper surface turned to the under surface of
the first stratum, and will give out more heat by radiation towards
that stratum than it can receive by radiation from it. The result
of this unequal exchange will thus be a loss of temperature for the
second stratum of leaves also. A similar operation will continue from
stratum to stratum until all the leaves of the tree, by greater or
less radiation as modified by their diversity of position, have passed
into a state of stable equilibrium of which the law can be deduced by
mathematical analysis. In this manner, in the long and clear nights of
the equinoctial zone, the forest air contained in the intervals between
the strata of leaves becomes cooled by the process of radiation; and by
reason of the great quantity of its thin appendicular organs or leaves,
a tree, the horizontal section of whose summit would measure for
example 2000 square feet, would act in diminishing the temperature of
the air equivalently to a space of bare or turf-covered ground several
thousand times greater than 2000 square feet (Asie Centrale, T. iii.
p. 195-205). I have sought thus to develop in detail the complicated
effects which make up the total action of extensive forests upon the
atmosphere, because they have been so often touched upon in reference
to the important question concerning the climates of ancient Germany
and Gaul.