Extent.
a belt of water, roughly of an S-shape, between the western coasts of
Europe and Africa and the eastern coasts of North and South America. It
extends northward to the Arctic Basin and southward to the Great
Southern Ocean. For purposes of measurement the polar boundaries are
taken to be the Arctic and Antarctic circles, although in discussing the
configuration and circulation it is impossible to adhere strictly to
these limits. The Atlantic Ocean consists of two characteristic
divisions, the geographical equator forming a fairly satisfactory line
of division into North and South Atlantic. The North Atlantic, by far
the best-known of the main divisions of the hydrosphere, is remarkable
for the immense length of its coast-line and for the large number of
enclosed seas connected with it, including on the western side the
Caribbean Sea and Gulf of Mexico, the Gulf of St Lawrence and Hudson
Bay, and on the eastern side the Mediterranean and Black Sea, the North
Sea and the Baltic. The North Atlantic is connected with the Arctic
Basin by four main channels: (1) Hudson Strait, about 60 m. wide,
communicating with the gulfs and straits of the North American Arctic
archipelago; (2) Davis Strait, about 200 m. wide, leading to Baffin Bay;
(3) Denmark Strait, between Greenland and Iceland, 130 m. wide; and (4)
the "Norwegian Sea," about 400 m. wide, extending from Iceland to the
Faeroe Islands, the Shetland Islands and the coast of Norway. The width
of the North Atlantic in lat. 60 deg., approximately where it breaks up
into the branches just named, is nearly 2000 m.; in about lat. 50 deg.
N. the coasts of Ireland and Newfoundland approach to 1750 m.; the
breadth then increases rapidly to lat. 40 deg. N., and attains its
maximum of 4500 m. in lat. 25 deg. N.; farther south the minimum breadth
is reached between Africa and South America, Cape Palmas being only 1600
m. distant from Cape St Roque. In marked contrast to this, the South
Atlantic is distinguished by great simplicity of coast-line; inland seas
there are none, and it attains its greatest breadth as it merges with
the Southern Ocean; in lat. 35 deg. S. the width is 3700 m.
The total area of the North Atlantic, not counting inland seas connected
with it, is, according to G. Karstens, 36,438,000 sq. kilometres, or
10,588,000 sq. m.; including the inland seas the area is 45,641,000 sq.
kilometres or 13,262,000 sq. m. The area of the South Atlantic is
43,455,000 sq. kilometres, or 12,627,000 sq. m. Although not the most
extensive of the great oceans, the Atlantic has by far the largest
drainage area. The "long slopes" of the continents on both sides are
directed towards the Atlantic, which accordingly receives the waters of
a large proportion of the great rivers of the world, including the St
Lawrence, the Mississippi, the Orinoco, the Amazon, the rivers of the La
Plata, the Congo, the Niger, the Loire, the Rhine, the Elbe and the
great rivers of the Mediterranean and the Baltic. Sir J. Murray
estimates the total area of land draining to the Atlantic to be
13,432,000 sq. m., or with the Arctic area nearly 20,000,000 sq. m.,
nearly four times the area draining to the Pacific Ocean, and almost
precisely four times the area draining to the Indian Ocean. Murray's
calculations give the amount of precipitation received on this area at
15,800 cub. m. annually, and the river discharge from it at 3900 cub. m.
Relief of the bed.
The dominant feature of the relief of the Atlantic basin is a submarine
ridge running from north to south from about lat. 50 deg. N. to lat. 40
deg. S., almost exactly in the central line, and following the S-shape
of the coasts. Over this ridge the average depth is about 1700 fathoms.
Towards its northern end the ridge widens and rises to the plateau of
the Azores, and in about 50 deg. N. lat. it merges with the "Telegraph
Plateau," which extends across nearly the whole ocean from Ireland to
Newfoundland. North of the fiftieth parallel the depths diminish towards
the north-east, two long submarine ridges of volcanic origin extend
north-eastwards to the south-west of Iceland and to the Faeroe Islands,
and these, with their intervening valleys, end in a transverse ridge
connecting Greenland, through Iceland and the Faeroe Islands, with
North-western Scotland and the continental mass of Europe. The mean
depth over this ridge is about 250 fathoms, and the maximum depth
nowhere reaches 500 fathoms. The main basin of the Atlantic is thus cut
off from the Arctic basin, with which the area north of the ridge has
complete deep-water communication. This intermediate region, which has
Atlantic characteristics down to 300 fathoms, and at greater depths
belongs more properly to the Arctic Sea, commonly receives the name of
Norwegian Sea. On both sides of the central ridge deep troughs extend
southwards from the Telegraph plateau to the Southern Ocean, the deep
water coming close to the land all the way down on both sides. In these
troughs the depth is seldom much less than 3000 fathoms, and this is
exceeded in a series of patches to which Murray has given the name of
"Deeps." In the eastern trough the Peake Deep lies off the Bay of Biscay
in 20 deg. W. long., Monaco Deep and Chun Deep off the north-west of
Africa, Moseley Deep off the Cape Verde Islands, Krech Deep off the
Liberian coast, and Buchanan Deep off the mouth of the Congo. The
western trough extends northwards into Davis Strait, forming a
depression in the Telegraph plateau; to the south of Newfoundland and
Nova Scotia are Sigsbee Deep, Libbey Deep and Suhm Deep, each of small
area; north-east of the Bahamas Nares Deep forms the largest and deepest
depression in the Atlantic, in which a sounding of 4561 fathoms was
obtained (70 m. north of Porto Rico) by the U.S. ship "Blake" in 1883.
Immediately to the south of Nares Deep lies the smaller Makarov Deep;
and off the coast of South America are Tizard Deep and Havergal Deep.
Before the Antarctic expeditions of 1903-1904 our knowledge of the form
of the sea bottom south of 40 deg. S. lat. was almost wholly derived
from the soundings of the expedition of Sir J.C. Ross in the "Erebus"
and "Terror" (1839-1843), and the bathymetrical maps published were
largely the result of deductions based on one sounding taken by Ross in
68 deg. 34' S. lat., 12 deg. 49' W. long., in which he recorded a depth
exceeding 4000 fathoms. The Scottish Antarctic expedition has shown this
sounding to be erroneous; the "Scotia" obtained samples of bottom, in
almost the same spot, from a depth of 2660 fathoms. Combining the
results of recent soundings, Dr W.S. Bruce, the leader of the Scottish
expedition, finds that there is a ridge "extending in a curve from
Madagascar to Bouvet Island, and from Bouvet Island to the Sandwich
group, whence there is a forked connexion through the South Orkneys to
Graham's Land, and through South Georgia to the Falkland Islands and the
South American continent." Again, the central ridge of the South
Atlantic extends a thousand miles farther south than was supposed,
joining the east and west ridge, just described, between the Bouvet
Islands and the Sandwich group.
The foundations of our knowledge of the relief of the Atlantic basin may
be said to have been laid by the work of H.M.S. "Challenger"
(1873-1876), and the German ship "Gazelle" (1874-1876), the French
expedition in the "Travailleur" (1880), and the U.S. surveying vessel
"Blake" (1877 and later). Large numbers of additional soundings have
been made in recent years by cable ships, by the expeditions of H.S.H.
the prince of Monaco, the German "Valdivia" expedition under Professor
Chun (1898), and the combined Antarctic expeditions (1903-1904).
Islands.
The Atlantic Ocean contains a relatively small number of islands. The
only continental groups, besides some islands in the Mediterranean, are
Iceland, the British Isles, Newfoundland, the West Indies, and the
Falklands, and the chief oceanic islands are the Azores, Madeira, the
Canaries, the Cape Verde Islands, Ascension, St Helena, Tristan da Cunha
and Bouvet Island.
Mean depth, and bottom deposits.
The mean depth of the North Atlantic is, according to G. Karstens, 2047
fathoms. If we include the enclosed seas, the North Atlantic has a mean
depth of 1800 fathoms. The South Atlantic has a mean depth of 2067
fathoms.
The greater part of the bottom of the Atlantic is covered by a deposit
of Globigerina ooze, roughly the area between 1000 and 3000 fathoms, or
about 60% of the whole. At a depth of about 3000 fathoms, i.e. in the
"Deeps," the Globigerina ooze gradually gives place to red clay. In the
shallower tropical waters, especially on the central ridge, considerable
areas are covered by Pteropod ooze, a deposit consisting largely of the
shells of pelagic molluscs. Diatom ooze is the characteristic deposit in
high southern latitudes. The terrigenous deposits consist of blue muds,
red muds (abundant along the coast of Brazil, where the amount of
organic matter present is insufficient to reduce the iron in the matter
brought down by the great rivers to produce blue muds), green muds and
sands, and volcanic and coral detritus.
The question of the origin of the Atlantic basin, like that of the other
great divisions of the hydrosphere, is still unsettled. Most geologists
include the Atlantic with the other oceans in the view they adopt as to
its age; but E. Suess and M. Neumayr, while they regard the basin of the
Pacific as of great antiquity, believe the Atlantic to date only from
the Mesozoic age. Neumayr finds evidence of the existence of a continent
between Africa and South America, which protruded into the central North
Atlantic, in Jurassic times. F. Kossmat has shown that the Atlantic had
substantially its present form during the Cretaceous period.
Distribution of temperature.
In describing the mean distribution of temperature in the waters of the
Atlantic it is necessary to treat the northern and southern divisions
separately. The heat equator, or line of maximum mean surface
temperature, starts from the African coast in about 5 deg. N. lat., and
closely follows that parallel to 40 deg. W. long., where it bends
northwards to the Caribbean Sea. North of this line, near which the
temperature is a little over 80 deg. F., the gradient trends somewhat to
the east of north, and the temperature is slightly higher on the western
than on the eastern side until, in 45 deg. N. lat., the isothermal of 60
deg. F. runs nearly east and west. Beyond this parallel the gradient is
directed towards the north-west, and temperatures are much higher on the
European than on the American side. From the surface to 500 fathoms the
general form of the isothermals remains the same, except that instead of
an equatorial maximum belt there is a focus of maximum temperature off
the eastern coast of the United States. This focus occupies a larger
area and becomes of greater relative intensity as the depth increases
until, at 500 fathoms, it becomes an elongated belt extending right
across the ocean in about 30 deg. N. lat. Below 500 fathoms the western
centres of maximum disappear, and higher temperatures occur in the
eastern Atlantic off the Iberian peninsula and north-western Africa down
to at least 1000 fathoms; at still greater depths temperature gradually
becomes more and more uniform. The communication between the Atlantic
and Arctic basins being cut off, as already described, at a depth of
about 300 fathoms, the temperatures in the Norwegian Sea below that
level are essentially Arctic, usually below the freezing-point of fresh
water, except where the distribution is modified by the surface
circulation. The isothermals of mean surface temperature in the South
Atlantic are in the lower latitudes of an ~-shape, temperatures being
higher on the American than on the African side. In latitudes south of
30 deg. S. the curved form tends to disappear, the lines running more
and more directly east and west. Below the surface a focus of maximum
temperature appears off the coast of South America in about 30 deg. S.
lat., and of minimum temperature north and north-east of this maximum.
This distribution is most marked at about 300 fathoms, and disappears at
500 fathoms, beyond which depth the lines tend to become parallel and to
run east and west, the gradient slowly diminishing.
Salinity.
The Atlantic is by far the saltest of the great oceans. Its saltest
waters are found at the surface in two belts, one extending east and
west in the North Atlantic between 20 deg. and 30 deg. N. lat., and
another of almost equal salinity extending eastwards from the coast of
South America in 10 deg. to 20 deg. S. lat. In the equatorial region
between these belts the salinity is markedly less, especially in the
eastern part. North of the North Atlantic maximum the waters become
steadily fresher as latitude increases until the channels opening into
the Arctic basin are reached. In all of these water of relatively high
salinity usually appears for a long distance towards the north on the
eastern side of the channel, while on the western side the water is
comparatively fresh; but great variations occur at different seasons and
in different years. In the higher latitudes of the South Atlantic the
salinity diminishes steadily and tends to be uniform from east to west,
except near the southern extremity of South America, where the surface
waters are very fresh. Our knowledge of the salinity of waters below the
surface is as yet very defective, large areas being still unrepresented
by a single observation. The chief facts already established are the
greater saltness of the North Atlantic compared with the South Atlantic
at all depths, and the low salinity at all depths in the eastern
equatorial region, off the Gulf of Guinea.
Meteorology.
The wind circulation over the Atlantic is of a very definite character.
In the South Atlantic the narrow land surfaces of Africa and South
America produce comparatively little effect in disturbing the normal
planetary circulation. The tropical belt of high atmospheric pressure is
very marked in winter; it is weaker during the summer months, and at
that season the greater relative fall of pressure over the land cuts it
off into an oval-shaped anticyclone, the centre of which rests on the
coolest part of the sea surface in that latitude, near the Gulf of
Guinea. South of this anticyclone, from about the latitude of the Cape,
we find the region where, on account of the uninterrupted sea surface
right round the globe, the planetary circulation is developed to the
greatest extent known; the pressure gradient is steep, and the region is
swept continuously by strong westerly winds--the "roaring forties."
In the North Atlantic the distribution of pressure and resulting wind
circulation are very largely modified by the enormous areas of land and
frozen sea which surround the ocean on three sides. The tropical belt of
high pressure persists all the year round, but the immense demand for
air to supply the ascending currents over the heated land surfaces in
summer causes the normal descending movement to be largely reinforced;
hence the "North Atlantic anticyclone" is much larger, and its
circulation more vigorous, in summer than in winter. Again, during the
winter months pressure is relatively high over North America, Western
Eurasia and the Arctic regions; hence vast quantities of air are brought
down to the surface, and circulation must be kept up by ascending
currents over the ocean. The Atlantic anticyclone is, therefore, at its
weakest in winter, and on its polar side the polar eddy becomes a trough
of low pressure, extending roughly from Labrador to Iceland and Jan
Mayen, and traversed by a constant succession of cyclones. The net
effect of the surrounding land is, in fact, to reverse the seasonal
variations of the planetary circulation, but without destroying its
type. In the intermediate belt between the two high-pressure areas the
meteorological equator remains permanently north of the geographical
equator, moving between it and about 11 deg. N. lat.
Currents.
The part of this atmospheric circulation which is steadiest in its
action is the trade winds, and this is, therefore, the most effective in
producing drift movement of the surface waters. The trade winds give
rise, in the region most exposed to their influence, to two
westward-moving drifts--the equatorial currents, which are separated in
parts of their course by currents moving in the opposite direction along
the equatorial belt. These last may be of the nature of "reaction"
currents; they are collectively known as the equatorial counter-current.
On reaching the South American coast, the southern equatorial current
splits into two parts at Cape St Roque: one branch, the Brazil current,
is deflected southwards and follows the coast as a true stream current
at least as far as the river Plate. The second branch proceeds
north-westwards towards the West Indies, where it mingles with the
waters of the northern equatorial; and the two drifts, blocked by the
<-shape of the land, raise the level of the surface in the Gulf of
Mexico, the Caribbean Sea, and in the whole area outside the West
Indies. This congestion is relieved by what is probably the most rapid
and most voluminous stream current in the world, the Gulf Stream, which
runs along the coast of North America, separated from it by a narrow
strip of cold water, the "cold wall," to a point off the south-east of
Newfoundland. At this point the Gulf Stream water mixes with that from
the Labrador current (see below), and a drift current eastwards is set
up under the influence of the prevailing westerly winds: this is
generally called the Gulf Stream drift. When the Gulf Stream drift
approaches the eastern side of the Atlantic it splits into two parts,
one going southwards along the north-west coast of Africa, the Canaries
current, and another turning northwards and passing to the west of the
British Isles. Most of the Canaries current re-enters the northern
equatorial, but a certain proportion keeps to the African coast, unites
with the equatorial return currents, and penetrates into the Gulf of
Guinea. This last feature of the circulation is still somewhat obscure;
it is probably to be accounted for by the fact that on this part of the
coast the prevailing winds, although to a considerable extent monsoonal,
are off-shore winds, blowing the surface waters out to sea, and the
place of the water thus removed is filled up by the water derived either
from lower levels or from "reaction" currents.
The movements of the northern branch of the Gulf Stream drift have been
the object of more careful and more extended study than all the other
currents of the ocean put together, except, perhaps, the Gulf Stream
itself. The cruises of the "Porcupine" and "Lightning" which led
directly to the despatch of the "Challenger" expedition, were altogether
within its "sphere of influence"; so also was the great Norwegian
Atlantic expedition. More recently, the area has been further explored
by the German expedition in the ss. "National," the Danish "Ingolf"
expedition, and the minor expeditions of the "Michael Sars," "Jackal,"
"Research," &c., and since 1902 it has been periodically examined by the
International Council for the Study of the Sea. Much has also been done
by the discussion of observations made on board vessels belonging to the
mercantile marine of various countries. It may now be taken as generally
admitted that the current referred to breaks into three main branches.
The first passes northwards, most of it between the Faeroe and Shetland
Islands, to the coast of Norway, and so on to the Arctic basin, which,
as Nansen has shown, it fills to a great depth. The second, the Irminger
stream, passes up the west side of Iceland; and the third goes up to the
Greenland side of Davis Strait to Baffin Bay. These branches are
separated from one another at the surface by currents moving southwards:
one passes east of Iceland; the second, the Greenland current, skirts
the east coast of Greenland; and the third, the Labrador current already
mentioned, follows the western side of Davis Strait.
The development of the equatorial and the Brazil currents in the South
Atlantic has already been described. On the polar side of the
high-pressure area a west wind drift is under the control of the
"roaring forties," and on reaching South Africa part of this is
deflected and sent northwards along the west coast as the cold Benguella
current which rejoins the equatorial. In the central parts of the two
high-pressure areas there is practically no surface circulation. In the
North Atlantic this region is covered by enormous banks of gulf-weed
(_Sargassum bucciferum_), hence the name Sargasso Sea. The Sargasso Sea
is bounded, roughly, by the lines of 20 deg.-35 deg. N. lat. and 40
deg.-75 deg. W. long.
The sub-surface circulation in the Atlantic may be regarded as
consisting of two parts. Where surface water is banked up against the
land, as by the equatorial and Gulf Stream drift currents, it appears to
penetrate to very considerable depths; the escaping stream currents are
at first of great vertical thickness and part of the water at their
sources has a downward movement. In the case of the Gulf Stream, which
is not much impeded by the land, this descending motion is relatively
slight, being perhaps largely due to the greater specific gravity of the
water; it ceases to be perceptible beyond about 500 fathoms. On the
European-African side the descending movement is more marked, partly
because the coast-line is much more irregular and the northward current
is deflected against it by the earth's rotation, and partly because of
the outflow of salt water from the Mediterranean; here the movement is
traceable to at least 1000 fathoms. The northward movement of water
across the Norwegian Sea extends down from the surface to the
Iceland-Shetland ridge, where it is sharply cut off; the lower levels of
the Norwegian Sea are filled with ice-cold Arctic water, close down to
the ridge. The south-moving currents originating from melting ice are
probably quite shallow. The second part of the circulation in the depth
is the slow "creep" of water of very low temperature along the bottom.
The North Atlantic being altogether cut off from the Arctic regions, and
the vertical circulation being active, this movement is here practically
non-existent; but in the South Atlantic, where communication with the
Southern Ocean is perfectly open, Antarctic water can be traced to the
equator and even beyond.
The tides of the Atlantic Ocean are of great complexity. The tidal wave
of the Southern Ocean, which sweeps uninterruptedly round the globe from
the east to west, generates a secondary wave between Africa and South
America, which travels north at a rate dependent only on the depth of
the ocean. With this "free" wave is combined a "forced" wave, generated,
by the direct action of the sun and moon, within the Atlantic area
itself. Nothing is known about the relative importance of these two
waves. (H. N. D.)
See also OCEANS AND OCEANOGRAPHY.
ATLANTIS, ATLANTIS, or ATLANTICA, a legendary island in the Atlantic
Ocean, first mentioned by Plato in the _Timaeus_. Plato describes how
certain Egyptian priests, in a conversation with Solon, represented the
island as a country larger than Asia Minor and Libya united, and
situated just beyond the Pillars of Hercules (Straits of Gibraltar).
Beyond it lay an archipelago of lesser islands. According to the
priests, Atlantis had been a powerful kingdom nine thousand years before
the birth of Solon, and its armies had overrun the lands which bordered
the Mediterranean. Athens alone had withstood them with success. Finally
the sea had overwhelmed Atlantis, and had thenceforward become
unnavigable owing to the shoals which marked the spot. In the _Critias_
Plato adds a history of the ideal commonwealth of Atlantis. It is
impossible to decide how far this legend is due to Plato's invention,
and how far it is based on facts of which no record remains. Medieval
writers, for whom the tale was preserved by the Arabian geographers,
believed it true, and were fortified in their belief by numerous
traditions of islands in the western sea, which offered various points
of resemblance to Atlantis. Such in particular were the Greek Isles of
the Blest, or Fortunate Islands, the Welsh Avalon, the Portuguese
Antilia or Isle of Seven Cities, and St Brendan's island, the subject of
many sagas in many languages. These, which are described in separate
articles, helped to maintain the tradition of an earthly paradise which
had become associated with the myth of Atlantis; and all except Avalon
were marked in maps of the 14th and 15th centuries, and formed the
object of voyages of discovery, in one case (St Brendan's island) until
the 18th century. In early legends, of whatever nationality, they are
almost invariably described in terms which closely resemble Homer's
account of the island of the Phaeacians (_Od._ viii.)--a fact which may
be an indication of their common origin in some folk-tale current among
several races. Somewhat similar legends are those of the island of
Brazil (q.v.), of Lyonnesse (q.v.), the sunken land off the Cornish
coast, of the lost Breton city of Is, and of Mayda or Asmaide--the
French _Isle Verte_ and Portuguese _Ilha Verde_ or "Green Island"--which
appears in many folk-tales from Gibraltar to the Hebrides, and until
1853 was marked on English charts as a rock in 44 deg. 48' N. and 26
deg. 10' W. After the Renaissance, with its renewal of interest in
Platonic studies, numerous attempts were made to rationalize the myth of
Atlantis. The island was variously identified with America, Scandinavia,
the Canaries and even Palestine; ethnologists saw in its inhabitants the
ancestors of the Guanchos, the Basques or the ancient Italians; and even
in the 17th and 18th centuries the credibility of the whole legend was
seriously debated, and sometimes admitted, even by Montaigne, Buffon and
Voltaire.
For the theory that Atlantis is to be identified with Crete in the
Minoan period, see "The Lost Continent" in _The Times_ (London) for
the 19th of February 1909. See also "Dissertation sur l'Atlantide" in
T.H. Martin's _Etudes sur le Timee_ (1841).
ATLAS, in Greek mythology, the "endurer," a son of the Titan Iapetus and
Clymene (or Asia), brother of Prometheus. Homer, in the _Odyssey_ (i.
52) speaks of him as "one who knows the depths of the whole sea, and
keeps the tall pillars which hold heaven and earth asunder." In the
first instance he seems to have been a marine creation. The pillars
which he supported were thought to rest in the sea, immediately beyond
the most western horizon. But as the Greeks' knowledge of the west
increased, the name of Atlas was transferred to a hill in the north-west
of Africa. Later, he was represented as a king of that district, rich in
flocks and herds, and owner of the garden of the Hesperides, who was
turned into a rocky mountain when Perseus, to punish him for his
inhospitality, showed him the Gorgon's head (Ovid, _Metam._ iv. 627).
Finally, Atlas was explained as the name of a primitive astronomer, who
was said to have made the first celestial globe (Diodorus iii. 60). He
was the father of the Pleiades and Hyades; according to Homer, of
Calypso. In works of art he is represented as carrying the heavens or
the terrestrial globe. The Farnese statue of Atlas in the Naples museum
is well known.
The plural form ATLANTES is the classical term in architecture for the
male sculptured figures supporting a superstructure as in the baths at
Pompeii, and in the temple at Agrigentum in Sicily. In 18th-century
architecture half-figures of men with strong muscular development were
used to support balconies (see CARYATIDES and TELAMONES).
A figure of Atlas supporting the heavens is often found as a
frontispiece in early collections of maps, and is said to have been
first thus used by Mercator. The name is hence applied to a volume of
maps (see MAP), and similarly to a volume which contains a tabular
conspectus of a subject, such as an atlas of ethnographical, subjects or
anatomical plates. It is also used of a large size of drawing paper.
The name "atlas," an Arabic word meaning "smooth," applied to a smooth
cloth, is sometimes found in English, and is the usual German word, for
"satin."
ATLAS MOUNTAINS, the general name for the mountain chains running more
or less parallel to the coast of North-west Africa. They extend from
Cape Nun on the west to the Gulf of Gabes on the east, a distance of
some 1500 m., traversing Morocco, Algeria and Tunisia. To their south
lies the Saharan desert. The Atlas consist of many distinct ranges, but
they can be roughly divided into two main chains: (1) the Maritime
Atlas, i.e. the ranges overlooking the Mediterranean from Ceuta to
Cape Bon; (2) the inner and more elevated ranges, which, starting from
the Atlantic at Cape Ghir in Sus, run south of the coast ranges and are
separated from them by high plateaus. This general disposition is seen
most distinctly in eastern Morocco and Algeria. The western inner ranges
are the most important of the whole system, and in the present article
are described first as _the Moroccan Ranges_. The maritime Atlas and the
inner ranges in Algeria and Tunisia are then treated under the heading
_Eastern Ranges_.
_The Moroccan Ranges._--This section of the Atlas, known to the
inhabitants of Morocco by its Berber name, Idraren Draren or the
"Mountains of Mountains," consists of five distinct ranges, varying in
length and height, but disposed more or less parallel to one another in
a general direction from south-west to north-east, with a slight
curvature towards the Sahara.
1. The main range, that known as the Great Atlas, occupies a central
position in the system, and is by far the longest and loftiest chain. It
has an average height of over 11,000 ft., whereas the loftiest peaks in
Algeria do not exceed 8000 ft., and the highest in Tunisia are under
6000 ft. Towards the Dahra district at the north-east end the fall is
gradual and continuous, but at the opposite extremity facing the
Atlantic between Agadir and Mogador it is precipitous. Although only one
or two peaks reach the line of perpetual snow, several of the loftiest
summits are snowclad during the greater part of the year. The northern
sides and tops of the lower heights are often covered with dense forests
of oak, cork, pine, cedar and other trees, with walnuts up to the limit
of irrigation. Their slopes enclose well-watered valleys of great
fertility, in which the Berber tribes cultivate tiny irrigated fields,
their houses clinging to the hill-sides. The southern flanks, being
exposed to the hot dry winds of the Sahara, are generally destitute of
vegetation.
At several points the crest of the range has been deeply eroded by old
glaciers and running waters, and thus have been formed a number of
devious passes. The central section, culminating in Tizi n 'Tagharat or
Tinzar, a peak estimated at 15,000 ft. high, maintains a mean altitude
of 11,600 ft., and from this great mass of schists and sandstones a
number of secondary ridges radiate in all directions, forming divides
between the rivers Dra'a, Sus, Um-er-Rabia, Sebu, Mulwiya and Ghir,
which flow respectively to the south-west, the west, north-west, north,
north-east and south-east. All are swift and unnavigable, save perhaps
for a few miles from their mouths. With the exception of the Dra'a, the
streams rising on the side of the range facing the Sahara do not reach
the sea, but form marshes or lagoons at one season, and at another are
lost in the dry soil of the desert.
For a distance of 100 m. the central section nowhere presents any passes
accessible to caravans, but south-westward two gaps in the range afford
communication between the Tansift and Sus basins, those respectively of
Gindafi and Bibawan. A few summits in the extreme south-west in the
neighbourhood of Cape Ghir still exceed 11,000 ft., and although the
steadily rising ground from the coast and the prominence of nearer
summits detract from the apparent height, this is on an average greater
than that of the European Alps. The most imposing view is to be obtained
from the plain of Marrakesh, only some 1000 ft. above sea-level,
immediately north of the highest peaks. Besides huge masses of old
schists and sandstones, the range contains extensive limestone, marble,
diorite, basalt and porphyry formations, while granite prevails on its
southern slopes. The presence of enormous glaciers in the Ice Age is
attested by the moraines at the Atlantic end, and by other indications
farther east. The best-known passes are: (1) The Bibawan in the upper
Wad Sus basin (4150 ft.); (2) the Gindafi, giving access from Marrakesh
to Tarudant, rugged and difficult, but low; (3) the Tagharat, difficult
and little used, leading to the Dra'a valley (11,484 ft.); (4) the Glawi
(7600 ft.); (5) Tizi n 'Tilghemt (7250 ft.), leading to Tafilet
(Tafilalt) and the Wad Ghir.
2. The lower portion of the Moroccan Atlas (sometimes called the Middle
Atlas), extending north-east and east from an undefined point to the
north of the Great Atlas to near the frontier of Algeria, is crossed by
the pass from Fez to Tafilalt. Both slopes are wooded, and its forests
are the only parts of Morocco where the lion still survives. From the
north this range, which is only partly explored, presents a somewhat
regular series of snowy crests.
3. The Anti-Atlas or Jebel Saghru, also known as the Lesser Atlas,
running parallel to and south of the central range, is one of the least
elevated chains in the system, having a mean altitude of not more than
5000 ft., although some peaks and even passes exceed 6000 ft. At one
point it is pierced by a gap scarcely five paces wide with walls of
variegated marbles polished by the transport of goods. As to the
relation of the Anti-Atlas to the Atlas proper at its western end
nothing certain is known.
The two more or less parallel ranges which complete the western system
are less important:--(4) the Jebel Bani, south of the Anti-Atlas, a low,
narrow rocky ridge with a height of 3000 ft. in its central parts; and
(5) the Mountains of Ghaiata, north of the Middle Atlas, not a
continuous range, but a series of broken mountain masses from 3000 to
3500 ft. high, to the south of Fez, Taza and Tlemcen.
_The Eastern Ranges._--The eastern division of the Atlas, which forms
the backbone of Algeria and Tunisia, is adequately known with the
exception of the small portion in Morocco forming the province of
Er-Rif. The lesser range, nearer the sea, known to the French as the
Maritime Atlas, calls for little detailed notice. From Ceuta, above
which towers Jebel Musa--about 2800 ft.--to Melilla, a distance of some
150 m., the Rif Mountains face the Mediterranean, and here, as along the
whole coast eastward to Cape Bon, many rugged rocks rise boldly above
the general level. In Algeria the Maritime Atlas has five chief ranges,
several mountains rising over 5000 ft. The Jurjura range, extending
through Kabylia from Algiers to Bougie, contains the peaks of Lalla
Kedija (7542 ft.), the culminating point of the maritime chains, and
Babor (6447 ft.). (See further ALGERIA.) The Mejerda range, which
extends into Tunisia, has no heights exceeding 3700 ft. It was in these
coast mountains of Algeria that the Romans quarried the celebrated
Numidian marbles.
The southern or main range of the Eastern division is known by the
French as the Saharan Atlas. On its western extremity it is linked by
secondary ranges to the mountain system of Morocco. The Saharan Atlas is
essentially one chain, though known under different names: Jebel K'sur
and Jebel Amur on the west, and Jebel Aures in the east. The central
part, the Zab Mountains, is of lower elevation, the Saharan Atlas
reaching its culminating point, Jebel Shellia (7611 ft. above the sea),
in the Aures. This range sends a branch northward which joins the
Mejerda range of the Maritime Atlas, and another branch runs south by
Gafsa to the Gulf of Gabes. Here Mount Sidi Ali bu Musin reaches a
height of 5700 ft., the highest point in Tunisia. In the Saharan Atlas
the passes leading to or from the desert are numerous, and in most
instances easy. Both in the east (at Batna) and the west (at Ain Sefra)
the mountains are traversed by railways, which, starting from
Mediterranean seaports, take the traveller into the Sahara.
_History and Exploration._--The name Atlas given to these mountains by
Europeans--but never used by the native races--is derived from that of
the mythical Greek god represented as carrying the globe on his
shoulders, and applied to the high and distant mountains of the west,
where Atlas was supposed to dwell. From time immemorial the Atlas have
been the home of Berber races, and those living in the least accessible
regions have retained a measure of independence throughout their
recorded history. Thus some of the mountain districts of Kabylia had
never been visited by Europeans until the French military expedition of
1857. But in general the Maritime range was well known to the Romans.
The Jebel Amur was traversed by the column which seized El Aghuat in
1852, and from that time dates the survey of the mountains.
The ancient caravan route from Mauretania to the western Sudan crossed
the lower Moroccan Atlas by the pass of Tilghemt and passed through the
oasis of Tafilalt, formerly known as Sajilmasa ["Sigilmassa"], on the
east side of the Anti-Atlas. The Moroccan system was visited, and in
some instances crossed, by various European travellers carried into
slavery by the Salli rovers, and was traversed by Rene Caille in 1828 on
his journey home from Timbuktu, but the first detailed exploration was
made by Gerhard Rohlfs in 1861-1862. Previous to that almost the only
special report was the misleading one of Lieut. Washington, attached to
the British embassy of 1837, who from insufficient data estimated the
height of Mount Tagharat, to which he gave the indefinite name of
Miltsin (i.e. _Mul et-Tizin_, "Lord of the Peaks"), as 11,400 ft.
instead of about 15,000 ft.
In 1871 the first scientific expedition, consisting of Dr (afterwards
Sir) J.D. Hooker, Mr John Ball and Mr G. Maw, explored the central part
of the Great Atlas with the special object of investigating its flora
and determining its relation to that of the mountains of Europe. They
ascended by the Ait Mizan valley to the Tagharat pass (11,484 ft.), and
by the Amsmiz valley to the summit of Jebel Tezah (11,972 ft.). In the
Tagharat pass Mr Maw was the only one of the party who reached the
watershed; but from Jebel Tezah a good view was obtained southward
across the great valley of the Sus to the Anti-Atlas, which appeared to
be from 9000 to 10,000 ft. high. Dr Oskar Lenz in 1879-1880 surveyed a
part of the Great Atlas north of Tarudant, determined a pass south of
Iligh in the Anti-Atlas, and penetrated thence across the Sahara to
Timbuktu. He was followed in 1883-1884 by Vicomte Ch. de Foucauld, whose
extensive itineraries include many districts that had never before been
visited by any Europeans. Such were parts of the first and middle
ranges, crossed once; three routes over the Great Atlas, which was,
moreover, followed along both flanks for nearly its whole length; and
six journeys across the Anti-Atlas, with a general survey of the foot of
this range and several passages over the Jebel Bani. Then came Joseph
Thomson, who explored some of the central parts, and made the highest
ascent yet achieved, that of Mount Likimt, 13,150 ft., but broke little
new ground, and failed to cross the main range (1888); and Walter B.
Harris, who explored some of the southern slopes and crossed the Atlas
at two points during his expedition to Tafilalt in 1894. In 1901 and
again in 1905 the marquis de Segonzac, a Frenchman, made extensive
journeys in the Moroccan ranges. He crossed the Great Atlas in its
central section, explored its southern border, and, in part, the Middle
and Anti-Atlas ranges. A member of his expeditions, de Flotte
Rocquevaire, made a triangulation of part of the western portion of the
main Atlas, his labours affording a basis for the co-ordination of the
work of previous explorers. (See also MOROCCO, ALGERIA, TUNISIA and
SAHARA.)
AUTHORITIES.--Vicomte Ch. de Foucauld, _Reconnaissance au Maroc
1883-1884_ (Paris, 1888, almost the sole authority for the geography
of the Atlas; his book gives the result of careful surveys, and is
illustrated with a good collection of maps and sketches); Hooker, Ball
and Maw, _Marocco and the Great Atlas_ (London, 1879, a most valuable
contribution, always scientific and trustworthy, especially as to
botany and geology); Joseph Thomson, _Travels in the Atlas and
Southern Morocco_ (London, 1889, valuable geographical and geological
data); Louis Gentil, _Mission de Segonzac, &c._ (Paris, 1906; the
author was geologist to the 1905 expedition); Gerhard Rohlfs,
_Adventures in Morocco_ (London, 1874); Walter B. Harris, _Tafilet, a
Journey of Exploration in the Atlas Mountains, &c._ (London, 1895),
full of valuable information; Budgett Meakin, _The Land of the Moors_
(London, 1901), first and last chapters; Dr Oskar Lenz _Timbuktu:
Reise durch Marokko_, vol. i. (Leipzig, 1884).
ATMOLYSIS (Gr. [Greek: atmos], vapour: [Greek: lyein], to loosen), a
term invented by Thomas Graham to denote the separation of a mixture of
gases by taking advantage of their different rates of diffusion through
a porous septum or diaphragm (see DIFFUSION).
ATMOSPHERE (Gr. [Greek: atmos], vapour; [Greek: sphaira], a sphere), the
aeriform envelope encircling the earth; also the envelope of a
particular gas or gases about any solid or liquid. Meteorological
phenomena seated more directly in the atmosphere obtained early
recognition; thus Hesiod, in his _Works and Days_, speculated on the
origin of winds, ascribing them to the heating effects of the sun on the
air. Ctesibius of Alexandria, Hero and others, founded the science of
pneumatics on observations on the physical properties of air. Anaximenes
made air the primordial substance, and it was one of the Aristotelian
elements. A direct proof of its material nature was given by Galileo,
who weighed a copper ball containing compressed air.
Before the development of pneumatic chemistry, air was regarded as a
distinct chemical unit or element. The study of calcination and
combustion during the 17th and 18th centuries culminated in the
discovery that air consists chiefly of a mixture of two gases, oxygen
and nitrogen. Cavendish, Priestley, Lavoisier and others contributed to
this result. Cavendish made many analyses: from more than 500
determinations of air in winter and summer, in wet and clear weather,
and in town and country, he discerned the mean composition of the
atmosphere to be, oxygen 20.833% and nitrogen 79.167% The same
experimenter noticed the presence of an inert gas, in very minute
amount; this gas, afterwards investigated by Rayleigh and Ramsay, is now
named argon (q.v.).
The constancy of composition shown by repeated analyses of atmospheric
air led to the view that it was a chemical compound of nitrogen and
oxygen; but there was no experimental confirmation of this idea, and all
observations tended to the view that it is simply a mechanical mixture.
Thus, the gases are not present in simple multiples of their combining
weights; atmospheric air results when oxygen and nitrogen are mixed in
the prescribed ratio, the mixing being unattended by any manifestation
of energy, such as is invariably associated with a chemical action; the
gases may be mechanically separated by atmolysis, i.e. by taking
advantage of the different rates of diffusion of the two gases; the
solubility of air in water corresponds with the "law of partial
pressures," each gas being absorbed in amount proportional to its
pressure and coefficient of absorption, and oxygen being much more
soluble than nitrogen (in the ratio of .04114 to .02035 at 0 deg.); air
expelled from water by boiling is always richer in oxygen.
Various agencies are at work tending to modify the composition of the
atmosphere, but these so neutralize each other as to leave it
practically unaltered. Minute variations, however, do occur. Bunsen
analysed fifteen examples of air collected at the same place at
different times, and found the extreme range in the percentage of oxygen
to be from 20.97 to 20.84. Regnault, from analyses of the air of Paris,
obtained a variation of 20.999 to 20.913; country air varied from 20.903
to 21.000; while air taken from over the sea showed an extreme variation
of 20.940 to 20.850. Angus Smith determined London air to vary in oxygen
content from 20.857 to 20.95, the air in parks and open spaces showing
the higher percentage; Glasgow air showed similar results, varying from
20.887 in the streets to 20.929 in open spaces.
In addition to nitrogen and oxygen, there are a number of other gases
and vapours generally present in the atmosphere. Of these, argon and its
allies were the last to be definitely isolated. Carbon dioxide is
invariably present, as was inferred by Dr David Macbride (1726-1778) of
Dublin in 1764, but in a proportion which is not absolutely constant; it
tends to increase at night, and during dry winds and fogs, and it is
greater in towns than in the country and on land than on the sea. Water
vapour is always present; the amount is determined by instruments termed
hygrometers (q.v.). Ozone (q.v.) occurs, in an amount supposed to be
associated with the development of atmospheric electricity (lightning,
&c.); this amount varies with the seasons, being a maximum in spring,
and decreasing through summer and autumn to a minimum in winter.
Hydrogen dioxide occurs in a manner closely resembling ozone. Nitric
acid and lower nitrogen oxides are present, being formed by electrical
discharges, and by the oxidation of atmospheric ammonia by ozone. The
amount of nitric acid varies from place to place; rain-water, collected
in the country, has been found to contain an average of 0.5 parts in a
million, but town rain-water contains more, the greater amounts being
present in the more densely populated districts. Ammonia is also
present, but in very varying amounts, ranging from 135 to 0.1 parts
(calculated as carbonate) in a million parts of air. Ammonia is carried
back to the soil by means of rain, and there plays an important part in
providing nitrogenous matter which is afterwards assimilated by
vegetable life.
The average volume composition of the gases of the atmosphere may be
represented (in parts per 10,000) as follows:--
Oxygen 2065.94 Ozone 0.015
Nitrogen 7711.60 Aqueous vapour 140.00
Argon (about) 79.00 Nitric acid 0.08
Carbon dioxide 3.36 Ammonia 0.005
In addition to these gases, there are always present in the atmosphere
many micro-organisms or bacteria (see BACTERIOLOGY); another invariable
constituent is dust (q.v.), which plays an important part in
meteorological phenomena.
Reference should be made to the articles BAROMETER, CLIMATE and
METEOROLOGY for the measurement and variation of the pressure of the
atmosphere, and the discussion of other properties.
ATMOSPHERIC ELECTRICITY. 1. It was not until the middle of the 18th
century that experiments due to Benjamin Franklin showed that the
electric phenomena of the atmosphere are not fundamentally different
from those produced in the laboratory. For the next century the rate of
progress was slow, though the ideas of Volta in Italy and the
instrumental devices of Sir Francis Ronalds in England merit
recognition. The invention of the portable electrometer and the
water-dropping electrograph by Lord Kelvin in the middle of the 19th
century, and the greater definiteness thus introduced into observational
results, were notable events. Towards the end of the 19th century came
the discovery made by W. Linss (6)[1] and by J. Elster and H. Geitel (7)
that even the most perfectly insulated conductors lose their charge, and
that this loss depends on atmospheric conditions. Hard on this came the
recognition of the fact that freely charged positive and negative ions
are always present in the atmosphere, and that a radioactive emanation
can be collected. Whilst no small amount of observational work has been
done in these new branches of atmospheric electricity, the science has
still not developed to a considerable extent beyond preliminary stages.
Observations have usually been limited to a portion of the year, or to a
few hours of the day, whilst the results from different stations differ
much in details. It is thus difficult to form a judgment as to what has
most claim to acceptance as the general law, and what may be regarded as
local or exceptional.
2. _Potential Gradient._--In dry weather the electric potential in the
atmosphere is normally positive relative to the earth, and increases
with the height. The existence of _earth currents_ (q.v.) shows that the
earth, strictly speaking, is not all at one potential, but the natural
differences of potential between points on the earth's surface a mile
apart are insignificant compared to the normal potential difference
between the earth and a point one foot above it. What is aimed at in
ordinary observations of atmospheric potential is the measurement of the
difference of potential between the earth and a point a given distance
above it, or of the difference of potential between two points in the
same vertical line a given distance apart. Let a conductor, say a
metallic sphere, be supported by a metal rod of negligible electric
capacity whose other end is earthed. As the whole conductor must be at
zero (i.e. the earth's) potential, there must be an induced charge on
the sphere, producing at its centre a potential equal but of opposite
sign to what would exist at the same spot in free air. This neglects any
charge in the air displaced by the sphere, and assumes a statical state
of conditions and that the conductor itself exerts no disturbing
influence. Suppose now that the sphere's earth connexion is broken and
that it is carried without loss of charge inside a building at zero
potential. If its potential as observed there is -V (volts), then the
potential of the air at the spot occupied by the sphere was +V. This
method in one shape or another has been often employed. Suppose next
that a fixed insulated conductor is somehow kept at the potential of the
air at a given point, then the measurement of its potential is
equivalent to a measurement of that of the air. This is the basis of a
variety of methods. In the earliest the conductor was represented by
long metal wires, supported by silk or other insulating material, and
left to pick up the air's potential. The addition of sharp points was a
step in advance; but the method hardly became a quantitative one until
the sharp points were replaced by a flame (fuse, gas, lamp), or by a
liquid jet breaking into drops. The matter leaving the conductor,
whether the products of combustion or the drops of a liquid, supplies
the means of securing equality of potential between the conductor and
the air at the spot where the matter quits electrical connexion with the
conductor. Of late years the function of the collector is discharged in
some forms of apparatus by a salt of radium. Of flame collectors the two
best known are Lord Kelvin's portable electrometer with a fuse, or F.
Exner's gold leaf electroscope in conjunction with an oil lamp or gas
flame. Of liquid collectors the representative is Lord Kelvin's
water-dropping electrograph; while Benndorf's is the form of radium
collector that has been most used. It cannot be said that any one form
of collector is superior all round. Flame collectors blow out in high
winds, whilst water-droppers are apt to get frozen in winter. At first
sight the balance of advantages seems to lie with radium. But while
gaseous products and even falling water are capable of modifying
electrical conditions in their immediate neighbourhood, the "infection"
produced by radium is more insidious, and other drawbacks present
themselves in practice. It requires a radium salt of high radioactivity
to be at all comparable in effectiveness with a good water-dropper.
Experiments by F. Linke (8) indicated that a water-dropper having a
number of fine holes, or having a fine jet under a considerable
pressure, picks up the potential in about a tenth of the time required
by the ordinary radium preparation protected by a glass tube. These fine
jet droppers with a mixture of alcohol and water have proved very
effective for balloon observations.