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Division of Mollusies
. Sectional Library
ON THE

Depth, Temperature, and Currents of the Ocean

BY

LOLEN | Avie Sa WwW Lib

MEMBER OF THE CIVILIAN SCIENTIFIC STAFF OF H.M.S. ‘‘ CHALLENGER”

GHith Charts and Diagrams by the Author

- <NTHSONIA N

MAY 0 3 1988

LSS KAKICS

London:
MARCUS WARD & CO., 67 & 68, CHANDOS STREET, STRAND

AND ROYAL ULSTER WORKS, BELFAST

1877

CONTENTS.

PAGE
PREFACE, . : : 5 : : ° ee

CE NE Eat
DEPTH OF THE OCEAN.

Distribution of Land and Water, 11—Principal Areas of Elevation and of
Depression, 12—Results of Soundings obtained to present Date, 13—Average
Depth of the Ocean, 14—Greatest Depths ascertained in the Atlantic, 14; in
the Indian Ocean and in the Pacific, 15 Configuration of the Sea-bottom, 16
—The Basin of the Atlantic, 17—Central Atlantic Plateau, 18—The Basin of
the Indian Ocean, 19—The Kerguelen Plateau, 19—The Basin of the
Pacific Ocean, 2o—Submerged Plateau in the South-Eastern Pacific, 2t—
Large Area of Depression upon the Limits of the South Pacific and the
Southern Ocean, 22—The Basin of the Southern Ocean, 23—The Basin of
the Antarctic Sea, 24—The Basin of the Arctic Sea, 25 —Area of Depression
between Greenland and Norway, 26.

Cl Arnis Rey aie
TEMPERATURE OF THE OCEAN.

Surface Temperature, 27—-Total Range of Oceanic Temperatures, 28—Influence
of Currents upon the Distribution of Temperature, 29—Oceanic Currents
an Adequate Cause of Alterations of Climate, 29 Deep-Sea Temperature,
30—Earliest Systematic Observation of Deep-Sea Temperature, 30—
Effect of Pressure upon the ordinary Thermometer, 31—The Miller-
Casella Thermometer, 32—Serial Soundings and Temperature Curves, 34—
Construction of Temperature Curves, 35—Deductions from the Curve, 36
—General Decrease of Temperature from the Surface to the Bottom, 37—

ho

Contents.

Distribution of Temperature in nearly land-locked Basins, 38—Increase of
Temperature from the Surface towards the lower Strata in the Polar
Regions, 39—Distribution of Oceanic Temperature effected through the
Agency of Currents, 4o.—Formation of Intermediate Strata, 41—Connection
between the Gradients of the Curve and the relative Motion of the different
Strata, 42—Illustrations from the Temperature Soundings of H.MLS.
*“‘ Challenger,” 42-45.

CHA Pa ER lh.
CURRENTS OF THE OCEAN.

The Aqueous and the Aerial Oceans, 46—Resemblance between the Phenomena
in the two Terrestrial Envelopes, 47—Thermal Circulation, 47—Disproportion
between the Vertical and the Horizontal Extension of the Aqueous and Aerial
Envelopes, 48—Conclusions based upon this Disproportion, 49—Influence
of the several Areas of the Earth’s Surface upon Oceanic and Atmospheric
Circulation, 50—Parallelism between Oceanic and Atmospheric Currents,
50—Formation of Belts of Calms and Belts of Currents, 51—The Critical
Latitudes, 52—Effect of the present Distribution of Land and Water upon
Atmospheric and Oceanic Circulation, 52—Subdivision of the latter into
Distinct Areas of Circulation, 52—Atmospheric and Oceanic Currents
revolve round Areas of High Barometric Pressure, 53—Effect of the
Sun’s apparent Progress from Tropic to Tropic upon Atmospheric and
Oceanic Currents, 53—Surface and Under-Currents, 54—Relation between
the Colour of Sea-water and the Percentage of Salt held in solution, 54—
Limits of the Specific Gravity of Salt Water according to Temperature and
to Percentage of Salt, 55—-Change of a Surface-Current into an Under-
Current, or Vertical Circulation of the Oceanic Waters, 56— Diagram of
the System of Circulation in an Oceanic Basin, 57.

CTA Pah

THE TEMPERATURE SECTIONS SURVEYED BY H.M.S. ‘*CHALLENGER” IN THE
ATLANTIC.

Explanation of Diagrams and Tables, 5S—Section from Teneriffe to Sombrero,
59—Contrast between the Eastern and Western Basin of the North Atlantic,
61—Observed Rise of the Isotherms with the Sea-bottom, 62—Section

Contents.

ios)

from St. Thomas to Halifax, 63—The Gulf Stream a branch of the North
Atlantic Equatorial Current, 65—Parallelism between the Gulf Stream and
the Kuro-Siwo Current, 66—Area of Alternate Streaks of Warm and Cold
Water between Halifax and Bermudas, 67—Section between Cape May
and Madeira, 69—Temperature of the Gulf Stream, 71—Cold Current off
the Azores, 72—The Sargasso Sea, 73—Section from Madeira to Tristan
d’Acunha, 74—The Equatorial Belt, 75—Cold Under-Current across the
Equator, 77—Isothermal and Isobathymetrical Lines, 79—Section from
Cape Palmas to Cape S. Roque, 802—Contrast between the Eastern and
Western Basin of the South Atlantic, 81—Section between Cape S. Roque
and Tristan d’Acunha, 81—Section from the Falkland Islands to the Cape
of Good Hope, 83—The Antarctic Under-Current of the South Atlantic, 85
—The Equatorial Current of the South Atlantic, 86—The Antarctic Surface-
Current, 86.

Gall Ae Re ON.

THE TEMPERATURE SECTIONS SURVEYED BY H.M.S. ‘“‘CHALLENGER” IN THE
SOUTHERN OCEAN, THE INDIAN ARCHIPELAGO, AND THE PACIFIC.

Section from the Cape of Good Hope to the Ice-barrier and to Melbourne, 88—
The Agulhas Current, 89—Sudden Changes of Temperature in Simons
Bay and Table Bay, 91—Encounter of Warm and Cold Currents off the
Cape of Good Hope, 93—Icebergs in the Southern Ocean, 95—The South
Australian Current, 96—Section from Sydney to Cook Strait, New Zealand,
and from Cook Strait to Tonga Tabu, 97—The Basin between Australia
and New Zealand, 99—Sections from Tonga Tabu to Torres Strait, too —
The Melanesian Sea, 101—Conclusions derived from the Temperature
Conditions of the latter, 103—Sections from Torres Strait to Hong-kong, and
from Hong-kong to the Admiralty Islands, rog—The Arafura Sea, t0o4—
Existence of a Deep Channel between the Papua-Australian Plateau and
the Plateau of the Indian Archipelago, 105—The Banda Sea, 105-—
The Molucca Passage, 107—The Sea of Celebes, 107—The Sulu Sea,
107—The Philippine Inland Seas, 108 —The China Sea, 108—The Sea of
Papua, 108—Section from the Admiralty Islands to Japan, 109—The
Sea of Magallanes, tog—The Kuro-Siwo Current and the Gulf Stream
compared, rr1—The Arctic Current of the North Pacific, 113—Area of
Alternate Streaks of Warm and Cold Water south of Japan, 115—-Section
from Yokohama to Station 253, 115—The Kuro-Siwo Current and

4 Contents.

the Arctic Current, 116—Area of Alternate Streaks of Warm and Cold
Water east of Japan, r16—Section from Station 253, along the Meridian
of Honolulu and Tahiti, to Station 288, 117—The Pacific and the Atlantic
Oceans compared, 117—The Equatorial Counter Currents of the Atlantic,
the Indian, and the Pacific Oceans, 119g—Section from Station 288 to
Valparaiso and Magellan Straits, 121.

GH APT Eine TV a.
THE BED OF THE OCEAN.

Changes in the Distribution of Land and Water, 124—-Average Height of the
Dry Land, 126—Low Angle of Inclination of the Sea-bottom, 127—Forma-
tion of Sub-oceanic Strata, 128—Marginal and Central Deposits, r2g—
Formation of Central Oceanic Plateaux, 130—Deposits composed of
Inorganic and Organic Particles, 131—Absence of Organic Remains no
Evidence of the Antiquity of a Geological Formation, 133—Formation of
Areas of Elevation and of Areas of Depression, 134—Formation and Trans-
formation of Continents, 135—-Primary and Secondary Areas of Elevation,
136—Formation of Mountain Ranges and Submarine Ridges, 137—
Oceanic Pressure a Cause of Internal Heat, 138—Elevation of Mountain
Ranges and Creation of Axis of Volcanic Eruption, due to Lateral Pressure,
139—Water the Principal Agent in the Disintegration, the Re-distribution,
and the Accumulation of the Solid Matter composing the Earth-Crust,
140.

ist of Illustrations.

CHARTS.

PiaTE 1.—The Northern and Southern Hemispheres, with Track of H.M.S.
“ Challenger,” :

PLate 2.—Contour-Chart of the nie of a Ocean, .

PLATE 3.—Contour-Chart of the Bottom of the Atlantic,

PLaTE 4.—Surface-Currents and Surface-Temperatures,

PLATE 4a.—Lines of Equal Barometric Pressure (Isobars) for ite ee
September,

PLATE 5.—Current Chart of the Olea

OCEANIC SECTIONS.

Approximate Section through the Atlantic, : : frontispiece.
PLATE 6.—From Teneriffe to Sombrero,

PLATE 7.—From St. Thomas to Halifax,

PiaTE §8.—From Cape May to Madeira,

PLaTE 9.—From Madeira to Tristan d’Acunha,

PLATE 10.—From Cape Palmas to Cape S. Roque and Tristan d’ oan
PiaTE 11.—From the Falkland Islands to the Cape of Good Hope,

PLATE 12.—From the Cape of Good Hope to Cape Otway,

PLaTE 13.—From Lat. 50° S. to the Antarctic Circle,

PLATE 14.—From Port Jackson to Cook Strait and the Friendly ends
PLATE 15.—From the Fiji Islands to Torres Strait, ‘
PiaTE 16.—From Torres Strait to Hong-kong and to the Admiralty Tignes
PiaTe 17.—From the Admiralty Islands to Japan,

PuLaTE 18.—From Yokohama to Station 253,

PiaTE 19.—From Station 253 to Station 288,

PLATE 20.—From Station 288 to the Coast of Chile,

PAGE

If
14
17
27

46
53

PAGE

60
64
70
76
82

6 List of Illustrations.

DIAGRAMS.
PAGE
The Miller-Casella Thermometer, . ° . . - 32
Fic. 1.—Temperatures in the South Agente
Fic. 2.—Temperatures in the Gulf Stream and Arctic Current, a
Fic. 3.— : :
Rider \ Temperatures in the North Atlantic, . ; 5° Az
Fic. 5.—Temperatures in the Agulhas Current,
Fic. 6.—Temperatures in the South Australian Current, \ =

Fic. 7.—Temperatures in the Antarctic Current, )
Fic. 8.—Temperatures in the Southern Ocean, {° : ‘ shee
Fic. 9.—Atlantic Equatorial Temperatures,

Fic. 10.—Pacific Equatorial Temperatures, 45
Fic. 11.—Diagram of Oceanic Isotherms,

Fic. 12.—Diagram of Oceanic Circulation, } 57
Fic. 13.—Diameter of Rotation,

Fic. 14.—Gradients of the Sea-bottom, \ an:

PREFACE,

THE numerous and successful efforts made in recent times to
extend, and as far as possible to complete, our knowledge of the
globe we inhabit, constitute one of the most characteristic
features of the present age. The central parts of great con-
tinents, hitherto untrodden by the foot of civilised man, are only
now commencing to be systematically explored, and, while the
interest of the general reader is stimulated from time to time
by the description of newly-discovered regions, a rich harvest
of fresh materials is placed at the disposal of the scientific
student.

The work carried on with so much energy and success on
terra firma has been supplemented in the domain of the sea by
several naval expeditions, sent out for the especial purpose of
fathoming the depths of the ocean, of observing the currents
and the physical and chemical condition of the water, of
bringing up from the bottom samples of the deposits now in
process of formation, and of gathering specimens of the countless
forms of animal life with which the sea abounds at all depths.

As a natural consequence of the new facts brought to light
day after day, opinions held until recently by the most com-
petent authorities in almost every branch of Natural Science,
but especially in Biology, Geology, and Physical Geography,
have undergone considerable modifications, or have had to be
abandoned altogether. On the other hand, the numerous
carefully-made observations collected in every quarter of the
globe furnish an opportunity for attempting, with renewed

8 Preface.

chances of success, the solution of several problems, for which
no complete answer has as yet been found. One of these
problems is that of “ Oceanic Currents.” Ever since Lieutenant
Maury, in his admirable work on the Physical Geography of
the Sea, undertook to explain the probable causes of the
“beautiful system of circulation” by which “cooling streams
are brought from the Polar Seas to temper the heat of the
Torrid Zone,” this question has been the subject of frequent
controversy ; but I believe I am correct in stating that none of
the theories advanced in explanation of this gigantic natural
phenomenon have met with general acceptance, nor has any
satisfactory solution of the problem been offered.

One of the principal causes of this failure will be found in
the want which existed, until within late years, of the necessary
appliances and opportunities for ascertaining the conditions
which prevail in every part of the ocean, from its surface down
to the bottom. The scientific explorer possessed neither proper
sounding apparatus, nor instruments capable of resisting the
enormous pressures to which they are subject at great depths,
nor the aid cf steam-power for expediting the hauling in of
miles of sounding-line or dredge-rope; and last, not least, he
found no Government willing to open the national purse in
favour of scientific experiments planned on a scale the expense
of which exceeded the resources of even wealthy individuals,
and which were absolutely beyond reach of the proverbially
poor devotee of Science. In the absence of these indispensable
helps, it was next to impossible to secure observations sufficiently
reliable to afford a criterion in judging between the numerous
theories which have been advanced, however sound the premises
and logical the deductions on which they were based.

Another source of failure may be traced to the attempt to
explain a highly-complex phenomenon by the operation of a
single cause, and to the consequent neglect of one or other of

Preface. 9

the numerous conditions which must more or less determine the
direction, velocity, volume, and persistency of oceanic currents—
such, for example, as the unequal distribution of solar heat over
the surface of our planet, the agency of the winds or atmospheric
currents, the difference of temperature, specific gravity and
chemical composition of the water, the direction of the coast-
lines, the distribution of land and water in general, the
configuration of the sea-bottom, &c. It remains to the
student to ascertain, with the help of the observations at his
disposal, the part which is to be assigned to each of these
conditions in the great phenomenon of oceanic circulation,
considered as the final product of various causes, all acting and
reacting upon each other. |

Amongst the principal efforts in the domain of deep sea
exploration, we have the labours of the officers of the United
States Coast Survey along the course of the Gulf Stream
(1845 to 1859), the soundings of the U.S.S. “ Mercury” between
Barbadoes and Sierra Leone in 1871, the observations made on
board H.M.S. “Lightning” and H.M.S. “Porcupine” in the
seas extending from the Feroe Islands to the Mediterranean
(1868-1870), culminating finally in the two voyages of circum-
navigation made by H.M.S. “Challenger” (1872-1876) and the
German frigate “Gazelle” (1874-1876). The extensive series
of soundings for which we are indebted to these expeditions
has received further additions through the operations of H.M.S.
“Valorous ” between the English Channel and Davis Strait in
1875, of the Norwegian ship “ V6ringen” between Norway and
Iceland, of the U.S.S. “Gettysburg” in the North Atlantic
in 1876, the.U.S.S. “Tuscarora” in the Pacific Ocean (1874
to 1876), and the latest Arctic Expedition under the command
of Sir G. S. Nares (1875-1876).

In the charts and diagrams which accompany the following

pages, I have endeavoured to combine the results of recent
B

10 Preface.

observations, and more especially of the sounding operations
carried on on board H.M.S. “Challenger” during her cruise
round the world, in so far as they throw any light upon the
distribution of depth, temperature, and currents in the different
oceanic basins which have been explored.

The advance made in our days towards a satisfactory solu-
tion of the problem of oceanic circulation will probably be
recorded with.due completeness by abler hands; meantime, I
am not without hope that the contents of this essay may find a
welcome amongst those who have followed with interest the
progress of the numerous expeditions sent out of late years to
clear up the mysteries of the ocean.

JOHN JAMES WILD.
Lonpon, May, 1877.

06 ae
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T Ad

fra EAS S2\.

Cl A Pork: I
DEPLE OF THE OCEAN:

Distribution of Land and Water—Depth of the Ocean—Configuration of the Sea-
bottom—Description of the Basin of the Atlantic—The Indian Ocean—The
Pacific—The Southern Ocean—The Arctic Sea—The Antarctic Sea.

DistTRIBUTION OF LanpD and WatTeER.—Our conception of
the relative distribution of land and water over the surface of
the Earth has been hitherto limited to a comparison of the
superficial areas occupied by these two elements, such as they
are presented to us on a chart of the world. In this sense we
speak of the different continents and islands which constitute
the sum total of dry land, and of the different oceans and seas
which compose the water-surface of our planet. But if we
wish to form a more perfect idea of the distribution of land and
water, we must consider not only the length and breadth of the
areas occupied, but also the height of the land and the depth
of the water; in other words, the volume of those portions of
the solid crust of the earth which are raised above the level
of the sea, and the volume of the masses of water which fill
up the depressed portions of the earth’s crust. We are thus
led to regard the surface of the solid crust of our planet as
composed of heights and hollows, of areas of elevation and
areas of depression, and, as a next step, to discriminate between
these areas—not according to the usual standard of the level
of the sea, but according to their relative distance from the
centre of the earth. In this sense we may conceive an area of

12 Depth of the Ocean.

elevation—z.e., a raised portion of the earth’s surface, which
may be partially or entirely covered with water, and an area
of depression—z.e., a hollow in the same surface, which may
be raised above the level of the sea, and form dry land or the
basin of an inland sea-or lake.

If we examine the chart of the world (Plates 1 and 2) in
the light which has been thrown upon this question by all the
reliable soundings obtained up to the present, it will be found
that continents and islands which we have been in the habit
of considering as separated from each other by wide seas and
deep straits virtually form part of the same area of elevation ;
and, in a similar manner, that certain oceans and seas, which we
are accustomed to distinguish by separate names, form part of
the same area of depression. It will also appear that, with
the exception of the islands scattered over the face of
the ocean and of the Antarctic region, all the dry land at
present existing may be reduced to one large area of elevation,
gravitating towards the North Pole, as the common centre of
the principal land masses; similarly, if we except the Arctic
region and other inland basins, all the oceans and seas compose
a single vast area of depression with the South Pole for
common centre of the larger accumulations of water ‘on this
globe. The Arctic region forms a distinct area of depres-
sion placed in the centre of the great area of elevation,
and the Antarctic region, according to the evidence we at
present possess, is an area of elevation, surrounded on all
sides by the above-described great area of depression. The
numerous small islands that crop up in the middle cf the
oceanic basins are generally found associated in groups,
and they belong to areas of elevation at the present time
submerged, that is to say, in the condition in which we know
the dry land to have been at an epoch more or less remote in
the history of our planet. |

Distribution of Land and Water. ia

In support of the above generalisation, we may point to the
following facts as established by recent soundings (Plates 2
and 3). The 1oo-fathom line, as is well known, joins the
whole of the British Islands, including the Hebrides, Orkneys,
and Shetland Islands, to the continent of Europe. It forms a
broad band connecting the Asiatic and American continents
across Behring Strait. It unites Australia, Papua, and
Tasmania in a single area of elevation, which, together with
the intervening archipelago of Java, Sumatra, Borneo, Celebes,
the Moluccas, and the Philippines, may be looked upon as a
prolongation of the continent of Asia. It joins Ceylon to
Hindostan, and the Falkland Islands to the South American
continent. The 500-fathom line connects North America,
Greenland, Iceland, the Faeroe Islands, and the continent
of Europe, the only unexplored space being Denmark Strait,
between Iceland and Greenland, where the soundings may
exceed the above depth. The 1ooo-fathom line unites New
Zealand with Australia, Madagascar with Africa, and nearly
exhausts the depth of the more or less land-locked seas which lie
between Australia and Asia, Africa and Europe, South and
North America, and of the seas situated within the Arctic
and Antarctic Circles. The Cape de Verde Islands and the
Canaries belong to Africa, Madeira to Europe, and less than
500 fathoms divide Norway from Spitzbergen.

Depths from 100 to 1000 fathoms may be considered as
shallow in comparison with the prevailing depths from 2000 to
3000 fathoms of the principal oceanic basins, and sufficient to
establish a connection between islands and continents, the more
so as we generally find one or more islands occupying the
intervening space, thus betraying the common link between
them.

The result of this examination is that all the larger land
masses compose an area of elevation, which, after nearly com-

14 Depth of the Ocean.

pleting the circuit of the world in the latitude of the Arctic
circle, subdivides itself into two parts, an eastern and a western
one—the former embracing Europe, Africa, Asia, and Australia,
the latter North and South America. In a similar manner the
different oceans combine into an area of depression, which, after
making the circuit of the world along the parallel of lat. 60°
South under the name of the Southern Ocean, divides itself into
three large basins, respectively designated as the Pacific, the
Atlantic, and the Indian Oceans. Thus the two elements, land
and water, starting from opposite hemispheres, extend their
arms across the Equator, holding each other in close embrace,
like two champions wrestling for the mastery of the world.

DeptH oF THE: OcEAN.—A comparison of the deep-sea
soundings obtained up to present date shows that, if we omit
the seas situated beyond the parallels of lat. 60° N. and lat.
60 S.—no depths exceeding 2000 fathoms having as yet
been ascertained beyond these latitudes—the average depth of
the ocean between these parallels may be estimated at about
2500 fathoms, or more roughly at three English miles, and the
average depth of all seas on the surface of the globe at probably
two miles.

Contrary to the ideas formerly entertained of the enormous
depth of the ocean, the soundings of H.M.S. “ Challenger,”
Sails. “Gazelle,” and of the U'S:S: “WTuscatora~ and“ Gettys-
burg,” indicate that depths of five miles, or over 4000 fathoms,
are but seldom met with, and are as exceptional as heights of
the same amount on land.

The greatest depth ascertained in the Atlantic was found by
H. MoS.“ (Challenoer,” in fat ro wi IN, long, G5 7 W., about
eighty miles north of the island of St. Thomas, in the West
Indies. It is 3875 fathoms, or about four and a-half miles. In
May, 1876, the “ Gettysburg” found 3593 fathoms in lat. 19° 30/
N., long. 65° 5’ W., or only eleven nautical miles south of the

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Depth of the Ocean. | 15

Challenger sounding, and 3697 fathoms in lat. 21° 53’ N., long.
65° 9’ W., about 132 nautical miles north of the same sounding,
A depth of 3370 fathoms was obtained by the American ship in
lat. 25° 47’ N., long. 65° 0’ W., which shows that tHe deepest
area in the Atlantic is placed to the northward of the Virgin
Islands, and extends over 400 miles along the meridian of
65 W.

The greatest depth observed in the Indian Ocean was
discovered by the “ Gazelle” in May, 1875. Two soundings of
3020 and 3010 fathoms were taken in the eastern extremity of
this ocean between the north-west coast of Australia and the
line of islands extending from Java to Timor.

The greatest of all depths of which we have reliable evidence
was found by the “Challenger” on the 23rd March, 1875, in the
comparatively narrow channel which separates the Caroline
Islands from the Mariana or Ladrone Islands. This sounding
is situated in lat. 11° 24’ N., long. 143° 16’ E., and amounts to
4575 fathoms, or about five miles and a quarter. Several
soundings exceeding 4000 fathoms were obtained by the
‘‘Tuscarora” to the eastward of the islands of Niphon and
Yezo, and another close to the most westerly of the Aleutian
Islands. Two of these soundings are over 4600 fathoms, but,
as it appears that no sample of the bottom was brought up, there
is no evidence of the latter having been reached. H.M.S.
“Challenger,” shortly after her departure from Yokohama,
sounded in 3950 and 3625 fathoms, and in doing so seems to
have just touched the southern border of this deep but narrow
area of depression, which runs parallel to the eastern coasts of
Japan and the Kuril Archipelago as far as the entrance to the
Behring Sea (Plate 2).

It will be observed that the above exceptional depths in the
Atlantic, Indian, and Pacific Oceans are not placed, as one
might be inclined to conjecture, in or near the centre of these

16 Depth of the Ocean.

oceanic basins, but, on the contrary, upon their confines, and
in close proximity to the land. This remarkable circumstance
suggests the idea that such areas of maximum depression may
be the effect of a sinking of the bottom of the sea in compensa-
tion for an upward movement of the land in their immediate
vicinity.

CONFIGURATION OF THE SEA-Botrom.—Just as the results of
the recent soundings have rendered the existence of depths from
six to nine miles, as formerly reported, highly improbable, so
have they modified our ideas of the shape of the sea-bottom.
The latter was generally represented as a repetition of the dry
land with its combination of mountain, valley, and plain, a view
not a little encouraged by the necessarily exaggerated scale of
the oceanic sections which have appeared in print. The vertical
scale is frequently from twenty-five to a hundred times in
excess of the longitudinal distance, since otherwise it would be
difficult to represent graphically a rise or fall of a few miles in
distances measuring several hundred miles. No doubt the
sea-bottom within a short distance of the shore naturally forms
a continuation of the leading features of the adjoining land.
Thus a large plain or a low-lying country will, as a rule,
continue its almost level slopes to a considerable distance
out to sea, whilst a range of hills or a chain of mountains
often extends its steep inclines below the surface of the water.
A comparison of the eastern and western coasts of the British
Islands will afford a ready illustration of the above remarks, and
examples abound in every part of the world. With regard,
however, to the more central portions of the bottom of our
oceanic basins, this conception of steep slopes and abrupt
changes of level within short distances is not borne out by the
form of the numerous oceanic sections which have been sur-
veyed. Those who have followed day after day the results of
sounding operations in mid-ocean, along sections measuring

Plate 3.

MARCUS WARD & CO

CONTOUR ~ CHART ortHe BOTTOM ortue ATLANTIC.

“Basin of the Atiantic. 17

several thousand miles, will easily recail the to them familiar fact
that, with rare exceptions, and those chiefly occurring in the
vicinity of land, the result of one day’s sounding gave a tolerably
approximate idea of the depth to be encountered on the following
day. The alteration of level in mid-ocean between two points
as much as a hundred miles apart is generally so slight that, to
an observer standing at the bottom of the sea, the latter would
appear a perfect plain. Thus the bottom of our larger oceanic
basins is composed of gentle undulations rising and falling from
a few fathoms to two or three miles, in distances extending over
many hundred miles. This view accords with the experience
of the geologist, who finds that the bulk of the dry land consists
of sedimentary strata originally laid down in a horizontal, or
nearly horizontal, position at the bottom of the sea, and there can
be little doubt but that the depths of the ocean are at the
present time the scene of the formation of sedimentary strata
which some day may be converted into dry land, and contain
embedded in their folds traces of the animal life with which
they abound.

Tue Basin oF THE ATLANTIC.—One of the most remarkable
results in connection with the exploration of the sea is the
discovery of several extensive submarine plateaux, which
interrupt what was until lately supposed to be an unbroken
waste of fathomless.abyss. One of these plateaux traverses the
Atlantic Ocean in its whole length from north to south,
repeating in its form the S-shaped contour of the eastern and
western shores of this ocean (Plate 3). After attaching itself
by its northern end to the plateau which connects Europe and
Iceland, and separates the Atlantic from the Arctic basin, it
runs southward towards the Azores, and, gradually con-
tracting in width, sweeps round towards St. Paul’s Rocks.
Reduced, comparatively speaking, to a narrow ridge, it follows
the line of the Equator as far as the meridian of Ascension

18 Depth of the Ocean.

Island, where, resuming its southward course, it widens out
until in lat. 30° S. it occupies nearly half the space between
South America and Africa, uniting the island of Ascension
with St. Helena in the east, Trinidad in the west, and the
group of Tristan d’Acunha and Gough Island at its southern
end. Inthe absence of soundings to the southward of Gough
Island, it is difficult to form an opinion as to whether this
plateau connects itself by its southern end with the Antarctic
plateau, or whether it is separated from the latter by an area of
depression extending along the parallel of lat. 50° S. from the
Falkland Islands to the meridian of the Cape of Good Hope.
Certain indications derived from the nature of the currents, and
from the deep-sea temperatures observed in that region of the
South Atlantic, are in favour of the latter hypothesis.

Considerable portions of this plateau are within 1500
fathoms, or a mile and a-half, of the surface of the sea, and
most of the islands are of volcanic origin. An extinct volcano,
8300 feet in height, forms the island of Tristan d’Acunha;
Ascension Island rises to 2800 feet, and the summit of Pico
in the Azores to 7600 feet above the level of the sea. The
northern end of the plateau joins, as already stated, the plateau
of Iceland with its still active focus of eruption.

By this central plateau, the Atlantic Ocean is divided
into two longitudinal areas of depression or channels, one
following the shores of North and South America, the other
the shores of Europe and Africa. The depths vary from
2000 fathoms to nearly 4000 fathoms, the average depth
being about three miles. The deepest portion of the eastern
channel is situated to the westward of the Cape de Verde
Islands, and forms an area of depression of over 3000
fathoms (Plate 3). In the western channel there are two
such depressions, one placed between the Antilles, Bermudas,
and the Azores, the other between Cape St. Roque, Ascension,

Basin of the Indian Ocean. 19

and Trinidad. They are divided from each other in lat. 10°
N. by a submarine elevation, which apparently connects the
central plateau with the South American continent. The
supposed existence of this dividing ridge is founded, not so
much upon soundings, which are very few in this part of the
North Atlantic, as upon the difference of bottom-temperature
observed in the two depressions. The higher temperatures
ascertained by the “Challenger” at the bottom of the basin
north of the Equator seem to indicate that the inflow of cold
water from the southern basin is arrested. The distance
between the observing-stations, amounting to about 20 of lat.
however, is sufficiently great to justify the conclusion that the
difference of the bottom-temperature in the two areas of
depression, which does not exceed 1° C. may be due to a
difference of latitude, since the gradual increase from South to
North of the bottom-temperature is one of the characteristic
features of the Atlantic.

Tue Basin oF THE INDIAN OcEAn.—The soundings taken
in this ocean prove the existence of a submerged plateau on the
limit between the Indian Ocean and the Southern Ocean. It
rises in many parts to within 1500 fathoms of the sea-surface,
and forms the common foundation of all the islands situated
in this part of the world—viz., Prince Edward Islands, the
Crozet Islands, the Kerguelen group, the Heard Islands, and
the islands of St. Paul and Amsterdam. The origin of all these
islands is probably volcanic. The plateau occupies the space
comprised between the meridians of 35° and 80° long. E., and
tie patallels) ef tat. 35, ‘and 55 9S. and ‘the soundings
obtained by H.M.S. “Challenger” between the Heard Islands
and the Antarctic Circle establish a connection with the
Antarctic plateau, of which the above plateau seems to be an
extension towards lower latitudes.

The main basin of the Indian Ocean, with an average depth

20 Depth of the Ocean.

of over 2500 fathoms, stretches from the meridian of the Cape
of Good Hope towards the angle between Java and north-
western Australia, where it attains its greatest depths, forming
a depression of over 3000 fathoms. It communicates with the
Arabian Sea by two narrow channels, situated north and south
of the Chagos Archipelago, being nearly cut off from that sea
by a line of islands and shallow soundings, which connect
Africa, Madagascar, Bourbon and Mauritius, the Chagos
Islands and the Maldivh Islands, with the Asiatic continent.
The 2500-fathom. line does not enter the Bay of Bengal, but it
penetrates, as we have just mentioned, into the Arabian Sea, as
far north as the latitude of Cape Guardafui. The 2000-fathom
line forms the Bay of Bengal, and also defines the limits of a
_ basin situated between Madagascar, Mauritius, the Seychelle
Islands, and the shallow banks which unite the latter with
Mauritius. The rooo-fathom line stops outside the Mozambique
Channel, the Red Sea, and the Persian Gulf. The 2500-fathom
area of the Indian Ocean crosses the parallel of lat. 4o° S.,,
between St. Paul and Amsterdam Islands and Cape Leeuwin
in Australia, and forms, between the south coast of Australia
and the forty-fifth parallel, an area of depression which extends
beyond the southern end of Tasmania, includes the deepest
portion of the basin between New South Wales and New
Zealand, and probably communicates with the depths of the
Pacific by a channel situated off the southern extremity of New
Zealand.

Tue Bastn of THE Pactric Ocean.—If we divide the Pacific
Ocean into an eastern and a western half by a line passing
from Honolulu to Tahiti, or by the meridian of long. 150° W.
(Plate 2), we observe a remarkable contrast between the two
portions thus formed. While the eastern half, extending
towards America, presents a vast unbroken sheet of water,
almost devoid of islands, the western half, towards Asia and

Basin of the Pacific Ocean. 21

Australia, and enclosed between the parallels of lat. 30° N. and
lat. 30° S., is composed of a labyrinth of seas, separated from
each other by chains of islands, the projecting points of numerous
submarine ridges. Although extensive tracts in the Pacific
Ocean remain as yet untouched by the sounding-line, the obser-
vations made by the “ Challenger,” the “ Gazelle,” and the “ Tus-
carora,” along several sections which traverse the length and
breadth of this ocean, enable us to form an idea of the general
contours of its bottom. From the shores of North and South
America, the depths of the eastern half of the Pacific gradually
increase until, upon the line between Honolulu and Tahiti, they
attain 3000 fathoms. The latter depth forms extensive areas of
depression in the western half of this ocean, and increases to
4000 fathoms in the already described hollow extending along
the Japanese and Kuril Islands towards the entrance of the
Behring Sea. Thus the idea formerly entertained of the inferior
depths of the Pacific in comparison with the Atlantic, founded
apparently upon the large number of islands scattered over its
surface, is proved to be erroneous. Many of these islands,
especially in the north-western half, rise immediately from
depths of 3000 fathoms and more.

In the south-eastern portion of the Pacific there are indica-
tions of the existence of a submerged plateau connecting the
Society Islands, the Low Archipelago, the Marquesas, and the
intervening islands of Easter Island and Juan Fernandez with
the coast of Chili and Patagonia. H.M.S. “Challenger,” after
leaving Tahiti, seems to have sounded along the southern edge
of this plateau down to the parallel of lat. 30° S., and as far as
long. 140° W. Thence, running south towards the fortieth
parallel, the ship entered the area of depression discovered by
the “Gazelle,” but in her eastward course along the latter
parallel she once more touched the plateau in about he
long. W., with a sounding of 1600 fathoms. In long. 94°

23 | Depth of the Ocean.

W., she crossed the apex of a plateau which rises to less
than 1500 fathoms from the surface, and the base of which,
extending from Juan Fernandez to Magellan Strait, attaches
itself to the South American continent. It seems, therefore, as
if an almost uninterrupted area of elevation crossed the whole
basin of the Pacific in a north-westerly direction from Patagonia
to Japan. The tendency of most of the submerged ridges of
this ocean to follow the same direction has been frequently com-
mented upon, and, as is the case with the submerged plateaux of
the Indian and Atlantic Oceans, their association with centres of
volcanic activity is equally evident.

The track of S.M.S. “Gazelle” in the South Pacific lay to
the westward and southward of that of the “Challenger,” and
her soundings have proved the existence of an area of depression
with depths of from 2600 to 2900 fathoms, bounded on the west
by New Zealand, the Kermadec group, the Friendly Islands,
and the Samoan Islands, and on the north by the Cook Islands,
and the Tibuai or Austral Islands. It extends with lessening
depths eastward towards Patagonia, along the southern edge of
the above-described plateau, and probably communicates with
the deep areas to the northward in the space between the
Samoan group and the Society Islands. This southern area of
depression, however, may be considered as belonging not so
much to the Pacific as to the Southern Ocean.

A line passing from Kamtschatka over Japan, the Ladrone,
Caroline, Marshall, Gilbert, Ellice, Samoa, Tonga, and Kermadec
Islands to New Zealand, divides the main basin of the Pacific,
of an average depth of 3000 fathoms, from the much shallower
seas lying to the westward, and may possibly have formed the
coast-line of a large continent which existed at a remote epoch
in the history of the surface of our planet, and the boundaries
of which have since been driven back to the present confines of
Asia and Australia.

Basin of the Southern Ocean. 23

Tue Basin oF THE SOUTHERN Ocean.—This ocean, which
makes the circuit of the world along the parallel of lat. 60° S., in
length equal to half the circumference of the Earth at the
Equator, may be considered as occupying the space between
the Antarctic Circle and the parallel of lat. 40°S. Owing to the
limited number of soundings as yet obtained within its limits,
we can only form a general idea of the distribution of its depths.

The boundary-line of the fortieth parallel, which separates
the Southern Ocean from the Pacific, Atlantic, and Indian
Oceans, is occupied, as has been shown in the previous pages,
alternately by areas of depression, with depths ranging from
2500 to nearly 3000 fathoms, and by areas of elevation, or sub-
marine plateaux, approaching to within 1500 fathoms of the sea
surface. On the side of the Pacific, we have the deep area
explored by the “Gazelle” stretching up towards the Samoan
Islands, and the submerged plateau between Tahiti and Pata-
gonia. On the side of the Atlantic, we find the southern end of
the central Atlantic plateau, with the island of Tristan d’Acunha
and Gough Island, flanked on the east and west by the deep
areas explored by H.M.S. “Challenger.” Upon the limit of the
Indian Ocean, we observe what may be called the Kerguelen
plateau, extending from Marion and Prince Edward Islands to
St. Paul and Amsterdam Islands, as proved by the soundings
both of the “Challenger” and of the “Gazelle,” and the deep
area, an extension of the main basin of the Indian Ocean, which
passes to the southward of Australia and New Zealand, as
established by the soundings of the officers of both ships. The
plateaux on the border of the Atlantic and Indian Oceans may
turn out to be mere extensions of the Antarctic plateau, whilst
the deep areas leading to the three oceanic basins may be con-
sidered, in accordance with the observed bottom-temperatures,
as the main channels by which the cold water of the Antarctic
region flows northward into the Pacific, Atlantic, and Indian

24 Depth of the Ocean.

Oceans. With regard to the general distribution of depth in
the Southern Ocean, its bottom appears to rise gradually from
nearly 3000 fathoms at the fortieth parallel (with the exception
of the intervening plateaux) to little over 1500 fathoms at the
Antarctic circle. There are also indications of an area of
depression, of an average depth of 2000 fathoms, making the
circuit of the globe between the parallels of 50° and 60° lat. S.
The whole surface of the Southern Ocean is strewn with masses
of floating ice, some of them forming islands many miles in
extent, and rising from 100 to 300 feet above the level of the
sea—an imposing spectacle, but fraught with much danger to the
navigator in these regions. Jt is this central ocean which
supplies the masses of cold water that fill up nearly two-
thirds of the total depth of the Atlantic, Pacific, and Indian
Oceans.

Tue Basin oF THE ANTARCTIC SEA.—We are indebted to Sir
James Ross for the only soundings procured within the Antarctic
circle. They are situated in the wide inlet, discovered by that
illustrious navigator in the year 1840, which extends along the
meridian of New Zealand, and terminates at the foot of Mount
Erebus and Mount Terror. These soundings, which are all
under 500 fathoms, viewed in combination with the above-
mentioned gradual rise of the bottom of the Southern Ocean
towards the Antarctic Circle, justify the assumption that the seas
included within the latter do not exceed 1500 fathoms in depth,
their average depth probably falling below this estimate. The
extensive formation of ice in this region, as well as the numerous
indications of land reported by the daring sailors who have
penetrated so far south, suggest the hypothesis of the existence,
if not of an Antarctic continent, at all events of a considerable
extent of land, rising in the mountain ranges and volcanoes of
Victoria: Land to 10,000 and 15,000 feet above the level of
the sea.

Basin of the Arctic Sea. 25

Tue Basin oF THE Arctic SEA.—It has been already
observed in the commencement of this chapter that the region
enclosed within the Arctic Circle forms an area of depression,
almost completely surrounded by the land-masses of the great
eastern and western continents. A. shallow strait of less than
fifty fathoms in depth connects it with the Pacific Ocean, and
it is separated from the depths of the Atlantic by the plateau
between the British Islands and Iceland, which rises to within
500 fathoms of the sea-surface. Greenland is probably the
largest land-mass belonging to this basin, and next in importance
we have the group of Spitzbergen, of Franz Joseph Land, dis-
covered by the Austrian expedition; Novaya Zemblya, the
Liakhov Islands, Kellett Land, off Behring Strait, discovered
by the Americans in 1867; and finally the extensive archipelago,
a continuation of the American continent, the extreme northern
limit of which has been recently determined by the officers of
the English Polar Expedition, commanded by Captain Sir
George S. Nares.

The few soundings taken within the Arctic Circle leave much
to conjecture, but we are tolerably safe in stating that the
average depth of the Arctic basin is probably under 1000
fathoms. The immense plains of. Northern Asia and America
seem to continue beneath the surface of the Arctic Sea, as
indicated by the numerous islands which skirt the coasts of these
continents, and the greatest depths to be found inside the
Arctic Circle are probably confined to the basin situated between
Greenland and Norway, Iceland and Spitzbergen. This basin
has been explored at different times, amongst others by Lord
Dufferin in 1856, who collected a series of valuable temperature
observations; by the Swedish ship the “Sophia” in 1868, to
whom we are indebted for the only existing deep-sea soundings
in the neighbourhood of Spitzbergen; and in the course of
August, 1876, by the Norwegian Exploring Expedition on

he

26 Depth of the Ocean.

board, the “ Véringen.” The latter obtained a sounding of 1800
fathoms about half-way between Iceland and Norway. As a
general result of the soundings taken in this basin, it appears that
an area of depression of over 1000 fathoms in depth extends
southward from the strait between Spitzbergen and Greenland,
in lat. 80° N., as far as the Feroe Islands, and occupies the
central part of the space between Greenland, Iceland, and Nor-
way, gradually shelving up towards their shores. The volcanic
‘island of Jan Mayen rises from the bottom of this area to a.
height of 6870 feet above the sea-level. The only deep-sea
communication between this basin and the Atlantic seems to be
effected by Denmark Strait, the unexplored space between
Iceland and Greenland. According to the temperature-sound-
ings of the “Véringen,” a mass of cold water under o° C.
fills up this basin to within 4oo.fathoms of the sea-surface on
the Norwegian side, and to. within 200 fathoms towards the
east coast of Iceland. An extension of this cold stratum was
discovered by H.M.S. “Lightning” and “ Porcupine” in the
500-fathom channel between the Feroe Islands and the British
plateau.

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TEMPERATURE OF THE OCEAN.

Surface Temperature—Deep-Sea Temperature—The Miller-Casella Thermometer—
Serial Soundings and Temperature Curves—Deductions from the Curve.

Surrace %TEmpPeraTure.—The temperature of the ocean
depends mainly on three conditions—latitude, currents, and the
season of the year. Owing to the unequal exposure of the
different portions of the spherical surface of our planet to the
rays of the sun, the amount of solar heat received gradually
diminishes from a maximum between. the tropics to a minimum
in the polar regions. We find, in consequence, that the tem-
perature of the surface-layer of the ocean decreases as we
proceed from the Equator towards the Poles, slowly at first
between the tropics, more rapidly in the temperate zones, until
it falls to zero and even below zero before we reach the Arctic
and Antarctic Circles (Plate 4).

In the absence of any other disturbing cause, the
isotherms, or lines of equal temperature, would therefore form —
a system of lines running parallel with the Equator. This,
however, is found not to be the case. Warm currents flowing
from the tropical towards the polar regions, and cold currents
issuing from the ice-bound confines of the Arctic and Antarctic,
and penetrating into warmer latitudes, considerably interfere
with the uniform decrease of temperature between the Equator
and the Poles, although this decrease continues to be the
leading feature “in the distribution of oceanic temperature; but
the isotherms, as a general rule, assume an oblique direction,
as shown in the annexed diagram (Fig. 11).

The sun, in his apparent progress from tropic to tropic,

28 Temperature of the Ocean.

_ draws the whole system of terrestrial, including atmospheric,
emperatures after him from one hemisphere to the other; but
the water of the ocean, absorbing heat much more slowly and
retaining it much longer than the atmosphere, is less subject
than the latter to those extremes of temperature caused by the
change of the seasong A range of 5°C. or 10° F., reduced to
a few degrees between the tropics and within the polar circles,
measures the difference between the mean temperature of the
surface-layer of the sea in the warmest and in the coldest months
of the year. :

The alteration of the seasons, if it does not cause a wide
difference between the winter and summer temperatures of the
open ocean, considerably affects the distribution of oceanic
temperature not only in the surface-layer, but also in the lower
strata. Both equatorial and polar currents present considerable
variations in’ volume, velocity, and direction according to the
season of the year; the warm currents acquiring a preponde-
rance towards the end of summer, the cold currents towards the
end of winter. There is thus a perpetual contest between polar
and equatorial currents, especially perceptible in the latitudes
where they meet, and which, on that account, may be called the
critical latitudes, of which more hereafter.

The total range of oceanic temperatures embraces about 36
degrees of the Centigrade scale, commencing at —3°.67 C., the
freezing-point of salt water, and rising above 32° C. The
highest observed surface-temperatures do not belong to the
Equator, but are found only in a chain of more or less land-
locked seas ranged along the parallel of lat. 20° N. They are
the Carribean Sea and the Gulf of Mexico in the Atlantic, the
Arabian Sea, with the Red Sea and the Persian Gulf, and the
Bay of Bengal in the Indian Ocean, the China Sea and the
numerous smaller seas extending down to Torres Strait, and
the large basin between the Philippine and Ladrone Islands in

Surface Temperature. 29
the Pacific. The surface-temperature of these seas in the
warmest part of the year ranges above 28° C., and in the Red
Sea rises to 32°C. The highest surface-temperature observed
on board H.M.S. “Challenger” was 31°.1 C. (88° F.), and was
recorded on: the 21st October, 1874, in the Sea of Celebes,
in lat. 4° 14” N., long. 124° 18’ E. The lowest, of —2°.8 C.
(27° F.), was registered on two occasions, on the 18th and 24th
February of the same year, in about lat. 65° S., when surrounded
by icebergs; as the temperature of the water a few hours
before and after these observations was several degrees higher,
the ship must have passed through what may be called pools of
water of this low temperature.

One of the most remarkable examples of the influence of
oceanic currents upon the distribution of temperature occurs in
the North Atlantic and the adjoining portion of the Arctic
basin. The cold current which flows down along the east coast
of Greenland, and, after rounding Cape Farewell, joins the
Labrador current, carries the western end of the isotherm of
o° C. down to the latitude of Newfoundland, while its eastern
extremity is advanced as far:as lat. 80° N. by a warm current
which, after bathing the west coast of the British Islands and of
Norway, penetrates into the North Polar regions along the
coast of Spitzbergen. Thus this isotherm forms an oblique
dine extending from Newfoundland over Iceland and Jan Mayen
to the northern end of Spitzbergen. The effect of the above-
mentioned warm current, generally considered an extension of
the Gulf Stream current, upon the climate of the British Islands
is well known. In its absence, or if its place were occupied by
a polar current—which probably was the case in a former
period in the history of our planet—the climate of these islands
would resemble the actual climate of Labrador.

If we consider the narrow limits of temperature, including
but a few degrees of the thermometric scale, which determine

30 Temperature of the Ocean.

the presence or absence of certain plants and animals, the power
of adaptation more or less inherent in all living organisms, as
well as the accumulative tendency of climatic conditions, the
‘influence of oceanic currents upon climate, and hence indirectly
upon the development of the fauna and flora in any given
region, seems to afford an ample basis to account for the
former presence of tropical species in latitudes now subjected
to the rigours of a cold climate, and for the existence in
past geological times of arctic forms in regions at present
belonging to the temperate zone. In endeavouring to explain
these anomalies of climate, it appears hardly necessary to go in
search of vast cosmic changes, such as an alteration in the
position of the terrestrial axis, a diminution in the amount of solar
heat, a gradual cooling of the earth’s crust, which, if they have
occurred—and we have as yet no positive evidence in their
favour—would imperil the existence of all life on our planet ;
while we have, close at hand, an agency whose effect upon
climatic conditions may be said to be a matter of daily experi-
ence, and which is sufficiently powerful to establish, in almost
any region on the earth’s surface, the small difference of tem-
perature which is a decree of life or of death to numerous animal
and vegetable organisms.

Derp-sEA TEMPERATURE.—We are indebted to the officers of
the United States Coast Survey for the first systematic investi-
gation of the conditions of temperature below the sea-surface.
Their operations, extending over fourteen years, from 1845 to
1849, determined the course of the two great currents which,
under the name -of the Gulf Stream and the Labrador
Current, were known to flow along the United States coast
between Nantucket Island and the Gulf of Mexico. Although
not provided with the more perfect appliances and instruments
placed at the disposal of recent.explorers, their observations
furnished ample evidence of the existence below the sea-surface

Deep-sea Temperature. 31

of vast bodies of moving water thousands of miles in length,
hundreds of miles in breadth, and hundreds of fathoms in depth,
now flowing side by side, now passing one above the other. It
was shown, for the first time, that a polar current may. prolong
its course along the bottom of the sea into tropical latitudes, as
it was known before that warm currents of: equatorial origin
traverse the temperate zone and extend their life-giving influence
into the polar regions. ©

Apart from the obvious difficulties tae stand in the way
of carrying on a series of scientific observations on the high’
seas with delicate instruments, the chief desideratum in
"these earlier experiments was felt to be in the possession of a
thermometer constructed to register with sufficient accuracy the
temperature of the water at great depths, and to minimise the
effects of the enormous pressure, amounting to about a ton on
the square inch for every thousand fathoms. This pressure,
exercised on the bulb of an ordinary thermometer, if it did not
effect the total destruction of the instrument, always caused the
latter to record a higher temperature than that of the water at
the depth to which it had been lowered, the difference increasing
with the depth to as much as 5° C. for 2000 fathoms, and often
more. . |
The want of an efficient deep-sea SReRAGRISESE which more
or less vitiates all the observations of earlier explorers, was
especially experienced during the cruise of H.M.S. “ Light-
ning” in 1868, the. first expedition fitted out for sounding and
dredging purposes by the joint co-operation of the Royal Society
and the Admiralty of England. Of the thermometers used on
this occasion, several returned to the surface broken by the
pressure to which they had been exposed, and the indications
given by the rest varied so much as to render the discovery of
a thermometer free from this error a matter of paramount
importance for the success of future deep-sea exploration. By

32 Temperature of the Ocean.

a happy coincidence, the desired improvement was effected at
the moment when a second expedition was being prepared. In
April, 1869, previous to the departure of H.M.S. “ Porcupine”
on her first cruise, at a meeting of the Deep-sea Committee of
the Royal Society of London, Dr. W. A. Miller, V.P.R.S.,
suggested a simple expedient for protecting thermometers from
the effects of pressure, which, ably.carried out by Mr.’ L. P.
Casella, F.R.A.S., the eminent scientific instrument maker to
the Admiralty, resulted in the construction of an almost perfect
instrument for recording the temperature at great depths. Since
all the observations made on board H.M.S. “Challenger,”
during her cruise round the world, were obtained with the help
of this instrument,- now known as the Miller-Casella_ther-
mometer, a description of it in these pages may not be out of
place. .

THe MILier-CaseLLA THERMOMETER.—It will be seen from
the accompanying figure that this thermometer is designed to
register the maximum and minimum temperature of the water
to which it is exposed during its descent from the surface of the
sea to the bottom. For this purpose the glass tube is bent in
the shape of UJ, each arm of the tube terminating in a bulb.
The larger bulb, A, is surrounded by another bulb, B, and about
three-fourths of the space between the two bulbs is filled with
alcohol. It is by the addition of this outer bulb, B, that the
protection of the instrument from the effects of pressure is
secured. On immersion, the outer bulb receives the pressure
of the water, and forces the enclosed alcohol into the portion of
the intervening space previously not occupied by this liquid,
thus relieving the inner bulb, A. The latter is completely filled
with a mixture of creosote, alcohol, and water, which rests upon
the mercury contained in the bend of the tube, and also fills up
the other arm and part of the bulb C. The upper part of bulb
C is occupied by air, introduced, with the help of a freezing

THE MILLER _CASELLA THERMOMETER.

Scale 2 Nat. Size.

10

3in.

The Miller-Casella Thermometer. | 4 3

mixture, at a very low temperature in order to increase its
density. It acts as a sort of elastic cushion intended to over-
come the friction of the liquids in the tubes, and to assist the
mercury in following the mixture when contracting under the
influence of cold. The indications of the thermometer depend
upon the expansion by heat and contraction by cold of the
mixture contained in bulb A. When expanding, the mercury is
forced down in the arm attached to this bulb and rises in the
other arm towards bulb C ; when contracting, the mercury falls
on the side of bulb C, and rises towards bulb A. Two metal
indices, a a, mark the maximum height which the mercury has
reached in either arm, and a hair attached to each index pro-
duces the friction necessary to retain them at the level to which
they have been raised. Before the thermometer is lowered into
the sea, the indices are brought down upon the mercury by
passing a magnet along the tube. One of the essential qualities
of an instrument intended for use on long voyages is that it
should be portable—a quality especially realised by Mr. Casella
in the construction of his thermometer. By contracting the
bore of the glass tube as much as possible, the quantity of the
liquids, particularly of the mercury, has been reduced to a
minimum ; and the liability to accident, almost inseparable from
instruments containing large quantities of this heavy substance,
has thus been greatly reduced.

The cruise of H.M.S. “Challenger” afforded ample oppor-
tunities for testing the capabilities of the Miller-Casella_ ther-
mometer. It resisted all the pressures to which it was exposed
down to a depth of about 4000 fathoms, when some of the
instruments were found to give way under a pressure of four
tons to the square inch ;' but as depths of from four to five miles
are exceptional, such accidents will be of rare occurrence.
Under a pressure of three tons, equivalent to a depth of three
miles, the error amounts: to less than 1° C., whilst that of the

34 Zi emperature of the Ocean.

unprotected thermometers previously in use sometimes exceeded
10° C. for the same depth; and a comparison of the data
furnished by the Miller-Casella thermometer ‘with the corre-
sponding temperature curves shows that the mean error of all
the deep-sea observations made on board H.M.S. “ Challenger,”
not much under 10,000, is probably less than 0.5 C. The
thermometers, before being sent out, are subjected to pressures
varying from one to four tons to the square inch in a hydraulic
press especially designed for this purpose by Mr. Casella, and
the amount of error ascertained for each instrument. When in
actual use, they are enclosed in a copper cylinder perforated
at both ends to allow free ingress and egress to the water.
Several thermometers may be attached to the same sounding-
line whilst it is being paid out, at distances of five, ten, twenty-
five, fifty, or one hundred fathoms as required; and it is found
that an immersion of from five to ten minutes is sufficient to
secure the desired record of temperature.

SERIAL SOUNDINGS AND TEMPERATURE Curves.—lIn order to _
ascertain the distribution of temperature at any selected station
upon the ocean, it is necessary to obtain as many observations
at various depths between the surface and the bottom as circum-
stances will permit. The amount of time absorbed in hauling
in the sounding-line from great depths, even with the help of a
steam-engine, as well as the uncertain state of the weather,
which may interrupt the operations at any moment, necessarily
limit the number of observations which can be taken in the
course. of a stoppage on the high seas. However, the first
experiments made of this kind establish the fact that, beyond a
depth of from 1000 to 1500 fathoms, the temperature of the
water decreases very slowly, often not more than o°.1 C. in 100
fathoms ; and that, beyond these depths, observations taken at
intervals of 100, 200, or 250 fathoms are sufficient for all
purposes. A series of observations at every 10 fathoms from

Serial Soundings and Temperature Curves. 35

the surface to 100 fathoms, every 25 fathoms down to 300
fathoms, and every too fathoms down to 1000 or 1500 fathoms,
furnishes, with the addition of the bottom-temperature, sufficient
materials for scientific enquiry ; and, although it may be secured —
in the course of a few hours, it can only be accomplished under
favourable’conditions of weather. The number of serial sound-
ings obtained by H.M.S. “Challenger” during her three and
a-half years’ cruise amounts to about 260, of which 120 belong
to the Atlantic, 110 to the Pacific, the remainder being about
equally divided between the Southern Ocean and the seas of
the Indian Archipelago. An important series of soundings was
taken almost simultaneously by the officers of the Imperial
German frigate the “ Gazelle,” whose operations in the Indian
Ocean and in the south-western portion of the Pacific, not visited
by H.M.S. “Challenger,” form a valuable supplement to the
results obtained by the officers of the English expedition.

When recording the observations made at a particular station,
it becomes at once apparent that a mere tabulated statement of
the temperature registered at each depth is but an imperfect mode |
of exhibiting the results of the soundings, and recourse is had to
the method of curves. It is not necessary here to insist on the
evident advantages of this method of representing graphically
the various stages of a phenomenon under observation, and on
the facilities which it offers not only for seizing at a glance its
leading features, but also for detecting the errors of the instru-
ments employed and those of the observer himself.

To convert a tabulated statement of a series of temperature
observations belonging to one station into a curve, it is sufficient
to lay down a scale of fathoms, say along a horizontal line, and,
at right angles to the latter, a scale of degrees of temperature
(Figs. 1 to 10). At the points on the horizontal scale corre-
sponding with the depths at which the observations were taken,
perpendicular lines are drawn, and their lengths made equal to

36 Temperature of the Ocean.

the number of degrees registered by the thermometer. Thus
these lines will be longer or shorter in proportion as the tempera-
ture increases or decreases. On joining the upper end of the
lines we obtain a curve of a more or less regular shape, gene-
rally assuming the form of the letter S, which is called the
temperature curve of the station, and it exactly represents the
tate of increase or decrease of the temperature of the water in
each stratum from the surface to the bottom.

To exhibit the conditions of temperature ascertained at a
~number of stations belonging to a certain section of an oceanic
basin, another class of curves may be constructed (Plates 6 to 20).
In this case the horizontal base-line forms a scale of miles, or of °
degrees of latitude or longitude, giving the distance between one
station and another. The vertical scale indicates the depth in
fathoms, and the curves thus obtained form isotherms exhibiting
by their rise and fall the decrease or increase of temperature
in the different strata of the oceanic section under consideration.
They also show the rate of increase or decrease, spreading out
when the temperature decreases more slowly at one station than
at another, and closing up when the decrease is more rapid.
Instead of a scale of fathoms, the vertical scale may be divided
into degrees of temperature, and the curves resulting from this
arrangement will form iso-bathymetric lines, exhibiting the
variation of temperature for the same depth at different stations.
In the plates and tables prepared for this essay, the former
arrangement has been given the preference, since, by using a
scale ot depths,. the diagram represents an actual section
of an oceanic basin, although on a scale necessarily much
exaggerated.

DEDUCTIONS FROM THE CurRVE.—An examination of the shape
of the curve, and of the modifications which it undergoes in the
different parts of the ocean, leads to certain conclusions respecting
the conditions which determine the distribution of oceanic tem-

Deductions from the Curve. a

perature, and the direction in which we must seek for a solution
of the problem of oceanic circulation.

In looking over the temperature curves and sections which
accompany these pages, we find ¢hat, as a general rule, the
temperature of the ocean decreases from the ‘surface to the bottom.
One of the first surprises in store for the early observers of
deep-sea temperature was the discovery that the temperature
of the deeper strata, and at the bottom of the ocean, is only a
few degrees removed from freezing-point. Subsequent observa-
tions have shown that, with certain exceptions, this is the case in
every part of the ocean, in tropical latitudes as well as in the
temperate and frigid zones, and that over some areas the tem-
perature of the lowest strata falls even below zero. The first
well-ascertained example of the presence of cold water in low
latitudes is due to the soundings of H.M.SS. “ Lightning” and
“Porcupine” in 1868-69, in the channel between the Feroe
Islands and Scotland. Over an area situated between lat. 60°
and» 617 N., lone, 4° sand ©: W:, the temperature of o C: is
veached at a depth of only 300 fathoms, descending to —1°.4 C.
at a depth of 640 fathoms. This mass of cold water—about
120 miles long, 60 miles broad, and a quarter-mile in depth—
must have come from the Arctic region, and the soundings of
the Norwegian ship “ Véringen,” in the course of last year, show
that it is a southern extension of the mass of cold water which
fills up the basin between Iceland and Norway to a depth of
1800 fathoms. A still:more remarkable instance of the existence
of a.cold area with a temperature below zero was discovered off
the mouth of the Rio de la Plata by H.M.S. “Challenger” on
her return voyage in 1876. Between lat. 36° and 42° S., long.
33° and 55° W., the bottom-temperature at seven different
stations was found below o° C., varying from —o*.3 to —o°.6
C. The maximum depth found in this area was 2900 fathoms,
and the zero-point was reached in 1900 fathoms (Station 323).

33 Temperature of the Ocean.

This cold area may be traced southwards into the Antarctic
region, and northwards as far as the Equator; for at Station ~
112, off Cape St. Roque, the temperature at a depth of 2200
fathoms is only 0°.5 C., and at Station 110, under the Equator,
o°.9 C. at a depth of 2275 fathoms. At the two latter stations,
the temperature of 5° C. is reached at a depth of little over 300
fathoms. ;

As early as 1859, the officers of the U.S..Coast Survey found
a temperature of 4°.4 C. (40° F.) at a depth of 300 fathoms in
the Strait of Florida; the water of the Gulf Stream at the
surface having, at the same date, a temperature of 26°.7 C.,
os 30°F.

The two principal exceptions to the above-stated rule of the
gradual decrease of temperature from the surface towards the
bottom are to be found, one in a number of small basins which
are cut off by submarine ridges or elevations from communica-
tion with the lower strata of the ocean outside, the other in the
Arctic and Antarctic regions, where the rule seems to be com-
pletely inverted, the temperature of the water increasing from
the surface towards the bottom.

The operations of H.M.S. “Porcupine” in the summer of
' 1870 in the Mediterranean, and the cruise of H.M.S. “ Chal-
lenger,” furnish examples of the first-mentioned exception. It
is well known that the Mediterranean Sea is cut off from the
depths of the North Atlantic Ocean by a submarine elevation
extending from Cape Spartel in Africa to Cape Trafalgar in
Spain, and rising to within 100 fathoms from the sea-surface.
From a series of temperature-soundings obtained by H.M.S.
“ Porcupine” at seven different stations between the meridian of
Malaga and Carthagena, it appears that the temperature of that
part of the Mediterranean falls from a mean of 22°.6 C. at the
surface to 13° C. at a depth of 100 fathoms, whence it remains
stationary at the latter temperature down to the bottom, at

_ Deductions from the Curve. 39

depths varying from 162 to 845 fathoms. From this it has
been inferred, that 7x a basin separated from the adjoining ocean
by a submarine elevation, the temperature of the water decreases
Jrom the surface down to the level of that elevation, and remains
stationary from that level down to the bottom.

The subsequent proceedings of H.M.S. “Challenger,” in the
Western Pacific and in the seas of the Indian Archipelago
(Plates 15 and 16), leave little room for doubting the correct-
ness of this conclusion. In the basin extending from the New
Hebrides to Torres Strait (which might appropriately be called
the Melanesian Sea), in the Banda Sea, Celebes, Sulu, and China
Sea, the temperature of the water was found to decrease from
the surface down to a certain level, and remain stationary
between that level and the bottom. The Sulu Sea furnishes
the most striking illustration of this hitherto unsuspected
pnenomenon. “At the time of the two: visits of. F1.M.S.
“Challenger,” in October, 1874, and January, 1875, the tem-
perature. was observed to fall from 28° and 27°C. at the
surface to 10°.2 C. at a depth of 4oo fathoms, and to remain
stationary at the latter temperature from 400 fathoms down to
the bottom, at depths of 2550 and 2225 fathoms, forming a
- stratum of more than two miles in depth of the comparatively
high temperature of 10°.2 C., or 50°4 F. Although we possess
no complete surveys of the bottom of the seas above mentioned,
a glance at a chart will show that they all are more or less
land-locked basins,.and, as the temperature-conditions seem to
prove, probably cut off from the colder strata of the Pacific and
Indian Oceans by submarine ridges rising to the level at which
the decrease of temperature is arrested.

The second exception, observed both in the South Polar
and North Polar regions, may require further research before it
can rank as an established scientific fact. . Besides the numerous
difficulties which beset thermometric observations under the

40 | Lemperature of the Ocean.

severe conditions of a polar climate, the limits of oceanic tem-
perature in these latitudes are so narrow as to render additional
caution necessary. There is, however, considerable agreement
between the observations made by the several explorers
who have penetrated into these inhospitable regions, who all
assert the discovery of warmer water below the cold surface-
stratum, so that the fact seems hardly doubtful; and the recent
experience of the officers of H.M.S. “Challenger” in the
vicinity of the Antarctic Circle points in the same direction
(Fig. 8, Curve B, and Plates 12 and 18). On theoretical
grounds it may be said that the existence of open water in the
polar regions, in contact with an atmosphere the temperature of
which is generally below freezing-point, shows that warm
currents from lower latitudes must find their way into these
regions. The constant melting of the ice floating in these
warm currents must tend to produce layers or pools of water
of a temperature near freezing-point and of a lower specific
gravity, which, for a time, must remain at or near the surface,
and form strata of a temperature lower than that of the strata
beneath.

It will appear from the previous remarks that the distribution
of temperature in the ocean depends, as a general rule, upon a
constant supply of heat at the surface and a constant supply of
cold at the bottom; and the temperature-curve will represent
a series of gradually decreasing temperatures from the surface
towards the bottom, as in Fig. 1. In those regions where the
supply of heat is reduced to a minimum, as we find is the case
in the higher latitudes, the stratum of cold water will be reached
within a.short distance from the surface (Fig. 2, Curve C, and
Fig. 7), and in some parts of the ocean it may be said to occupy
the whole depth of the sea (Fig. 8, Curve A). On the other
hand, where, as in the cases mentioned above, the supply of cold
is reduced or entirely cut off by submarine barriers, the tem-

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perature will remain stationary at or near the level of the
obstruction, and the stratum of warm water will extend to the
bottom, and thus fill up the whole space between the surface
and the bottom, whatever may be the depth of the basin
(Plate 16).

This constant supply of heat and of cold is effected, as is well
known, through the agency of currents. The latter are by no
means an exceptional phenomenon confined to certain parts of
the ocean. Varying in volume and velocity until they attain
the proportions of gigantic rivers flowing at a rate of several
miles an hour, they occupy every part of the ocean, no part of
which can be said to be in a condition of absolute rest. Com-
bined together they form, like the currents in the atmosphere,
and in intimate association with the latter, a complete system of
circulation, by which the physical and chemical equilibrium of
the ocean is maintained. From the principal storehouse of heat
in the tropics, warm currents proceed towards the temperate
and frigid zones, and return thence in the character of cold
currents towards the regions of the Equator. That this is so is
proved by the results of all observations made up to the present
day, and it is in perfect agreement with the well-known agency
of water as a storer-up and carrier of heat.

Two strata of different temperatures cannot remain in con-
tact for any time without the formation of an intermediate
stratum. This is presumably the reason why a series of deep-
sea temperature observations generally assumes the shape of a
curve, nowhere presenting a break or an abrupt transition from
one temperature to another. The depth of this intermediate
stratum will depend upon the duration of the contact. When
two masses of water, one warm the other cold, move in different
or opposite directions, the intermediate stratum will present a
rapid transition from the temperature of one stratum to the
temperature of the other, and the part of the curve which

D

42 Temperature of the Ocean.

represents the intermediate stratum will form a steep incline.
On the contrary, when two masses of water flow in the same
or nearly the same direction, the intermediate stratum will
appear in the curve as a gradual incline representing the slow
increase or decrease of temperature from one stratum to the
other. Hence the gradient of any part of the curve is not only
the measure of the rate of increase or decrease of temperature,
but also an indication of the relative motion or relative rest of
the strata in contact. A low gradient expresses the presence
of strata of equal or nearly equal temperature moving in the
same direction, 2z.¢., at relative rest towards each other; a steep
gradient indicates the existence of strata of different tempera-
tures, and moving in different directions.

Thus, in Fig. 1, we have a warm surface-stratum of con-
siderable thickness, the decrease of temperature in the first
200 fathoms amounting to only 2°4 C. The steep gradient
between 200 and 500 fathoms shows that this surface-stratum
moves in a direction different from that of the bottom-stratum,
which, at this station, is found to rise up to within 600 fathoms
from the surface. The temperature at the bottom, at a depth
of 2550 fathoms, is o.7 ‘C.) at 1560: fathoms, 27 C+ atGco
fathoms, 2°.9 C., or a difference of only o°.9 C. in 900 fathoms.

In Fig. 2, Curve A, belonging to Station 41, near the eastern
limit of the Gulf Stream, the hump extending down to 300
fathoms represents a stratum of nearly uniform temperature.
The latter is, at the surface, 18°.3 C.; at 125 fathoms, 18°.0 C.;
at 300 fathoms, 17°.0 C. In Curve B, at Station 43, in the
Gulf Stream itself, all this warm water below too fathoms
has disappeared; but a surface-stratum has been added,
of a temperature rising to 24°C. Still further on, at Station
44 (Curve C), a short distance beyond the western limits of
the Gulf Stream, we find ourselves in the midst of an Arctic
current rising up to within 300 fathoms from the surface, with a

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Deductions from the Curve. 43

temperature of 4°.1 C. at 300 fathoms, of 3° C. at 900 fathoms,
and of 2°C. at 1500 fathoms. The temperature of the surface
has fallen from 24° C. to 11°%1 C. The space which contains
the three stations covers 1° 20’ of latitude, and’ 1° 57’ of
longitude—truly an extraordinary change in the distribution
of temperature in so small a portion of the Atlantic.

In Fig. 3, the Curves A, B, C illustrate the gradual dis-
appearance towards the Azores of the same stratum, 300 fathoms
in thickness, which appears in Curve A, Fig. 2.

In Curve A, Fig. 4, the hump between 200 and 700
fathoms (12° C. to 7°.5 C.) marks the presence of a large
current flowing between the Azores and Madeira; and the low
gradients of Curve B indicate the existence, between the surface
and 1500 fathoms, of various strata of gradually decreasing
temperatures moving more or less in the same direction.

Curve A, Fig. 5, shows the rise of temperature caused by
the Agulhas current, to the southward of the Cape of Good
Hope; while Curve B of Station 139, situated but a short
distance westward of the Cape, affords little or no indication of
the proximity of this large current of warm water. The
unusually irregular shape of Curve B betrays the existence in
the vicinity of the Cape of numerous currents moving in different
directions one above the other.

The two curves in Fig. 6 illustrate the temperature-con-
ditions in the great South Australian current. H.M.S. “ Chal-
lenger,’ after a three months’ cruise, her stores of coal having
run short, was precluded from establishing numerous stations
on her way from the Antarctic Circle to Australia. Thus it
was found, on arriving at Station 159, that the ship had already
crossed the southern limit of this great current, of which, at
Station f58,.1m lat 50° S., long. 123° E., there had been: little
on no trace’ (Plate. 12), The isotherm of 5° €., which at the
latter station was reached at a depth of 200 fathoms, fell to 600

44 Temperature of the Ocean.

fathoms at Station 159, and rose again to 500 fathoms at Station
160, so that between Stations 159 and 160, the “Challenger”
must have crossed the axis of a current about 500 fathoms deep,
and from 500 to 600 miles broad. This current, coming from
the Indian Ocean, flows in a south-easterly direction to the
southward of Australia, and penetrates into the Antarctic region
along the meridian of New Zealand.

Fig. 7 presents a section, at Station 318, of the great
Antarctic current which flows as an under-current along the
east coast of South America, crosses the Equator, and pene-
trates into the North Atlantic. At the above station it rises to
within 100 fathoms of the surface. The steep gradient between
the surface and 100 fathoms is due to a branch of the Brazilian
current, which flows in a southerly direction towards the Falk-
land Islands.

Curve A, Fig. 8, furnishes a similar example of the presence
of a cold stratum at the depth of little more than 100 fathoms
from the surface. It is the temperature-curve of Station 147,
near the Crozet Islands. Curve B illustrates the case of a cold
surface-stratum, probably formed by melting ice, observed in
the vicinity of the Antarctic Circle. ‘The temperature falls
from —1°.2 C. at the surface to —1°7 C. at 50 fathoms, but
rises to —o°.8 C. at 200 fathoms, o°.0 C. at 300 fathoms, and
o°.4 C. at a depth of 500 fathoms (Plates 12 and 13),

Figs. 9 and 10 represent the conditions of temperature near
the Equator in the Atlantic and Pacific Oceans. The curve
of Fig. 9 belongs to Station 110, near St. Paul Rocks; the
curve of Fig. 10 to Station 221, in the basin between Papua
and. the Caroline Islands. In the former a surface-current,
retaining a nearly uniform temperature of 25° C. down to a
depth of 30 fathoms, is joined by a steep gradient to an
intermediate current which extends from 100 fathoms to 400
fathoms, the cold bottom-stratum being reached at a depth of

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Deductions from the Curve. 45

500 fathoms with a temperature of 4° C. At the station in the
Pacific, a surface-stratum, the temperature of which falls from
28°.8 C. to 28° C. at 30 fathoms, and to 26° C. at 80 fathoms, is
united by a steep gradient to an intermediate stratum, which
extends from 150 fathoms (11°.3 C.) to 800 fathoms (3° C.), the
bottom-stratum commencing at 900 fathoms with a temperature
of 2°.5 C. How soon the cold bottom-stratum is reached in
the equatorial belt is one of the unexpected discoveries due to
recent deep-sea exploration. In the warm seas which bathe the
British Islands, a temperature of 4° C. is not registered until we
arrive at a depth of 900 fathoms, and at 1500 fathoms the
temperature is still 2°.5 C.

CHAPTER. 1,
CURRENTS OF THE OCEAN.

The Aqueous and the Aerial Oceans—Thermal Circulation—Vertical and Horizontal
Extension of the Two Terrestrial Envelopes—Parallelism between Oceanic and
Atmospheric Currents—Surface and Under-Currents.

Tue AQurous AND THE AERIAL OcEaAns.—The aqueous
envelope, which, as we have seen, covers about three-fourths
of the surface of the solid crust of the earth to an average
depth of from two to three miles, is itself surrounded by and
everywhere in contact with another envelope termed the
atmosphere, which forms an “aerial ocean” covering the whole
surface of our planet to a depth supposed not to exceed
eighty miles. Whether this aerial ocean has a well-defined
surface like the aqueous ocean is a point which remains to be
settled by future research. What we know is, that the density
of the various strata into which it may be divided decreases so
rapidly that at a height or depth of 18,000 feet, or 3000 fathoms,
we have already left behind one-half of the mass of air of which
it is composed. It has also been ascertained by recent observa-
tions, that the proportion of aqueous vapour—upon the presence
of which in the air the agency of the atmosphere as a storer-up
of heat and moisture mainly depends—diminishes with equal
rapidity, and is, as far as observation goes, reduced to zero at
a distance of only a few miles from the earth’s surface. The
thickness of the atmospheric layer, considered as a meteorological
agent, may therefore be safely reduced to five miles, or even
less, for the greater number of the atmospheric phenomena with
which we are immediately concerned take place within a distance
of from two to three miles from the earth’s surface.

Ever since the movements of the atmospheric air and of the

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Thermal Circulation. 47

waters of the ocean have attracted the attention of the scientific
observer, the resemblance between the phenomena which occur
in the gaseous envelope and those observable in the aqueous
envelope of our planet has frequently been pointed out. This
resemblance is the obvious result of a similarity of conditions
and an identity of natural laws which govern the internal
economy: of the two envelopes. Both are composed of
fluids subject to the laws of gravity, and to the laws which
direct the movements of fluids in general, their expansion under
the influence of heat, their contraction under the action of cold.
The equilibrium of both is constantly disturbed in consequence
of the unequal distribution of solar heat between the Poles
and the Equator, and is as constantly restored through
the agency of currents, cold air and cold water unceasingly
flowing from high towards low latitudes, warm air and warm
water without intermission passing from the torrid zone
into the temperate and polar regions. As recent observations
have shown, in both, in the aqueous as well as in the aerial
ocean, the temperature rapidly decreases from the surface
towards the deeper strata (considering the stratum of the
atmosphere which is in close contact with the surface of the
earth as the virtual surface of the aerial ocean); and the
surface-stratum of both forms a stratum of maximum energy
which in the ocean extends to a depth of about 500 fathoms,
or half-a-mile (Plates 6 to 20), and in the atmosphere, to a
depth of 3000 fathoms, or three miles. Finally, the composition
of both fluids is altered under the influence of solar heat; that
of air throngh an increase in the quantity of moisture held in
suspense, that of water by an increase in the percentage of salt
held in solution.

THERMAL CircuLaTion.—It has been shown by Sir John
Herschel that the unequal exposure of the different zones of the
earth’s surface to the rays of the sun must result in a system

48 Currents of the Ocean.

of atmospheric circulation composed of equatorial and polar
currents; and by Lieutenant M. F. Maury, that the same
inequality must be considered as the primary cause of a system
of oceanic circulation also composed of polar and equatorial
currents. The two distinguished philosophers have proved that
these currents do not flow in the direction of the meridian, since,
under the influence of the diurnal rotation of our planet from
west to east, polar currents, as they move from a parallel of a
lesser to one of a greater rotatory velocity, have a tendency to lag
behind and to deviate in a westerly direction, while equatorial
currents, in their progress from a lower toa higher latitude, have
a tendency to deviate in the direction of the earth’s rotation,
z.é., towards the east. All observations agree in establishing
the fact that the internal economy of the atmosphere and of
the ocean is regulated by such a system of circulation composed
of equatorial and polar currents, and that the direction of these
currents is affected by the earth’s diurnal rotation in the manner
above described.

VERTICAL AND HorizontraL EXTENSION OF THE Two TEr-
RESTRIAL ENVELOPES.—Before entering upon an examination of
the phenomena of atmospheric and oceanic circulation, it is
necessary to attach due importance to a condition sometimes
overlooked in connection with occurrences which, in their
ensemble, embrace immense areas of the surface of our planet—
that is, the great disproportion which exists between the depth or
vertical extension and the lateral or horizontal extension of the two
terrestrial envelopes. The neglect of this condition is probably
due to the exaggerated scale which it is necessary to adopt in
graphical representations of oceanic and atmospheric sections,
and to the difficulty of placing before our mental vision
phenomena of such colossal proportions as we find realised in
the great currents of the air and of the sea.

The average depth of the ocean, whether we estimate it at

L:xtension of the Two Terrestrial Envelopes. 49

two or three miles, is but a minute fraction of the length and
breadth of an oceanic basin ; so is the depth of the more active
stratum of the atmosphere when compared with the areas of
sea and land with which it is in contact.

A due consideration of this disproportion between horizontal
and vertical extension leads to several conclusions of some
importance to the student of the phenomena of oceanic and
atmospheric circulation, namely :—1. That what in either system
of currents has been called horizontal circulation must be the
preponderating phenomenon, vertzca/ circulation only occupy-
ing the second place. 2. That the original direction, volume,
velocity, temperature, and composition of a current, considered
as part of a system of thermal circulation, must undergo im-
portant modifications under the influence of the terrestrial
areas with which the current comes in contact—an influence
depending upon the distribution of land and water, the direction
of the mountain ranges and coast lines, the configuration of the
surface of the land and of the bottom of the sea, and other
conditions present in a given area of the earth’s surface.
3. That the currents of the ocean and of the atmosphere, while
obeying their original tendency as thermal currents to flow in
a certain direction—towards the Equator in the case of polar
currents, towards the Poles in the case of equatorial currents
—will ultimately move in the direction of least resistance.
4. That under the influence of local conditions, the general system
of atmospheric or oceanic currents will resolve itself into as
many different systems of circulation as there are distinct areas
of land and water.

Sir John Herschel says, in his 7reatzse on Astronomy (sec.
197): “We have only to call to mind the comparative ¢hzzness
of the coating which the atmosphere forms around the globe,
and the immense mass of the latter, compared with the former
(which it exceeds at least 100,000,000 times), to appreciate

50 Currents of the Ocean.

fully the absolute command of any extensive territory of the
earth over the atmosphere immediately incumbent on it, in point
of motion.”

This remark, made with regard to the accelerating effect
which the friction of the earth’s surface exercises upon the
rotatory velocity of the superincumbent atmosphere, applies
with equal force to the powerful influence which the conforma-
tion of the surface of the solid earth’s crust must exercise upon
the atmospheric and oceanic currents with which this surface
comes in contact. |

Notwithstanding the daily accumulating mass of observa-
tions made in every part of the world, dissatisfaction has been
frequently expressed at our imperfect insight into the laws
which govern meteorological phenomena. Perhaps we look in
vain for a direct manifestation of those laws under conditions
constantly tending to modify the form under which they are
presented to us, and the observer must be content to discover
the expression of a general law under the disguise of ever
varying and often contradictory phenomena. Every terrestrial
area, both on sea and land, has its own system of atmospheric
and oceanic currents; as every part of a continent, every valley
has its own climate, subject to the general laws which govern
the circulation of currents and the distribution of climate over
the whole of our planet.

PARALLELISM BETWEEN OCEANIC AND ATMOSPHERIC CURRENTS.
—On account of the more uniform conditions which prevail
over oceanic areas in comparison with continental areas, the

phenomena of currents are less complicated in the former than .

in the latter, and can be studied to greater advantage.

If we consult a wind-chart, we find that the direction of the
prevailing currents of air which flow over the surface of the
ocean agrees with the direction of atmospheric currents, such
as would arise from the unequal distribution of solar heat over

Oceanic and Atmospheric Currents. Ble,

the surface of the rotating globe. We have currents of cold
air flowing from high into low latitudes in a westerly direction,
and currents of warm air passing in an easterly direction from
the Tropics into the temperate and the polar regions. The
effect of these currents moving in opposite directions is seen in
the creation .of several belts or areas of calms, one near
the Equator, one at each Pole, and one near the parallels
of lat. 30%. In accordance with theory, the belts of calms
are due to an encounter which takes place in about lat.
30° between equatorial and polar currents. The former,
coming from the Equator in the character of upper-currents,
are supposed to descend in that latitude to the. surface of the
ocean, and to continue their course towards the polar regions
as under-currents, having acquired an easterly tendency
owing to the gradually decreasing rotatory velocity of the areas
over which they flow, until, finally, they are arrested as they
approach the Poles by the friction of the earth’s surface. The
latter, coming from the polar regions as upper-currents, descend
near the same latitude towards the surface of the ocean, and
continue their course towards the Equator as under-currents,
gradually losing their westerly tendency until, in the vicinity of
the Equator, they commence to rotate with the earth’s surface
and are no longer felt as easterly winds, thus producing the
Equatorial belt of calms.

A current moving from the Equator towards the Pole will
not acquire a decided tendency towards the east until it reaches
the parallel of lat. 30°, as the diameter of rotation decreases very
slowly at first, its total decrease between lat. 30° and 45° being
greater than that between the Equator and lat. 30° (Fig. 13).
It will, therefore, not manifest itself as a strong easterly current
until it crosses the 30th parallel. On the other hand, a current
flowing from the Pole towards the Equator, while having from the
outset a strong tendency to lag behind in a westerly direction,

52 Currents of the Ocean.

on account of the steady increase of the diameter of rotation
between the Pole and the parallel of lat. 30°, will, after crossing
that parallel, gradually lose that tendency, and will, within
10° of the Equator, have acquired the rotatory velocity of the
earth’s surface, and therefore cease to show itself as a westerly
current.

It thus appears that the Equator, the parallels of lat. 30°,
and the Poles, constitute what may be termed the crdtzcal
latitudes in the system of atmospheric circulation, and for similar
reasons also in the system of oceanic circulation.

This state of matters, according to which we find the surface
of our planet, as regards its two fluid envelopes, divided into
belts of calms and belts of currents symmetrically distributed
on each side of the Equator, is subject to considerable modifi-
cations from various causes. The first and most important of
these causes is the division of the surface of our planet into
areas of land and water which, alternately stretching across the
Equator from one hemisphere into the other, intersect the
parallel belts of calms and of currents at right angles. We
have here carried out on a large scale one of those simple ex-
pedients by which Nature, in strict obedience to her laws, creates
that endless variety of contrasting phenomena, which the philo-
sopher, the poet, the artist, never cease to behold with wonder,
and which, while it is the source of all beauty, is, at the same
time, a necessary condition to the existence of all life. The
result, in the present case, is the creation of numerous areas of
atmospheric and oceanic circulation corresponding with the
different areas of land and of water distributed on each side
of the Equator, and the subdivision of the belts of calms into
distinct areas of calms, of which we find one in each of the
oceanic basins, in the North and South Atlantic, in the North
and South Pacific, and in the Indian Ocean. (Plate 4 A.)

A comparison of these areas of calms with a chart of

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Oceanic and Atmospheric Currents. 53

isobars shows that they form at the same time areas of high
barometric pressure round which the atmospheric currents
revolve—with the hands of the watch in the northern hemi-
sphere, against the hands of the watch in the southern hemi-
sphere, while the equatorial belt and the circumpolar regions
form areas of low barometric pressure. If we now compare a
chart of isobars and a wind chart with a chart of oceanic surface-
currents, we find that there is a remarkable agreement between
them. These currents are observed to revolve together with
the winds round the areas of calms and of high barometric
pressure placed near the centre of each oceanic basin, about the
parallel of lat. 30%. The conclusion at which we arrive, that the
winds are the cause of the surface-currents, seems obvious,
although we must not forget that even in the absence of wind,
the thermal circulation of the ocean would resolve itself into a_
system of surface and under-currents. It may be more in
accordance with facts to suppose that atmospheric and oceanic
currents mutually act and react upon each other. A current
of water will either raise or lower the temperature of a stratum
of air with which it remains in contact, and thus cause an
inflow of air from neighbouring colder regions, or an outflow of
air into adjoining warmer areas. A current of air will induce a
surface-current in the stratum of water with which it comes in
contact, or accelerate the velocity of a surface-current already
existing, or change its direction, or arrest its motion. That
the winds are a direct cause of oceanic surface-currents is a
fact too well established by actual observation to require further
proof. Perhaps the most striking example of their agency
will be found in the complete reversal of the currents in the
regions of the Monsoons.

There exists a second and hardly less important cause which
tends to modify the general system of atmospheric and oceanic
circulation. The sun, in its apparent progress from tropic to

54 Currents of the Ocean.

tropic, causes a change in the distribution of solar heat over
the surface of our globe, by transferring the zone of maximum
heat from one hemisphere to the other. As a necessary con-
sequence, the volume, rate, and direction of the different currents
are found to vary with the seasons; cold currents preponder-
ating at one time of the year, and warm currents at another.
At the same time the areas of calms and of high or low
barometric pressure expand and contract, and are, to a limited
extent, displaced.

SURFACE AND UNpDER-CurRENTS.—It may be taken for
granted that, water being a ponderable substance, and, as such,
subject to the laws of gravity, the different strata of the ocean
will be found arranged according to their weight, the heavier
strata below, the lighter strata above. The weight of salt
water varies with two conditions: temperature and percentage
of salt held in solution. In the tropical belt, the water of the
surface-stratum contains more salt, the increase being due to the
evaporation caused by the rays of the sun. In the polar
regions, the quantity of salt falls below the average on account
of the greater proportion of fresh water derived from the
melting of the ice and from precipitation.

The observations made on board the “Gazelle” have
shown that there is a direct relation between the colour of sea
water and the percentage of salt which it contains. The more
salt it holds in solution the more intensely blue is its colour; the
less salt it contains, the more greenish the colour is. In extra-
tropical latitudes, we sometimes observe water of a beautiful
blue colour, as, for example, in the Mediterranean and other
nearly land-locked basins, where, the inflow of fresher water
being more or less cut off, the percentage of salt is raised above
the average by evaporation. We also observe it when crossing
a current coming from tropical. latitudes, such as the Gulf
Stream. A green colour is sometimes met with in the tropics,

Surface and Under-Currents. 55

in places where great rivers pour their masses of fresh water
into the sea.

According to the extensive series of specific gravity observa-
tions on sea-water made on board H.M.S. “ Challenger,” by Mr.
T. Y. Buchanan, M.A., chemist to the expedition, it appears that
the specific gravity of salt water under the influence of tempera-
ture varies between a minimum of 1.021 and a maximum of 1.028
(to use round numbers), and the specific gravity, as affected by
the percentage of salt contained in the water, between 1.024 and
1.027. If we select the sections surveyed by the “ Challenger”
between St. Paul Rocks at the Equator, the Cape of Good
Hope, Kerguelen Land, and the Antarctic Circle, we find that
the specific gravity of the surface water, according to the percent-
age of salt, commences with 1.027 near the Equator, falls to
1.026 towards lat. 40° S., to 1.025 between lat. 40° and 50° S.,
remains at that figure as far as lat. 60° S., and finally sinks to
1.024 in the immediate vicinity of the ice-barrier. On the
other hand, the specific gravity of the surface-stratum, under the
influence of temperature, commences with 1.023 near the
Equator, rises to 1.026 towards lat. 4o° S., attains 1.027 near
lat. 50° S., and continues the same down to the ice-barrier.
The temperature of the bottom-stratum in the different oceanic
basins remains uniformly within a few degrees of o° C., hence
its specific gravity is generally about 1.028. The percentage
of salt in the bottom-water was found to decrease from the
Equator towards the polar regions, the specific gravity falling
front 1.027 tO 1.025;

It will thus be seen that the difference in specific gravity
due to temperature is more than double the difference arising
from the varying percentage of salt ; whence we conclude that the
order of the oceanic strata depends, in the first instance, upon
temperature, in the second, upon the amount of salt held in
solution.

56 Currents of the Ocean.

An equatorial surface-current will remain such so long as its
temperature is sufficiently high to render it lighter than the sur-
rounding waters, but as during its progress. towards higher
latitudes its temperature decreases, it will, on account of its
greater saltness, sink below the fresher waters of these latitudes,
and continue its course as a warm under-current towards the
polar regions. On the other hand, a polar surface-current,
although composed of fresher water, will, on arriving at a
certain latitude, sink below the tropical waters on account of its
low temperature and consequent greater specific gravity, and
continue its course towards the Equator as a cold under-current.

But the temperature of the ocean decreases not only from
the Equator towards the Poles, but also from the surface towards
the bottom. Hence, in the tropical regions, the warm but salt
surface-water will sink on becoming cooled by its contact with
the strata beneath, and impart some of its heat to the latter ;
while in the polar regions, the fresh but cold surface-water
produced by the melting of the ice will, on becoming more salt
by its admixture with the surrounding water, sink in its turn
and lower the temperature of the strata with which it comes in
contact.

The final result of these exchanges of temperature, which
constitute what may be called the vertical circulation of the
oceanic waters, appears in the oblique position, and the con-
sequent spreading out of the isotherms as we recede from the
Equator (Plates 9 and 19). The isotherm of 5° C., for example,
which near the Equator is found at a depth of 300 fathoms, is
met with at 600 fathoms in lat. 50° S. in the Southern Ocean,
and at 800 fathoms in lat. 50° N. in the North Atlantic.

The Southern Ocean is the main feeder of its three gigantic
offshoots—the Atlantic, the Pacific, and the Indian Oceans, which
it supplies through the medium of both surface and under-
currents. The former, driven by the westerly winds against

Fig. 12.
General Diagram of Oceanic Circulation.

Fig Il.
Diagram of Oceanic Isotherms .

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Surface and Under-Currents. By

the west coast of Africa, Australia, and South America, are
diverted northwards towards the Equator; the latter, piled up
by the rotating earth against the east coasts of these continents,
flow as under-currents in the same direction, both returning in
the character of warm currents towards their old home at the
Pole.

The annexed diagram represents the surface and under-
currents which, in accordance with the above theoretical deduc-
tions, compose the system of circulation in our principal oceanic
basins (Fig. 12).

CHAPTER ly.

THE TEMPERATURE SECTIONS SURVEYED BY H.M:S.
‘CHALLENGER? IN THE “ATLANTIC.

From Teneriffe to Sombrero and St. Thomas—From St. Thomas to Halifax—Between
Cape May, U.S., and Madeira—From Madeira to Tristan d’Acunha—Between
Cape Palmas and Cape S. Roque—Between Cape S. Roque and Tristan
d’Acunha—From the Falkland Islands to the Cape of Good Hope.

Tue OcEanic TEMPERATURE SECTIONS SURVEYED BY H.M.S.
“CHALLENGER.’—The accompanying diagrams and _ tables,
especially constructed by the author for this essay, embody the
principal results of the sounding operations carried on by the
officers of H.M.S. “Challenger” during her cruise round the
world between December, 1872, and May, 1876.

The isotherms of 2°.5, 5°, 10°, 15°, 20; and 25° C., have been
selected, partly as affording a sufficiently correct representation
of the distribution of temperature in the different oceanic sections
which have been explored, partly because the above degrees of
the Centigrade scale correspond with even numbers of the
Fahrenheit scale, namely, 36°.5, 41°, 50°, 59°, 68°, and 77°. The
intervening isotherms are, as a general rule, symmetrically
arranged between these limits. As the temperature of 10° C.
(50° F.) fairly marks the point which divides what may be called
warm water from cold water, the strata of a temperature above
10° C. have been coloured ved, those below that temperature
blue. The strata coloured wzo/e¢t are those in which the tem-
perature of the water has been found to remain unchanged down
to the bottom (Plates 15 and 16). The yellow or buff tint
indicates where bottom has been reached within 1500 fathoms
from the surface. It should also be observed that the scale of

Frrom Teneriffe to Sombrero. 59
depth is greatly in excess of the scale of distance marked in
degrees of latitude and longitude. For example, in Plate 9, a
division of 100 fathoms is equal to two divisions, or two degrees
of the horizontal scale, representing 120 nautical miles, or about
120,000 fathoms, so that the proportion between the two scales
is as 1 to 1200—in other words, the depths in that diagram are
1200 larger than the distances indicated by the horizontal scale.
The scale of depth, which stops at 1500 fathoms, represents
only about three-fourths, and often only one-half, of the total
depth of the oceanic basin; and from the lowest isotherm ot
2°.5 C., the temperature in most cases slowly decreases down to
the bottom, the depth and temperature of which at each station
are given in the table annexed to each diagram.

The station numbers are the same as those on the labels
attached to the natural history specimens brought up by the
dredge, the trawl, or the towing-net at each station. No doubt
these specimens, of which there are more than one hundred
thousand, embracing several hundreds of forms of animal life
never before beheld by the eye of man, and therefore highly
interesting not only to the student of zoology but to the public
in general, will be permanently exhibited in the shape of a
“Challenger Museum.” Collected, as they have been, at a
great sacrifice of time, money, and, sad to say, of life, including
the ever-to-be-regretted death of Dr. Rudolf von Willemoes-
-Suhm, the promising zoologist attached to the scientific staff of
H.M.S. “Challenger,” they will compose a lasting monument
of the generosity of the English nation, always ready to promote
the cause of knowledge, and prove of more enduring interest to
future generations than all the trophies of war bought at the price
of general ruin.

SECTION FROM TENERIFFE TO SOMBRERO (Plate 6, Table I.).
—This section stretches across the Atlantic in a west-south-
westerly direction, and crosses the parallel of lat. 20° N. near

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From Teneriffe to Sombrero. 61

the Antilles (Plate 2). It affords an instructive example of the
contrast which has been observed between the two portions of
the North Atlantic divided from each other by the central plateau,
as regards distribution of temperature. In the eastern basin the
temperatures are lower at the surface, higher in the deeper strata
than in the western basin ; while in the latter they are higher at the
surface, and lower in the deeper strata when compared with the
former. The isotherm of 1to° C., which throughout the section
remains at about the same level, marks the turning-point of the
change. Station 13, placed upon the central plateau, may be con-
sidered as dividing the two areas of circulation, which, however, as
might be expected, encroach upon each other. In the western
basin, the warm surface-stratum due to the North Atlantic
Equatorial Current, and extending down to 100 fathoms, stretches
eastwards beyond Station 13 as far as Station 10, and, gradually
thinning off, disappears near Station 8, where it makes room for the
North Atlantic Polar Current, which forms the surface-stratum
of the eastern basin. The cold stratum below 400 fathoms,
which in the west has a temperature of 5° C. at about 600
fathoms, shows in the east a fall of this isotherm down to 840
fathoms—in other words, a rise in the temperature of the lower
strata, which, as the difference in bottom-temperature (Table I.)
indicates, extends to the bottom. As we pass from the cold
water accumulated to westward of the central ridge by the Vorth
Atlantic Polar Under-current, we enter already, at Station 15,
into a warmer stratum caused by the mixture of the North Atlantic
Polar Current with the North Atlantic Equatorial Return Cur-
rent, flowing down together in the eastern basin. In the western
basin, the surface and the deeper strata flow in opposite direc-
tions—one north, the other south, and the curves show a more or
less abrupt transition from the warm upper strata to the cold lower
strata; whilst in the eastern basin, the currents flowing in the
same direction, z.¢., south, the heavier equatorial return current

62 Temperature Sections Surveyed.

sinks through the lighter polar current, and the curves present a ,
slow and gradual decrease of temperature from the surface to the
bottom (compare Curve B of Station 5, Fig. 4, with the Equatorial
Curve, Fig. 9, of Station 110, on the western slope of the
central plateau).

At Station 13 may be observed a remarkable phenomenon,
frequently noticed during the progress of the “Challenger”
expedition, namely, the semultaneous rise of the tsotherms with the
sea-bottom. This phenomenon first attracted the attention of the
officers of the U.S. Coast Survey as they were engaged in tracing
the course of the Labrador current along the coast of the United
States. This current was found to rise and fall with the sea-bottom
over which it flows, and finally to force its way into the Strait of
Florida at the high level of less than 300 fathoms from the
surface, immediately below and in a direction contrary to the
Gulf Stream current.

This circumstance seems to indicate that the great thermal
currents which, without ceasing, tend to restore the oceanic
equilibrium disturbed by the unequal distribution of solar heat,
force their way from north to south, and from south to north,
against every obstacle to their progress arising from the irregular
conformation of the sea-bottom, and from the direction of the coast-
lines which cross their path. They rise and fall with the sea-
bottom, and accumulate their waters against the shores of islands
and continents which stand in their way. There are also indica-
tions sufficient to show that the presence of land or submerged
areas of elevation is not indispensable to the production of this
phenomenon, and that currents flowing side by side but in
different directions accumulate their waters against each other,
in consequence of which the weaker current gives way to the
stronger, and the waters of the lighter current flow over
the surface of the heavier current, as is seen in the case of the
Gulf Stream, which, along the United States coast, flows over

Frrom St. Thomas to Halifax. 63

the Labrador current:as the latter forces its way southward
between the former and the coast of America.

The two depressions in the isotherms of 10° C. and 5°C,,
noticed in the western half of the section between Teneriffe and
Sombrero, may thus be accounted for—at Station 22 by an
accumulation of the water of the equatorial current against the
inclined base of the Caribbee Islands, and at Stations 18
and 17 by the same current forcing its way between the masses
of the polar under-current moving in an opposite direction.
The undulating form of these isotherms shows that in the
western half of the section there are two currents moving in
different directions and contending against each other—a form
which, at the surface of the ocean, assumes the character of
alternating streaks of warm and cold water flowing in opposite
directions. This phenomenon is realised on a large scale by
the Gulf Stream current, which, in its progress northward, is
split up into several branches by the Labrador current, and, as
the latter suffers the same fate at the hand of the former, the
scene of the contest is covered with alternate streaks of warm
and cold water, the one flowing north, the other south. The
Agulhas current, off the Cape of Good Hope, and the Kuro-
Siwo stream, off the coast of Japan, furnish illustrations of the
same phenomenon on a scale not much inferior, and it will occur
wherever currents of different origin, and therefore of different
temperature, weight, and chemical composition, meet each other.

For similar reasons, one current may present as solid an
obstacle to the progress of another current as if it were a
barrier of rock, and compel the latter either to alter its
direction or to flow above or below the former—a phenomenon,
as will be seen in the course of the following pages, also exhibited
on a large scale in the system of oceanic circulation.

SECTION FRom St. Tuomas To Hatirax (Plate 7, Table I1.).
—This section extends in a direction nearly due north along

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From St. Thomas to Halifax. 65

the meridian of long. 65° W., from St. Thomas, in the West
Indies, to Halifax in Nova Scotia, the group of the Bermudas
dividing the section into two nearly equal halves (Plate 2). An
examination of the isotherm of 20° C., as well as of the surface-
temperatures, shows that, between. Station 27 and Station 28,
we cross the northern limit of that portion of the North Atlantic
Equatorial Current which flows outside the West Indian Islands.
Reduced in depth, the warm surface-stratum continues towards
the Bermudas, beyond which group it suffers further reduction
by coming in contact with the Labrador current. At the
station of the 24th May, we once more fall in with the
equatorial current, namely, that portion of it which, after
entering the Caribbean Sea and after making the circuit of
the Gulf of Mexico, issues out of the latter through the Strait
of Florida and flows .along the U.S. coast under the name of
the Gulf Stream. At the above station, the “ Challenger” found
a surface-stratum 50 fathoms thick of a nearly uniform tem-
perature of 22°.8 C., only 1°.6 C. below the surface-temperature
of the current outside the West Indies. At Station 52 we
encounter the “cold wall” of the Labrador current, against which
the Gulf Stream banks itself up during the whole of its course
along the American coast; and at Station 51 and Station 50
we observe the rapid fall of temperature due to this cold
current. |

Those who have effaced the Gulf Stream off the Banks
of Newfoundland, and have attributed to the North Atlantic
Equatorial Current, or “ North Atlantic drift-current” as it has
been called, the vast masses of warm water which occupy the basin
of the North Atlantic as far north as Spitzbergen and Baffin
Bay, and those who supported the opposite theory giving all the
credit to the Gulf Stream, were probably both partly right and
partly wrong in their conclusions. The “Challenger” observations
leave little doubt but that the Gulf Stream is a branch of the

66 Temperature Sections Surveyed.

equatorial current, separated from the latter during its course
through the Caribbean Sea and the Gulf of Mexico, and joining
it again after coming out of Florida Strait, from which place it
forms the western edge of the great mass of equatorial waters
during its further progress towards the north.

The Channel of Yucatan, by which the Equatorial Current
enters the Gulf of Mexico, presents a much wider section than the
Florida Channel, whence the current issues under the name of
the Gulf Stream, and it seems as though more water flowed
into the gulf than out of it, unless it flow out with increased
velocity. It has been calculated that a difference of level
amounting to two feet—a difference which falls within the
error of even the most careful survey embracing so large an
area—creates sufficient pressure to force the water through the
Strait of Florida at the rate of four miles an hour, at which the
Gulf Stream is known to flow out of the Strait. A similar
phenomenon, occurring under similar conditions, may be
observed in connection with the Kuro-Siwo current. A branch of
the North Pacific Equatorial Current flows into the basin situated
between the Philippine and the Ladrone Islands, which basin, like
the Caribbean Sea, is separated from the ocean by a chain
of islands, the projecting points of a submarine ridge, and the
northern and narrow half of this basin stands in the same relation
to the southern half as the Mexican Gulf to the Caribbean Sea.
The current, after passing along the east coast of the Philippines,
of Formosa, and of the islands which connect the latter with
Japan, has to force its way, and, like the Gulf Stream, in the
face of a contending polar current, over the shallow barrier
which joins Japan to the chain of islands terminating with the
Ladrone Group. After crossing this barrier, it unites itself to
the portion of the North Pacific Equatorial Current which
flows along the eastern side of these islands, and the two com-
bined form the Kuro-Siwo current, whose waters are traced

From St. Thomas to Halifax. 67

through Behring Strait into the Arctic basin, and eastward as
far as the west coast of North America.

The isotherms of 15° C. and 1o° C., in the section between
St. Thomas and Halifax, continue to descend to a lower level
as the temperature of the intermediate strata increases with
the distance from the equator, until, at Station 52, we enter the
polar current. The portion of the Atlantic between Halifax
and Bermudas is occupied by alternate streaks of warm and
cold water, as will appear from the following observations
made on board H.M.S. “ Challenger.”

After leaving Halifax on the roth of May, the surface-tem-
perature marked a steady increase from 4° C., to 10 C., when,
between 3 and 7 a.m. of the 22nd May, a rapid rise of the
temperature betrayed the existence of a belt of warmer water.
The latter attained a temperature of 17° C. between 5 and 7 p.m.
of the same day, but at midnight it fell to 12°.2 C., to rise half-
an-hour afterwards, at 12.30 am. of the 23rd, to 15.2 C.
Between that hour until the arrival of the ship near Bermudas
several alternate streaks of warm and cold water were passed
through, the former of a temperature from 22° to 23° C,, the
latter from 18° to 20°C. It will be observed that the water of
the Gulf Stream Current was only cooled down to the extent of
1° C. during its passage from the section between Bermudas and
Sandy Hook to the section between Halifax and Bermudas.

The centre of the first warm belt was reached at 8.30 a.m.
of the 23rd May, that of the second at 1 a.m. of the 24th, of the
third at 8 a.m., of the fourth at midnight of the same day, and of
the fifth at 1.30 p.m. of the 26th, the whole of the 25th May
having been occupied in traversing a broad belt of colder water.
In the vicinity of Bermudas the surface-temperature once more
rose to: 237 CG; .

68 Temperature Sections Surveyed.

TABLE OF SURFACE-TEMPERATURES BETWEEN HALIFAX AND BERMUDAS.

Date, . Latitude at Surface .
ee Station. Nisan Hour. Temperature. Observations.
May 9 Halifax. 44° 39/N. os CO Os

5A LG, 99 ; ~- Avo: Cold streak.
eo) 49 43° 3'N = 5°.0 C

Sy 2 50 42° 8/N. — 8.0 C

ifn 22 5t 41° 19/N. 3 a.m. 10.0C

99 ” ” Hie 14.2 Cc

39 99 9 9° Noon 15 Ae Cc;

ass AS 3 5 to7 p.m. feo Ge Warm streak.
99 » » IO p.m. 15.3 Ce

ean a8 - Midnight 12 21C. Cold streak.
Meese) 52 39° 44/ N. 12.30 a.m. is 2 -€

9909 ” ry I ait 18°.2 (c

9999 ” ” 1.30 ,, 20)-0 iG:

> 99 ” ” 4 ” 21.0.

29 oog or) ” 8.30 ” 2IAOnes Warm streak.
ey) ” 9 9 ” 19°.3 Cc Cold streak.
Sa ” ” 11.30 p.m. 207-01€

299 ” ” Midnight Dina cE

ray 24: - 38° 32! N. Laan: 22770). Warm streak.
” ” ° ” ” 3 ” 21" 8 G f

” ’ 2 ” 3-30 ” 20% oe

oe 6 » a0 5 on TOR 4G. Cold streak.
39) 5 ” ” 5.30 ,, 21.1 c&

50) 6 5 > 8 - pati (Cp

9, oat as os 8.30 ., ota (Ce Warm streak.
” .3 ” 39 6 p.m. 22°.8 GC:

” ” ” 33 8 ” 2X ad (es

” ” ” 9 9 > 19 .4 eG Cold streak.
2” ” > ” ste) ” 21°.7 (Ge

” or ” ” 10.30 ,, 22.2) C:

9 » 55 Midnight 22n2e Warm streak.
35) 25 — 377 FN Tanne 22°.2 C

89 ” 9 1.30 ;, 20° ole:

2999 9 ” 6.30 a Longo.

9399 39 aC 7 oe 18°.0 C. Cold streak.
” ” ” ” The rest of 10°.0 c:

” ” ” ” the day to 20°.9 GC;

Feels: 53 36° 30/ N. 2 a.m, LOR ole

rr) ” ” Pry Ares 21°.4 (en

” ” ” %> Noon 22°.8 C.

m9 ” +5 1.30 p.m. 2x0.

5 ” » 4 - ey (Ce Warm streak.
99 aS “e Estoy ey 22°.8 CG

” ” 9 33 8 ” Dr atom Or

” ” : an Midnight 22°.2 (ee

» 2 54 34° 5D | 12.30 a.m. pas (Or

” ” ” ry 7 p-m. 22 <2 (Gs

” ” ” 33 8.30 ” ZL <i (Ge

25 55 B3520/0NE 1.30 a.m. ZOebIC: Cold streak.
oe) ” ” » 7-390 55 20.8 Cc.

9 29 3:9 ” 8 ” DUraw GC.

rr ” 1 Midnight 22q21C.

9, 29 56 Off — 22°.2 C. Warm streak.
33) 30 57 | Bermudas. — toj2s,.01C.

Me! Bermudas. ge0 rs iN:

Between Cape May and Madeira. 69

The isotherms of 5° C. and 2°.5 C. retain an almost uniform
level, the former at 600 fathoms, the latter at 1 500 fathoms, as
far as Station 52, where they both commence to rise rapidly and
-come to the surface off Halifax, as seen in the above table.
The isotherm of 2°5 C. rises with the sea-bottom around
Bermudas. The bottom-temperature of 1°5 C. is found at
Station 51 at 2920 fathoms, at Station 52 at 2800 fathoms, and
at Station 27 at 2960 fathoms.

SECTION BETWEEN Carpe May anp Maperra (Plate 8, Table
III.).—This section crosses the North Atlantic from west to east
between the latitudes of 32° and 38° N. It presents at Station
45, distant 140 miles from Sandy Hook, a section of the
Labrador ‘current, and at Station 43 a section of the Gulf
Stream current. The latter formed, at the time of the “ Chal-
lenger’s” visit, the 1st of May, 1873, a surface-current about 60
miles broad and 100 fathoms deep, flowing in an east-north-
easterly direction at the rate of three miles an hour. Later in
the season the volume and velocity of this great oceanic river
are much greater. As seen in the diagram, the Gulf Stream
constitutes the extreme western border of the enormous mass of
water which composes the North Atlantic Equatorial Current, and
virtually flows over the Labrador Current found immediately
below it. The temperature of the current was 23°.9 C. at the sur-
face, 20 C. at 80 fathoms, and 18°.3 C. at 100 fathoms. But at
125 fathoms it had fallen already to 13°.8 C., and at 350 fathoms
to 7.1 C. At Station 44, just beyond the edge of the Gulf
Stream, the temperature at the surface was 11.1 C., at 125
fathoms $.3 ©, and at 350 fathoms.3.9 €. The distance
between Station 43 and Station 44 is little over 60 miles.

The table on page 71 shows the remarkable changes of
temperature observed by the “Challenger” in crossing the Gulf
Stream.

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Between Cape May and Madeira. 7

ee ce | Pane irtoie ae
April 30th 35°53 70°39 I p.m. eS |

” ” ” ” 2 » 22.0
” ” ” ” Z ” 22.8
May 43 36°23' ial 6 a.m. 22.9
fens ” ” ” 7 ” 23-9
” ” 6 8 p.m. 20
” ” ” 9 ” 20.6

Ts 19.4
‘ : : Midnight | 133

Passing along the section from west to east, we find inside
the Gulf Stream an area of colder water extending from
Station 41 to Station 39, then a streak of warmer water at
Station 38, and one of colder water at Station 37, westward
of Bermudas. These currents of colder water are probably
a continuation of the streaks of low temperature met with
between Halifax and the latter islands. At Station 57, off Ber-
mudas, we enter an area of warm water of nearly the same
temperature as the Gulf Stream, which can be traced as far as.
Station 60. Another area of the same temperature is met with
further east at Station 64 and Station 65. The nearly uniform
surface-temperature, varying from 21 to 22° C., found between
these areas and in the rest of the section as far as Station 82,
may be considered as the surface-temperature of this region of
the North Atlantic in the months of June and July. The undu-
lating level of the isotherms of 15°, 10, and 5° C. betrays the
struggle going on between contending currents, the warm water
retaining the upper hand until we reach Station 71, on the
western slope of the plateau of the Azores. This Station, like
Station 10 on the eastern slope of the same plateau in the
section between Teneriffe and Sombrero (Plate 6), fixes the
limit between two areas totally different as regards the dis-

72 Temperature Sections Surveyed.

tribution of temperature, and a line drawn between the two
stations will divide this portion of the Atlantic into an eastern
area, in which the polar current predominates, and into a
western area, where the equatorial current has the upper hand.

A glance at the chart will show that the Azores are placed
in the direct continuation of the axis of Davis Strait, and there
can be little doubt but that the sudden rise of the isotherms
between Station 69 and Station 72 is due to a cold current, a
branch of the Labrador current. The latter, as it arrives off the
great bank of Newfoundland, spreads out like a fan—one branch,
running in a south-westerly direction, forms the cold current of
the coasts of Nova Scotia and the United States; the other
branch, a south-easterly continuation of the original current,
flows straight towards the Azores, to the westward of which
islands. it forms a powerful current more than a hundred miles
broad. One part of this current probably runs down along the
western slope of the plateau, while another part sweeps over
the plateau in a south-easterly direction, carrying along with it
a portion of the equatorial current, and thus composing that
mixture of cold and warm water which we find in the channel
between the Azores and Madeira, and which flows down outside
the Canaries and the Cape de Verde Islands (Plates 8 and 9,
Curves Figs. 3 and 4).

A line drawn between Newfoundland and the Azores passes
over the area where the equatorial and the polar currents en-
counter each other, both coming out of the conflict profoundly
altered in their constitution. To the northward of this line
we find the equatorial current flowing in a north-easterly
direction towards the Faeroe Islands, sending off branches into
Davis Strait and towards Iceland on one side, and towards the
coast of Portugal, the Bay of Biscay, and the western shores of
the British Islands on the other side. In this northern part of
the North Atlantic the equatorial current generally assumes the

Between Cape May and Madetra. 73

character of an intermediate current, which extends from a
depth of 100 fathoms down to 800 and goo fathoms from the
surface. The existence of this enormous mass of warm water,
nearly a mile thick, off the west coasts of Europe, is amply
proved by the temperature-soundings of H.M.S. “ Porcupine”
between the Fzroe Islands and the Straits of Gibraltar. Its
depth necessarily decreases as the current proceeds northward,
but it still forms a layer 500 fathoms thick, of a temperature
from 10 C. to 8° C. in the channel between Rockall and
Scotland, and between the Hebrides and the Feroe Islands.
It dwindles down to about 150 fathoms as it sweeps over the
area of Arctic water (below freezing-point) between the Feroes
and Scotland discovered by H.M.S. “Lightning,” then follows
the coast of Norway, and finally disappears.in the seas of
Spitzbergen and Novaya Zemlya. In about lat. 65° N., near
the coast of Norway, it forms a surface-stratum about 200
fathoms thick of an average temperature of 7° C. In the
same latitude, the zero-point is reached at a depth of 300 and
400 fathoms.

A comparison of the section between St, Thomas and
‘Halifax with the section between Cape May and the Azores
shows that the portion of the North Atlantic which they tra-
verse forms an area of nearly equal distribution of temperature,
and that this area coincides with what is known under the
name of the Sargasso Sea. It extends from Station 28 to
Station 52, or from lat. 25° to lat. go’ N. along the section
between St. Thomas and Halifax, and it occupies the whole
space between the edge of the Gulf Stream, near Station 41, and
Station 69, near the Azores, or between long. 70° and long. 38° W.
This area, about 1000 miles in diameter, may be fitly termed an
immense whirlpool in the middle of the North Atlantic, whose
waters revolve with the hands of the watch. It is covered

with a surface-stratum of warm water about 300 fathoms thick,
F

74 Temperature Sections Surveyed.

in which the temperature slowly decreases from about 22° C.
at the surface, to 16° C. at a depth of 300 fathoms. This
stratum is represented by the protuberance or hump in Curve
A, Fig. 2, and Curve A, Fig. 3. Throughout this area, the
isotherm of 15° C. is found at an average depth of 350 fathoms,
that of 10° C. at 450 fathoms, that of 5° C. at about 600 fathoms,
and the isotherm of 2°.5 C. at 1500 fathoms, occasionally falling
below 1600 fathoms.

SECTION FRoM Maperra To Tristan D’ Acunna (Plate 9,
Table IV.).—This section traverses the Atlantic Ocean from
the parallel of lat. 30° N. to the parallel of lat. 37° S. along
the meridian of long. 20° W., and may, as regards the distribu-
tion of temperature, be divided into three parts—a central
belt, extending across the equator from lat. 15° S. to lat. 15°
N., bounded on each side by the belts respectively belonging
to the South and to the North Atlantic.

The most remarkable feature of the central belt is the
rapid decrease of temperature in the surface-stratum of the
ocean, amounting to from 15° to 19 C. within less than 200
fathoms, in comparison with the much slower decrease in the
northern and southern belt, in the lower strata of which we
observe a gradual increase of temperature as we recede from the
equator, much more rapid, however, in the former than in the
latter. ; |

The maximum increase and decrease is seen in the diagram
of the section to extend from Station 346, or lat. 3° S., to Station
95, or lat. 13° N., limits which coincide within a few degrees with
the equatorial limits of the trade-winds, so that this belt may be
considered as identical with the equatorial belt of calms. An
examination of the section of the Pacific Ocean (Plate 19) shows
that the same phenomenon has been observed in that ocean also.
The increase is naturally due to the excess of solar heat in the
equatorial belt, the decrease to the presence of a stratum of cold

Frrom Madeira to Tristan d’Acunha. aig

water within a few hundred fathoms from the surface. It
appears from the diagram of the section between Cape Palmas
and Cape S. Roque (Plate 10) that this stratum of cold water
stretches right across the Atlantic, occupying the whole length
of the equatorial belt, and that it is colder in the western half
fiamein the eastern. , The isotherm of -5:‘C. rises between
Station 104 and Station 106 from 460 fathoms to 260 fathoms,
its average level in the western portion towards Cape S. Roque
being at a depth of 300 fathoms, and in the eastern portion
towards Cape Palmas at 400 fathoms. On the other hand, the
isotherms in the northern belt, from Station 95 to Station 84,
show that the stratum between 100 and 500 fathoms is warmer
than in the equatorial belt; while in the southern belt, between
Station 339 and Station 342, the water between 250 and 500
fathoms is colder than in the equatorial belt. Between 500
fathoms and 1000 fathoms the same difference may be observed,
the northern belt being warmer, the. southern colder, than the
central belt. The conclusion we arrive at seems obvious. As
the cold stratum between 100 and 500 fathoms in the equatorial
belt cannot come from the northern belt, which is warmer, it
must come from the southern belt, which is colder at the same
depths, and especially it must come from that portion of it which is
situated between Station 339 and Station 342, or, more correctly
speaking, from the belt which lies immediately to the southward
of the equatorial belt, between lat. 10° S. and lat. 17° or 18° S.
There must, therefore, be a current of cold water flowing from
the southern belt into the equatorial belt above the level of
500 fathoms, and it is this current which forms the stratum of
cold water found between 100 and 500 fathoms below the sur-
face of the equatorial belt. According to the theory of thermal
circulation, the superheated water of the equatorial region is con-
stantly being transferred to higher latitudes, and is as constantly
replaced by cold water pouring in from the polar regions. To

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From Madeira to Tristan a’ Acunha. hii

speak more definitely, the warm surface-water at the equator
is replaced by water flowing in from the colder strata im-
mediately adjoining, which must result in the formation and
maintenance of a stratum of cold water immediately below the
surface of the equatorial belt. This cold stratum, in the present
case, is found to extend to a depth of about 400 fathoms.

In this sense the cold water may be said to rise up towards
the surface at the equator, but if we compare the thickness of
the strata under consideration with their horizontal extension in
latitude and longitude, whatever movement may take place in a
vertical direction must be absolutely insignificant in comparison
with the horizontal currents thus created. This inflow of cold
water will take place more or less along the whole boundary
between the equatorial belt and the colder belt immediately
adjoining, and this agrees with the extension of the cold, stratum
along the whole section between Cape Palmas and Cape
S. Roque, the current being stronger in the western half than
in the eastern. There are several indications of a large mass of
cold water moving across the equator and the plateau of St. Paul
Rocks in a north-easterly direction, and forming an under-current,
which may be traced as far as the Cape de Verde Islands
along the West Coast of Africa, and between the latter and the
Canaries, Madeira, and even the Josephine Bank, off the Straits
of Gibraltar.

The abrupt gradient of the temperature-curve which is so
characteristic of the equatorial stations (Curve Fig. 9) is also
observed at Station 95, Station 96, and Station 97, and to a
slightly lesser extent at Stations 98, 99, and 100. In fact, this
form of curve makes its appearance immediately south of the
Cape de Verde Islands, and thence extends towards the equator.
The cold current, after crossing the equator in an oblique direc-
tion from S.W. to N.E., probably divides itself off the coast of
Africa into two branches, one flowing north, between the Cape

78 Temperature Sections Surveyed.

de Verde Islands and the African coast, the other flowing south,
past Sierra Leone towards Cape Palmas. At Stations 95, 96, and
97, in the direct line of this cold under-current, the decrease of
temperature between the surface and 100 fathoms amounts to
15. C. The temperature observations made by the “ Chal-
lenger” on the return journey at Stations 350, 351, 352, in the
same area, off Sierra Leone, prove the same extraordinary
decrease of temperature in the first hundred fathoms, amounting
respectively to 15.1, 14°.3, and 12°.6 C. ' The remarkable rise of
the isotherm of 2°.5 C. in this area also deserves to be noticed.
At Station 97, the temperature at 1500 fathoms was found to be
1.7 © ~ Proceeding fiirther north, at Stations 89 and, 90;
between the Cape de Verde Islands and the Canaries, the tem-
perature at 1100 fathoms was only 3.2 C. and 3° C. respec-
tively. .At a sounding taken on the 6th February, 1873,
between Teneriffe and Madeira, the temperature at 1975
fathoms was ascertained to be 1°.6 C.; on February 6th, to the
east of Madeira, 1°.8 at 2225 fathoms; and on January 30th of
the same year, a temperature of 1°.6 C. was registered in 1525
fathoms, immediately south of the Josephine Bank.

If we draw a line connecting the western extremities of the
plateaux of the Cape de Verde Islands, the Canary Islands, and
Madeira, it will be found that the isotherms of the stations west
of this line are lower than the isotherms of the stations east of
this line, and that, consequently, the water between this line and
the coast of Africa is colder than it is to the westward of the
above islands. The presence of this cold under-current along
the West Coast of Africa, moreover, would be a necessary con-
sequence of the general law which governs the circulation of
thermal currents. The quantity of cold water supplied to the
Atlantic by the Arctic basin is much less than the quantity
flowing into it from the Antarctic region, and the principal
supply is undoubtedly derived from the latter source. The cold

From Madeira to Tristan ad’ Acunha. 79

bottom-current traced by H.M.S. “Challenger” from the Falk-
land Islands along the east coast of South America as far as
Cape S. Roque, and thence across to St. Paul Rocks, and along
the southern slope of the equatorial plateau as far as Station
346, with the bottom-temperatures slowly rising from —o’.4 C. to
+0°.4 C., in this long course of several thousand miles, proves that
there is such an inflow of cold water from the south towards the
north. Such a current, after crossing the equator, would havea
tendency to flow eastward and press up against the coast of
Africa, owing to the same cause which compels the Arctic current
to press up against and to flow along the coast of North
America. Unfortunately, we do not possess on the African
coast such a complete series of temperature-soundings as that
furnished by the officers of the U.S. Coast Survey, and future
observations can alone show how far the above theoretical con-
clusions are supported by facts.

The portion of the Atlantic section from Stations 84 to 95 is
a continuation southwards of the eastern basin represented in
the section from Teneriffe to Sombrero, Station 1 to Station
10. It is probable that the combined warm and cold currents
flowing southwards from the Azores encounter, when off the
Cape de Verde Islands, the cold equatorial stratum, and turn
westwards, along the northern limit of the equatorial belt.

Before proceeding further, it may be necessary to allude to
an appearance in the Atlantic section (Plate 9) which is also
strongly marked in the Pacific section (Plate 19). It is the
increasing undulations of the isotherms, as we enter the lower
strata, at first sight suggesting the completely erroneous idea
that the fluctuations of temperature increase with the depth,
the very opposite being the case. This appearance is due partly
to the excess of the vertical over the horizontal scale, the former
being to the latter as 1200 to 1 in the Atlantic section, and as
1500 to 1 in the Pacific section—a disproportion inevitable on

80 Temperature Sections Surveyed.

account of the relative thinness of the aqueous envelope when
compared to the diameter of the areas which it occupies. It
arises partly also from the nature of a section composed of
tsotherms. A difference of o.1 C. in temperature, which in the
higher strata corresponds to only a few fathoms, and causes
no perceptible alteration in the form of the isothermal line,
is represented by 50 or too fathoms in the lower strata,
and causes the isothermal line to assume the shape of a big
wave. This defect, which requires only to be known to be
guarded against, is obviated by the employment of zso-
bathymetrical lines. As the latter represent the fluctuations of
temperature at the same depth along the section of an oceanic
basin, their undulations will keep pace with the fluctuations of
temperature, greater near the surface, smaller as we enter the
lower strata. But as in such a diagram the scale of depth must
be replaced by a scale of temperature, it ceases to be a
representation of an actual section (all but the horizontal dis-
tances) of an oceanic basin, and therefore the employment of a
diagram of isotherms is, for general purposes, to be preferred.
Of course, where space will allow it, both kinds of thermometric
sections may be used, as has been done in the published dia-
grams of the U.S. Coast Survey.

SECTION FROM Cape Patmas TO CapE S. Rogue (Plate 10,
Table V.).—The principal feature of this equatorial section has
already been discussed above. A comparison of the isotherms
between Station 106 and Station 102, and of the isotherms
between Stations 339 and 342 of the Atlantic section (Plate 9),
with the isotherms between Stations 129 and 119 of the
southern section (Plate 10), shows that the north-eastern
portion of the South Atlantic is warmer than its north-western
portion.

A line drawn from Station 106 to Station 339 and continued
down to the latitude of Tristan d’Acunha, which is a line pro-

Between Cafe S. Roque and Tristan d’Acunha. 81

ceeding in a south-south-easterly direction from a point to the
eastward of St. Paul Rocks and intersecting the meridian of
Ascension Island on the parallel of St. Helena, will divide the
South Atlantic into two halves—a colder western half as regards
the temperature of the lower strata, and a warmer eastern half.
The contrast is the same as that found in the North Atlantic,
and arises from a similar cause—a warming of the lower strata
in the eastern half caused by the return eastward as an under-
current of the South Atlantic Equatorial Current. There exists
probably an additional cause of the higher temperature of the
lower strata of the eastern half of the South Atlantic in com-
parison with the western half, namely, a return under-current cf
warmer water from the North Atlantic, which, finding the
depths of the western half occupied by the Antarctic Current,
flows down the eastern valley of the South Atlantic between
the central plateau and the coast of South Africa. |
We possess, in this part of the world, only a few soundings
of the German frigate “Gazelle,” which, however, prove the
higher temperature of the lower strata in the eastern half of the
South Atlantic, but further observations between Ascension,
St. Helena, and the Cape of Good Hope are necessary.
SECTION BETWEEN Care S. Rogue AND TRISTAN D ACUNHA
(Plate 10, Table V.).—This section passes in a south-easterly
direction from Cape S. Roque to Tristan d’Acunha. The arrange-
ment of its isotherms marks the progress of the South Atlantic
Equatorial Current along the coast of South America, and
especially of that branch of it which, turning eastwards, occupies
the space between Station 339 and Tristan d’Acunha (Plate 9),
and between Stations 131 and 132 (Plate 10). As seen in both
sections, this branch attains its maximum depth between lat. 30°
and 35° S. A comparison of the isotherm of 2°.5 C. in the two
sections shows that, while in the western half of the South
Atlantic it is found not lower than at 1000 fathoms from the

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From Falkland Islands to Cape of Good Hope. 83

surface as far north as lat. 15° S., it sinks below that level in
lat. 30° S. between Stations 335 and 336 (Table IV.), upon the
plateau which divides the western from the eastern half of the
South Atlantic—a further proof of the higher temperatures
which prevail in the lower strata of the latter as compared with
the former. At Station 340, towards Ascension, the isotherm
of 2°.5 C. is already below 1500 fathoms, and its remaining near
that level as far as the equator indicates the presence of a large
accumulation of warm water in the depths of the eastern half of
the South Atlantic between lat. 20° S. and the equator. This
circumstance has suggested the existence of a submarine ridge
connecting the Central Atlantic plateau with the coast of Africa
between the parallels of lat. 20° and 35° S., as shown in the
chart of Staff-Commander T. H. Tizard which accompanies
No. 7 of the Report on Ocean Soundings, by Captain Frank
T. Thomson of H.M.S. “Challenger,” and published by
the Hydrographic Office. There are indications of the
existence of such a ridge or area of elevation furnished by the
discovery of several shallow soundings of less than 2000
fathoms between the central plateau and the coast of Africa,
but the reasons stated above perhaps suffice to explain the
presence of higher temperatures in the eastern basin of the
South Atlantic, the more so as we observe a similar phenomenon
in the North Atlantic. We know, besides, that a current of
cold water opposes as effectual an obstacle to the further exten-
sion of warmer strata as a solid barrier formed by a submarine
ridge or protuberance of the earth’s crust. Further soundings
in this region of the South Atlantic will decide this question.
SECTION FROM THE FALKLAND ISLANDS TO THE CaPE OF Goop
Hore (Plate 11, Table VI.).—This section includes the Stations
317, 318, 319, and 320, situated between the Falkland Islands
and the mouth of the Rio de la Plata, and they were added as
belonging to the same thermal area. The section between

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From Falkland Islands to Cape of Good Hofe. 85

Station 323 and Station 140 traverses the whole of the South
Atlantic Ocean from the mouth of the Rio de la Plata to the
Cape of Good Hope, between the parallels of lat. 35° and 38°
S., and only a few degrees north of what may be considered as
the limit between the South Atlantic and the Southern Ocean.
It is a combination of two sections surveyed at two different
periods of the circumnavigation cruise of H.M.S. “ Challenger,”
and Station 333 of the homeward voyage in March, 1876,
nearly coincides with Station 133 of the outward voyage in
October of 1873. The former date being in those southern
latitudes the end of summer, when warm currents may be
expected to have attained their maximum volume and velocity,
while the latter date marks the beginning of spring, when cold
currents have acquired their greatest power, this difference in
the seasons ought not be entirely lost sight of in a comparison of
the distribution of temperature in the two portions of the section
east and west of Tristan d’Acunha. At Stations 317, 318, 319,
and 3.0, we find the Antarctic Current pressing up against the
coast of Patagonia and nearly coming to the surface at Station
317; while at Stations 318, 319, and 320, it is disguised by a
warm surface-stratum about 100 fathoms thick—an extension or
overflowing of the South Atlantic Equatorial Current between
Rio de la Plata and the Falkland Islands. An examination of
Curve Fig. 7, belonging to Station 318, shows that the Antarctic
current here forms a stratum of the enormous thickness of
about 1400 fathoms, or one and a-half English miles, with a
nearly uniform temperature of from 1° to 2° C. The abnormal
bottom-temperatures ascertained by the “Challenger” (varying
from —o.3 to —o.6'C.), between Station 319 and Station 330
(long. 55° W., and long. 33° W.), a distance of over 1000 nautical
miles, have been alluded to in a former chapter, as well as the
slight increase of these temperatures towards the equator, where
they are still found to vary between 0%4 C. and o%.9 C.—an

86 Temperature Sections Surveyed.

increase of less than 1° C. in a distance of about 40° of latitude,
or 2400 nautical miles.

Between Station 320 and Station 326, a distance of about
460 miles, we traverse the South Atlantic Equatorial Current,
forming a surface-stratum of an average depth of 50 fathoms,
with a temperature above 20°C. The undulating form of the
isotherms marks the struggle going on between the equatorial and
the polar current, betraying itself at the surface, as in the case
of the Gulf Stream or North Atlantic Equatorial Current, by the
formation of alternate streaks of warm and cold water. The
cold under-current which presses up at Station 324 and Station
329 comes to the surface at Station 326. The warm current
predominates at Stations 323, 325, 327 (at which latter station it
forms a warm surface-streak beyond the cold streak of Station
326), and 330. As far as Station 334 we trace the warming
influence of the equatorial current, a branch of which we have
seen (Plate 9) flows to the northward of Tristan d’Acunha
between the parallels of lat. 30° and 35° S. We have no
observations in a direct line between Cape Horn and the
Cape of Good Hope, but the main portion of the equatorial
current seems, after flowing in a southerly direction from the
place which it occupies in our section, to bend round between
the parallels of lat. 40° and 50° S., and, coming in conflict with
the Antarctic surface-current which flows towards the Cape of
Good Hope, to sink under it and continue its course into the
Antarctic regions as a warm under-current. The wide open
sea, discovered by Weddell in 1823 beyond the parallel of lat.
70'S. and in the meridian of South Georgia (long. 40° W.), is
probably an effect of this warm under-current.

The low surface-temperatures between Station 135 and
Station 140 are due to the Antarctic surface-current which,
flowing in a north-easterly direction, passes up in the space
between Tristan d’Acunha and the Cape. The rise of the

From Falkland Islands to Cape of Good Hope. 87

isotherms of 10° C. and of 2°.5 C. over the plateau of Tristan
d’Acunha and Gough Islands deserves notice. The depth of
the isotherms of to C. and 5° C. in the eastern portion of this
section differs but little from that in the western portion. The
concave shape of these isotherms between Tristan d’Acunha and
the Cape is due probably to the influence of a comparatively
warm surface-stratum, which, however, soon comes in contact
with the cold bottom-stratum, for at Station 136, 4° C. was
Bemistered, at 400) fathoms; at Station’ 137, 2.6 ‘C. at 7oo
fathoms ; at Station 138, 3°.3 C. at 500 fathoms; and at Station
139, 3.1 C. at 4oo fathoms. The high level of the isotherm
of 2.5 C.,, rising in this section to 600 fathoms from the surface,
while in the North Atlantic its position is generally at 1500
fathoms, proves the inflow of cold water from the Southern
Ocean into the Atlantic basin, and the gradual rise of the tem-
perature of the bottom strata as we proceed along the meridian
from South to North.

CHART Rw:

THE TEMPERATURE SECTIONS SURVEYED “BY H.W S:
*CHALLENGER” IN THE SOUTHERN OCEAN, THE
INDIAN ARCHIPELAGO; AND THE. PACIBIC

From the Cape of Good Hope to Melbourne—From Kerguelen Land to the Ice-barrier
—From Sydney to Cook Strait, New Zealand—From Cook Strait to Tonga
Tabu—From Tonga Tabu to Torres Strait—From Torres Strait to Hong-kong,
and from Hong-kong to the Admiralty Islands—From the Admiralty Islands
to Japan—From Yokohama to Station 253—From Station 253, in the Meridian
of Honolulu and Tahiti, to Station 288—From Station 288 to Valparaiso and
Magellan Straits.

SECTION FROM THE Care oF Goop Hope To THE ICE-BARRIER
AND TO MELBouRNE (Plates 12 and 18, Table VII.).—Much of
the interest attached to the voyage round the world of H.M.S.
“Challenger” centres in her cruise in the Southern Ocean.
The latter, already associated with the fame of such great
navigators as Cook (1773), Bellingshausen (1820), Weddell
(1823), Morrell (1823), Biscoe (1831), Kemp (1834), Balleny
(1839), D’Urville (1840), Wilkes (1840), Ross (1841 and 1843),
and Moore (1845), had been the scene of many a courageous
attempt to solve the mystery of the South Polar region, and
it was the good fortune of Captain Sir George S. Nares, the
leader of the recent Arctic expedition, to add his name to this
long list of hardy explorers. It was not the mission of the
“Challenger” to penetrate into the ice-bound regions of the
South Pole—a task for which her size and her unprotected hull
rendered her unfit—but she was able, thanks to the skilful
navigation of her captain and officers, and with the assistance of
a picked crew of England's sailors, to extend her dredging and
sounding operations beyond the limits which had baffled former
navigators, to cross the Antarctic Circle at a point not touched

From Cafe of Good Hope to Melbourne. 89

by them, and perhaps to show the direction in which a future
attempt to penetrate towards the South Pole might be suc-
cessfully accomplished.

The section represented on Plate 12 traverses the Southern
Ocean—not in a direction due east, but forms two sides of a
triangle, the apex of which just touches the Antarctic Circle
(Plate 1). The western side of the triangle is formed by the
track of the “Challenger” from the Cape of Good Hope to
Kerguelen Land and the Ice-barrier; the eastern side by the

track of the ship between the latter and Melbourne.
The following table gives the surface-temperatures observed

while crossing the “Agulhas Current”—the great equatorial

current of the Indian Ocean.

TEMPERATURES OBSERVED BETWEEN THE CAPE OF GOOD HOPE
AND MARION ISLAND, AND IN THE AGULHAS CURRENT.

|
mes Station. ag ee a Pongitude at Hour. Tene cne Observations.
Deck 17 141 gu oer S 18° 34’ E. .oC. | Warm streak.
” ” a6 aes ” ” ” ”
9 9 9 9 ” ” =
” ” ” ” oa 99 5 Cold streak.
9 9 ” ” ” ” -
99 ” 9 ” ” ” =
oR a8 ae ames 550.45 : Warm streak.
9 OS 142 Bom 20) 18° 40° : Cold streak.
» 9 143 30° 48 19° 24
Rae ae A ee ria : Warm streak.
” ” ” ” Noon 22°.8 Agulhas
» 20 ° 53 |During the IOS GD current.
24 hours to 19°.4 | | Cold streak.
» 21 2 a.m. 22°.2 ) | Warm streak.
” ” 2.30,,; 20°.6
” ” Sass 18°. 3
oss 40) 5. 15.8
» oo» 2 p.m. 15°.0
aera Midnight 13 Cold waiter.
» 22 During the 14°.2 max. 53 aA
8°.9 min. - “A
9°.O max. Pe 5
6°.1 min. 35 8
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From Cape of Good Hofe to Melbourne. gi

H.M.S. “Challenger” left her anchorage in Simons Bay at
6.30 am., the 17th December, 1873: Between 10 and 11
a.m., she traversed a current of cold water with a surface-
temperature of 13° C., or 5° below the temperature recorded at
4am. At noon the thermometer had again risen to 18.3 C,,
and at 3 p.m. to 19.2 C. After sailing at midnight on the
following day through a second streak of cold water, the ship
crossed the northern limit of the Agulhas Current between
1 and 2 a.m. of the 19th December, at which time the surface-
temperature was observed to rise to 22°.2 C. Towards noon of
the same day, and at Station 143, distant about 150 nautical
miles from the Cape, the thermometer recorded the maximum
of 22°8.C. It next fell to 22°2°C., and, with the exception
of a cold streak of 19°.4 C. observed in the course of the
2oth December, remained stationary until 2 a.m. on the 2Ist,
between which hour and 2.30 a.m. it fell from 22°.2 C. to
20.6 C. This, then, was the southern limit of the Agulhas
Current, and the distance run between the two limits amounted
to about 250 miles,

The existence of a streak of cold water in the opening of
Simons Bay and in the immediate vicinity of the Cape, agrees
with an explanation attempted by the author of the sudden
changes of temperature observed in Simons Bay, and of the
difference in the temperature of the water in the latter bay as
compared with Table Bay. It is contained in a short paper
published in Zhe Cape Monthly Magazine for January, 1874,
edited by the late Professor Noble, and its substance may be
repeated here, as it furnishes a good illustration of the incessant
contest going on between the equatorial current of the Indian
Ocean and the polar current of the Southern Ocean, in the seas
off the Cape of Good Hope.

The observations arranged in the following table were made
on board H.M.S. “Challenger,” supplemented by Surgeon

92 Temperature Sections Surveyed.

Thomas Bolster of H.M.S. “ Flora,” and kindly placed at the
author’s disposal by Captain Sir G. S. Nares.

TEMPERATURES OBSERVED IN

Simons Bay. TABLE Bay.
Date,
1873. Temperature Direction of Temperature Direction of
at 9 a.m. the Wind. at 9 a.m. the Wind.

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i 2 10 £7 25 'C. Syl 107°C, N.N.E.

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2 i ea Or Shas —- —

The great surface-current of the Southern Ocean, as it
flows from west to east, makes the circuit of the world between
the parallels of lat. 60° and go’ S., and is split up by the
projecting continents of South America, South Africa, and
Australia into several branches, which can be traced flowing
northwards along the western coasts of these continents, and
which constitute the cold surface-currents of the South Pacific, _
the South Atlantic, and the Indian Ocean. The rapid fall of
temperature—from 16° C. to 5° C. in the summer, from 10° C.
to 5. C. in the winter of the southern hemisphere between the
parallels of lat. 40° and lat. 50° S.—shows that the great surface-
current of the Southern Ocean forms between these latitudes

Frrom Cafe of Good Flope to Melbourne. 93

an effectual barrier, a “cold wall,” which arrests the further
progress southwards of the equatorial currents of the three great
oceanic basins. These warm currents—the Brazilian Current
in the South Atlantic, the Agulhas Current or Cape Current
of the Indian Ocean, and the currents which flow along the
east coast of Australia and of New Zealand—on meeting the
easterly current of the Southern Ocean, are split up into two,
or rather three portions: the first portion is bent round and
flows eastwards as a warm surface-current; the second, mixing
with the cold current, is also carried eastwards, and accounts for
the rise of temperature—from 5° C. to 10° C. or 16° C. according
to the season—which is observed between lat. 50 and qo’ S.;
the third sinks below the cold surface-current, and, taking a
south-easterly course, flows as a warm under-current into the
Antarctic Ocean. These are the warm currents which under-
mine the enormous ice-masses that rise, under the name of the
“Ice-barrier,” like a solid wall toa height of from 150 to 300
feet above the surface of the sea, and detach from them the
innumerable floating icebergs which strew the face of the
Southern Ocean down to 50° and qo latitude. The continual
melting of these icebergs between 60° and 4o” latitude supplies
the masses of cold water which, to a depth of several thousand
fathoms, fill up the basins of the Atlantic, the Pacific, and the
Indian Ocean.

This splitting-up of the currents assumes especially marked
features off the Cape of Good Hope. The Agulhas Current,
immediately after crossing the meridian of the Cape, flows
‘into the angle between the two branches of the Antarctic
Current, between which it is completely annihilated as a
surface-current, for few if any traces of it appear at the surface
further westward. At the time of the “Challenger’s” visit
to these latitudes, the surface-temperatures between Tristan
d’Acunha and the Cape ranged from 12° C. to 15° C., while the

94 ZLemperature Sections Surveyed.

temperature of the surface-stratum of the Agulhas Current to a
depth of 20 fathoms was found to be 22°.2 C. A portion of this
current, however, may be found in the shape of an under-current
for a distance of about 150 miles to westward of the Cape, as
will appear on comparing the two Curves A and B, Fig. 5.
The undulating form of Curve B indicates, as pointed out on a
previous occasion, the presence of contending currents, which,
being split asunder during the encounter, form alternate currents
of warm and cold water flowing side by side and one above
the other.

A portion of the Agulhas Current is probably carried by
the Antarctic Current past the Cape and northward along the
west coast of the African continent, but by far the greater portion
is turned back and flows eastward, partly mixing with the cold
current of the Southern Ocean, partly as an under-current.
The latter seems to divide off the Crozet Islands into two
branches, one of which continues its eastward course, while the
other flows in a south-easterly direction towards the opening
between Kemp Land and Wilkes’ Termination Land, explored
by H. M.S. “ Challenger.”

Near the Cape of Good Hope the forces of the Agulhas
Current and of the Antarctic Current are so evenly balanced that,
as appears from the above table, False Bay, which includes
Simons Bay, is alternately occupied by branches of the warm
or of the cold current, according as the wind blows from the
south-east or from the north-west. During the prevalence of
the former wind, a warm current is observed to flow from Cape
Agulhas towards Cape Point, or from east to west, and False
Bay is occupied by a branch of the Agulhas Current ; during the
prevalence of the latter, a cold current circles round the bay
from west to east, and the bay is taken possession of by the
Antarctic Current. A prevailing north or north-west wind
drives the warm water out of False Bay, the bottom of

From Cape of Good Hope to Melbourne. 95

which, from a line drawn between the Cape of Good Hope
and Cape Hangklip, gradually shelves up from a depth of 50
fathoms. A prevailing south or south-east wind brings the branch
of the Agulhas Current which flows over the Agulhas Bank into
False Bay, raising the temperature of the water in Simons Bay ©
6 or 7 C. (11 to 13° F.). This difference was observed not
only at the surface but at the depth of 9 fathoms, in which the
“Challenger” was anchored, and the change would be accom-
' plished in the short space of six hours. .

The peninsula, from Table Mountain to the Vasco de
Gama Hill, must at one time have been an island about thirty
miles long and five miles broad, now joined to the mainland of
Africa through the slow silting up of the strait which formerly
flowed between Table Bay and False Bay.

Eastward of the Crozet Islands, the temperature of the
Southern Ocean decreases rapidly. The isotherm of 5° C., which
at Station 146 is at 50 fathoms, rises to the surface at Station
147. The temperature at 100 fathoms, which at the latter
Station 1s-2..9-G,, falls to r.8 C. at Station 150, and to o.o C.
at Station 152. On the morning of the 11th February, and in
about lat. 60° 4o’ S., long. 80° 20’ E., the “ Challenger’ sighted
the first iceberg. At 4 a.m. it presented the appearance of a
silvery mass dimly visible towards the east-south-east, and
was found, by angular measurement, to be over 700 yards
long, and to rise vertically on all sides to a height of more than
200 feet above the surface of the water. From that date to the
end of the month, the narrow horizon commanded from the
_ ship’s deck offered the imposing spectacle of a sea studded with
icebergs of every size and shape, though generally assuming the
form of huge slabs, whose snow-white surface reflected every
hue of day—from the delicate silvery-grey of dawn to the
golden and crimson tints of sunset, and from the inky dark-
ness of their recesses when in the shadow of a cloud, to the

96 Temperature Sections Surveyed.

exquisite azure-blue light which filled the numerous caves
worn by the waves in their flanks. On the day when the
ship stopped in lat. 66° 40’ S., long. 78° 22’ E.—a distance
of 1400 nautical miles from the South Pole—more than
seventy of these floating islands of ice could be counted
from her deck, one of them over five miles long, and rising
from 150 to 200 feet above the sea.

The melting of these ice-masses produces a quantity of water,
which, being fresher, is of less specific gravity than the salt
water of the surrounding sea, and therefore floats in the immediate
vicinity of the ice on the surface of the latter. But as the fresher
water derived from the icebergs mixes by degrees with the sur-
rounding salt water, the mixture being of lower temperature is
rendered heavier, and sinks below the surface, forming an inter-
mediate stratum or wedge, as shown in Plates 12 and 13.

Owing to her supplies of coal running short, H.M.S.
“Challenger” was prevented from establishing as many stations
between the Ice-barrier and Australia as might have been
desirable, but sufficient observations were secured to confirm
the existence and to ascertain the proportions of the great
current which, under the name of the South Australian Current,
was already known to flow in an easterly direction at some
distance from the south coast of the Australian continent.

The temperature of the water, which at Station 157 was
2°.9 C. at the surface, 2°.6 C. at 60 fathoms, 0.6 C. at 7o
fathoms, and 0.3 C. at 80 fathoms—thus betraying the presence
of an overflowing warmer current—and —o’.6 C. at the bottom
in 195@ fathoms, had risen at Station 158 to 7.2 C. at the
surface, 5° C. at 200 fathoms, 2° C. at 700 fathoms, 1° C. at 1500
fathoms, and o°.3 C. at the bottom in 1800 fathoms. At Station
159 there appeared evident signs of the presence of a warm
under-current, for the temperature of the water, which between
the surface and roo fathoms had fallen from 10.8 C. to

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From Sydney to Cook Strait. 97

9°.3 C., remained almost stationary between 150 fathoms and
400 fathoms, descending from 8°.8 C. to 8°.2 C. at the latter
depth, whence it decreased more rapidly to 5° C, at 600 fathoms,
to 3° C. at 800 fathoms, to 2° C. at 1100 fathoms, and to 0.8 C.
at the bottom in 2150 fathoms. Similar conditions of tempera-
ture were observed at Station 160, nearer to the Australian
coast, where, however, the warm stratum, still commencing with
8°.8 C. at 150 fathoms, ended with 8°.2 C. in 300 fathoms,
while 5°.1 C. were registered at 500 fathoms, 2.4 C. at 800
fathoms, and 0.2 C. at the bottom in 2600 fathoms. It seems,
therefore, that at the latter station the ship had already crossed
the axis of this warm under-current.

The distance run between Station 158 and Station 159, and
between the latter station and Station 160, was about 350 miles,
so that this current cannot be less than 400 miles broad. The
distance between Station 160 and Cape Northumberland, the
nearest point on the south coast of Australia, is about 380
nautical miles, and the axis of the current may be laid down on
the chart at a distance of about 500 miles from the Australian
coast, bending round in a south-easterly direction towards the
wide space of open water at the foot of Victoria Land, discovered
by Sir James Ross. It may be taken for granted that the
South Australian current just described, as well as the currents
to the westward of the Crozet Islands and Kerguelen Land,
represent the outflow of the warm water of the Indian Ocean
through the Southern Ocean into the Antarctic Basin.

SECTION FROM SYDNEY TO Cook Strait, NEw ZEALAND, AND
FROM Cook Strait To Tonca Tasu (Plate 14, Table VITI.).—
Narrow as the sea between Australia and New Zealand seems
when compared with its great neighbour the Pacific, its average
width is a thousand miles, and a ship sailing at the rate of 150
miles a-day will consume a week in accomplishing the voyage from
Sydney to Wellington. Divided from the basin of the Pacific

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Frrom Gook Strait to Tonga Tabu. 99

Ocean by a submarine plateau which rises to within 1000 fathoms
of the sea surface, and unites Australia, New Zealand, New
Caledonia, and Papua into a single area of elevation, it may be
considered as forming a bight of the Southern Ocean (Plate 2).
The cross section of this area presents the not unfrequent con-
trast of deep soundings and a comparatively rapid fall of the sea-
bottom along its western boundary, and of shallow soundings
and a slowly rising bottom towards the east. The western half
of the basin is occupied by an area of depression of more than
2500 fathoms, or about three miles in depth, extending from the
south point of Tasmania along the east coast of Australia as far
as Great Sandy Island, where the coast turns towards the north-
east. The eastern half forms a broad plateau, which ultimately
rises above the level of the sea under the name of New Zealand.

-A branch of the South. Pacific Equatorial Current, after
passing to the southward of the Fiji Islands and New Caledonia,
strikes the Australian coast near Great Sandy Island, and,
bending round, flows as a surface-current, known under the name
of the East Australian Current, close along the shores of New
South Wales. With the exception of this current, the whole of
the basin between Australia and New Zealand is occupied by a
branch of the South Australian Current, which, crossing the
basin in a north-easterly direction, carries a portion of the East
Australian Current along with it, and divides into two
branches, one returning southwards along the west coast of
Middle Island, while the other flows round the North Cape and
probably down the north-east coast of North Island.

_ A comparison of the isotherms at Stations 165 and 166 west
of New Zealand, and at Stations 168 and 169 east of the
latter, seems to show that at the time of the “ Challenger’s”
visit, which was in the winter of the southern hemisphere, the
whole plateau of New Zealand was swept by a current from the
south-west, whose temperature was somewhat raised through

100 - Temperature Sections Surveyed.

carrying the waters of the East Australian current along with it.
No trace of a branch of the South Pacific Equatorial Current
flowing along the east coast of New Zealand appears at Stations
168 and 169 of the section between Cook Strait and Tonga
Tabu; but as these stations are near the coast, and properly
belong to the plateau of New Zealand, it is possible that such
a branch may be found further to the eastward.

The above section exhibits the usual contraction of the
isotherms as we approach the tropics. The soundings of the
“Gazelle” and of the “ Tuscarora” have proved that a channel
of more than 2000 fathoms in depth passes up between New
Zealand and the Kermadec Islands in a north-westerly direc-
tion towards New Caledonia. Split into two branches by the
plateau which supports New Caledonia and Loyalty Islands, the
eastern branch continues.in the same direction between the
latter islands and the New Hebrides, and finally communicates
with the basin which stretches from the New Hebrides as far as
Torres Strait.

Station 1714, with a depth of 2900 fathoms, belongs to the
area of depression discovered by the “Gazelle,” and which has
been traced and explored by the German expedition from the
Samoan Islands across the South Pacific as far as the Strait of
Magellan.

SECTION FROM Tonca Tasu To Torres Strait (Plate 15,
Table IX.).—The combined soundings of H.M.S. “ Challenger”
and of the U.S.S. “ Tuscarora” show that the Fiji Islands occupy
the centre of a plateau which comprises the Samoan Islands
in the north-east, Tonga Tabu or the Friendly Islands
in the east, the Kermadec group in the south, and the New
Hebrides in the west. This plateau may be considered as
the terminal knot which unites two extensive areas of eleva-
tion. One of these stretches from the Samoan group, first
in a north-north-easterly direction through the Ellice and

From Tonga Tabu to Torres Strait. IOI
Gilbert islands to the Marshall group, and afterwards in
a due westerly direction through the Caroline Islands as far
as the Pelew Islands. The other connects the New Hebrides
with the Santa Cruz Islands, the Solomon Islands, New
Ireland, New Britain, the Admiralty Islands, and Papua. The
two areas of elevation, or ridges as we may call them, enclose
an area of depression which extends from Fiji to the Philippines,
and which was partially explored by the “Challenger” in the
months of February’and March, 1875, and by the “Gazelle” in
June and July, 1875. The total length of this basin amounts to
3000 miles, and as the continent of Papua constitutes about
one-half of its southern boundary, it may with some show of
reason be distinguished by the name of the “Sea of Papua.”
To the southward of the last-described ridge we find another area
of depression, which stretches from the New Hebrides to Torres
Strait, and forms a continuation of the 2000-fathom channel
between the New Hebrides and New Caledonia. This is the
basin represented in the Section Plate 15. Bounded on the south
by the shallow coral-sea whose numerous reefs crowd the space
between Australia and New Caledonia, it forms an almost
land-locked basin, communicating with the depths of the South
Pacific only through the above-mentioned 2000-fathom channel,
which also divides the Fijian plateau from New Caledonia and
New Zealand. The almost uniform level of the isotherms between
Fiji and Torres Strait, a distance of about 2000 miles, is the
most prominent feature of this section. It was this basin
—named, at the time of its discovery by the “Challenger,” the
“Melanesian Sea” (partly on account of the dark complexion
of the natives of the islands by which it ts surrounded,
partly to revive a term used by the earlier geographers and
navigators)—which furnished the first example of the distribu-
tion of temperature in a sea separated from the depths of
the ocean by a submarine ridge or area of elevation. The

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From Tonga Tabu to Torres Strait. 103

officers on board H.M.S. “Challenger” were surprised to
find that, beyond a depth varying between 1200 and. 1400
fathoms, the thermometers ceased to register any further
decrease of temperature.. The latter was observed to fall
from 25 C. at the surface to 1.8 C. between 1200 and. 1400
fathoms, and to remain stationary or nearly stationary from that
level down to the bottom. At Station 184, the bottom-tem-
perature in 1400 fathoms was 1°.8 C.; at Station 183, in 1700-
fathoms, iy Gat Station 182, in 2275 fathoms, 1 .4.:C. ; at
Station 176, between the Hebrides and Fiji, in 1450 fathoms,
2 C.; and at Station 175, in 1350 fathoms, 1°.°38 C. The same
phenomenon was found to occur under similar conditions in the
seas of the Indian Archipelago, and tended to confirm the
opinion formed at the time—that its cause must be sought in
the partial or complete suspension of all deep-sea communica-
tion by intervening submarine ridges or submerged areas of
elevation.

This circumstance affords one of the most convincing
proofs of the existence of a system of thermal circulation in
a horizontal direction, which embraces the whole of the ocean
as well as the minor seas in proportion to the facilties of sub-
marine communication. The almost uniform level of the
isotherms observed in landlocked basins also shows ¢hat the
obliquity or gradient of the isotherms stands in direct relation
to the presence or absence of currents of different temperature,
and therefore of different origin, and moving in different direc-
tions in the various areas which compose the aqueous envelope
of our planet. \n the same manner, it has been explained on a
previous occasion that the gradients of the temperature-curve
stand im direct relation to the presence or absence of currents of
different temperature, origin, and atrection, in the various strata
between the surface and the bottom.

An examination of Table IX. shows that, although the

104 Temperature Sections Surveyed.

isotherms appear almost at the same level from one end of the
section to the other in the diagram of Plate 15, the distribution
of temperature varies from one station to another. There is a
gradual decrease of temperature between 100 fathoms and 500
fathoms as we proceed westwards until we arrive at Station 182,
which is the coldest, after which the temperatures rise again
towards Station 184. The warm surface-stratum above 25° C.
is only 10 fathoms deep between the Fiji Islands and the New
Hebrides at Stations 175 and 176, a decrease probably due to
the influence of a cold current from the southward, perhaps a
branch of the cold current flowing northwards along the west
coast of New Zealand. Its depth rapidly increases as we enter
the Melanesian basin at the New Hebrides, attains a maximum
of 55 fathoms at Station 179, falls to 25 fathoms at Station 182,
and increases again to 40 and 45 fathoms as we approach Torres
Strait. These alterations of level, which represent a difference
in the thickness of the warm surface-stratum of over 100 feet,
must be caused by warm and cold surface-currents flowing into
the basin from the north or south.

SECTIONS FROM TorRES STRAIT TO HONG-KONG, AND FROM
HoNG-KONG TO THE ADMIRALTY IsLanps (Plate 16, Table X.).—
These sections embrace the numerous seas between Papua and
China, and are chiefly interesting as offering several instances of
the influence of submarine and surface barriers upon the dis-
tribution of temperature, the former interfering with the gradual
decrease of temperature between the surface and the bottom, the
latter resulting in a superheating of the surface-strata, which
in these seas, as well as in others similarly circumstanced, attain
a temperature not observed in the open ocean.

Tue Ararura Sea.—Following the track of H.M.S. “ Chal-
lenger,” we enter the Arafura Sea through Torres Strait. The
soundings taken between the latter and the Arrou Islands prove
that the continents of Australia and Papua are bound together

From Torres Strait to Fong-kong. 105

by a plateau about 600 miles broad and less than 50 fathoms
below the sea-surface, which extends along the parallel of lat.
to S. This plateau is separated from the Ki Islands, Timor
Laut, and Timor bya channel varying from 1000 to 2000 fathoms
in depth, which commences in the Indian Ocean, and passes close
to the eastern shores of these islands. After separating Great Ki
Island from the Arrou group, this channel bends round towards
the west, flows between Ceram in the south, Mysole and Obi
Major in the north, then, turning due north, it continues between
Celebes and Gillolo under the name of the Molucca Passage,
and enters the North Pacific near the Tulur Islands. The
“Challenger” sounded in 1200 fathoms in the Molucca Pas-
sage, and in 800 and 580 fathoms on each side of the channel
between Great Ki Island and Dobbo, Arrou Islands (Stations
191 and 1914). The “Gazelle” found 1720 and 995 fathoms
north of Ceram. Older soundings mark depths of 1700 fathoms
and 1070 fathoms off Timor.

This channel establishes what seems to be the only deep-
sea communication between the Indian Ocean and the North
Pacific, and has all the appearance of a “fault” on a gigantic
scale (it is about 1500 miles long, and assumes the shape of §),
separating the Papua-Australian plateau from the plateau
of the Indian Archipelago. It is probably swept by powerful
currents, and may have some connection with the remarkable
contrast which the naturalist discovers between the fauna and
flora of the regions east and west of it.

Tue Banpa Sea.—The soundings of the “Gazelle” and
“Challenger” of 2320 fathoms and 2800 fathoms show that this
sea, notwithstanding its restricted area, covers a depression of
considerable depth. The temperature of the water was
found to decrease from 28°.6 C. at the surface to 10° C. at
200 fathoms, to 4° C. at 600 fathoms, and to 3° C. at goo fathoms,

from which depth it remained stationary down to the bottom in
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From Hong-kong to the Admiralty Islands. 107

2800 fathoms, forming a stratum of more than two miles in
thickness of the uniform temperature of 3, C. The existence of
this stratum seems to prove that if the Banda Sea has any deep-
sea communication with the Indian Ocean—of which there are
certain indications in the deep soundings of 2005 and 2320
fathoms found by the “Gazelle” close to and north of the
island of Timor—the depth of the channel or channels which
connect the two oceans cannot exceed goo fathoms. At Station
195, between Banda and Amboina, 4400 fathoms of dredge-
rope were paid out in anticipation of a depth of 4000 fathoms,
marked in the older charts, but the depth turned out to be only
1425 fathoms (bottom temperature, 3° C.)—a further proof of how
little reliance can be placed on soundings taken with the imper-
fect appliances formerly in use.

Tue Motucca Passace.—The bottom temperature of 1°.8 C.,
registered in a depth of 1200 fathoms at Station 197, contrasts
remarkably with the temperature of the corresponding depth in
the Banda Sea; and a comparison of the isotherms of this station
with those of Station 214 and Station 215, at the entrance of the
Molucca Passage, establishes the deep-sea communication
between this passage and the North Pacific.

Tue Sea or CeLrses.—This sea forms another of those
areas of narrow superficial extent, but of great depth, so charac-
teristic of the Indian Archipelago. The several soundings of
H.M.S. “Challenger” give depths of 2150, 2600, and 2050
fathoms, with bottom temperatures respectively of 3.7 C., 3.5 C.,
and 3°C. The decrease of temperature is arrested at a depth of
800 fathoms with a temperature of 3°.7 C. The line of islands
between Celebes and Mindanao, therefore, forms an effectual
barrier against the inflow of colder water from the Pacific.

Tue Sutu Sea.—The portion of this sea in the immediate
vicinity of the east coast of Mindanao forms a deep hollow, with
depths of 2225 and 2550 fathoms, and furnishes, with the Medi-

108 Zemperature Sections Surveyed.

terranean, the most remarkable illustration of the conditions of
temperature in a nearly land-locked basin. The two soundings
of Station 202 and Station 211 were obtained at an interval of
three months, the former in October, 1874, the latter in January,
1875. On both occasions the decrease of temperature ceased at a
depth of 400 fathoms, and remained stationary at 10.2 C. from
that level down to the bottom. It will be remembered that the
temperature of the Mediterranean was found by H.M.S. “ Porcu-
pine” to remain stationary from 100 fathoms downwards.

Tue Purippins InLanp Seas.—The high bottom-tem-
peratures observed in these seas prove that, like the Sulu Sea,
they are virtually inland basins, and do not communicate with
the China Sea in the west and with the Pacific in the east by
channels deeper than 150 or 200 fathoms.

Tue Cuina Sra.—This sea also forms a nearly land-locked
basin. Its greatest depths as yet ascertained are situated in the
north-eastern portion, near the island of Luzon. The obser-
vations made at Station 205 and Station 206 are about two
months apart, which may account for the comparatively higher
temperatures of Station 205. A typhoon had swept these seas
in the interval. The decrease of temperature is arrested at a
depth of about 1000 fathoms, below which the temperature of
the water remains at 2°.4 C. down to 2100 fathoms. This low
temperature shows that the China Sea communicates with the
Pacific by channels of considerable depth, probably situated
between Formosa and Luzon.

Tue Sea or Papua.—Only the western portion of this large
basin, which extends from the Philippines to the Fiji Islands,
has as yet been explored—by the “ Challenger” in February and
March, 1875, and by the “ Gazelle” in the course of June and July
of the same year. The sections from Mindanao to the Admiralty
Islands (Plate 16), and from the latter to Station 224 (Plate 17,
Tables X. and XI.), represent the temperature-observations made

From the Admiralty [stands to Fapan. 109

in this basin by the English expedition, the results of which
show perfect agreement with those of the German expedition,
making allowance for the difference of dates.

The position of Station 215, with a depth of 2500 fathoms,
is exceptional. Placed in the centre between the Sea of Papua
in the east, the Molucca Passage in the south, the Celebes Sea
in the west, and the portion of the Pacific extending from the
Philippines to the Pelew Islands in the north, it need cause no
surprise that its isotherms should mark a considerable dis-
turbance in the distribution of temperature, due to the presence
of several currents of different temperature, direction, and origin.
They give evidence of a colder current between the surface and
100 fathoms, and of a warmer current between roo fathoms and
250 fathoms. The former may be traced to the north-west, and
is probably a current from the east coast of Mindanao, the latter
to the south-west and the Molucca Passage.

The remaining stations in this section belong to the thermal
area of the Sea of Papua.

SECTION FROM THE ADMIRALTY IsLANDS To JAPAN (Plate 17,
Table XI.).—This section may be divided, both geographically
and as regards distribution of temperature, into three parts.
The first and most southern part, from Station 220 to Station
224 (including also Stations 216-220 of the previous section,
Plate 16), forms the western extremity of the Sea of Papua, being
bounded in the south by the latter continent, and in the north by
the line of the Caroline and Pelew Islands. The second part,
from Station 224 to Station 228, traverses the eastern limits of the
sea situated between the Philippines and the Mariana or Ladrone
Islands, and between the Pelew Islands and Japan. This sea,
which fills up a deep basin separated from the rest of the Pacific
by the line of islands extending from the Caroline Islands to Japan,
has as yet received no name, and might appropriately be called
the ‘Sea of Magallanes,” after the discoverer of the Mariana

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From the Admiralty [slands to Fapan. 111

Islands and of the Philippines, and in honour of the first
European who crossed the Pacific Ocean. The principal deep-
sea communication between this basin and the 3000-fathom
area to the eastward is in the narrow sea which flows between
the Caroline Islands and the Mariana Islands, where H.M.S.
“Challenger” obtained her deepest sounding in 4575 fathoms.
The third part of this section, from Station 228 to Station 232,
embraces the northern half of the Sea of Magallanes, and is the
scene of the encounter between the North Pacific Equatorial
Current, here assuming the name of Kuro-Siwo or Japanese
Current, and the Arctic Current from the Sea of Okhotsk and
the Behring Sea.

One of the most prominent features of this section is the
extensive surface-stratum of warm water of a temperature
beeween 20 ©. and 25. (84 Fo and) 77, F.), ‘and of a thick
ness of from 70 fathoms to 100 fathoms. This stratum, which is
evidence of a vast accumulation of warm water in the western
Pacific, is seen to commence at Station 216 with a depth of 75
fathoms, increase to roo and 105 fathoms at Stations 220 and
222 in the axis of the Sea of Papua, fall to 75 fathoms and 70
fathoms in the southern part of the Sea of Magallanes, and after
gradually thinning off to 50 and 15 fathoms at Station 228 and
229, to disappear altogether. We now enter the waters of the
Arctic Current which comes to the surface at Station 231, but
from Station 234 to Station 235 we once more find ourselves in
a warm surface-stratum, the northern continuation of the North
Pacific Equatorial Current known as the Kuro-Siwo, and which is
nothing but the Gulf Stream of the North Pacific Ocean, the Sea of
Magallanes being on a larger scale the equivalent of the Gulf
of Mexico.

No two natural phenomena could present a more complete
parallelism than that which can be traced between the origin,
progress, and ultimate fate of the great thermal currents of the

112 Temperature Sections Surveyed.

North Atlantic and North Pacific Oceans. It constitutes one of
the most remarkable proofs of the uniformity of laws and con-
ditions which determine the movements of the oceanic waters
from pole to pole. The pouring in of the North Pacific
Equatorial Current through the chain of islands which
separates the Sea of Magallanes from the main basin of the
Pacific, just as the North Atlantic Equatorial Current flows
into the Caribbean Sea through the Antilles—the progress
of the Pacific current through the southern portion of the Sea
of Magallanes, and the accumulation of its waters in the
northern and more restricted portion of this sea, as we observe
the circulation of the Atlantic current through the Caribbean
Sea and the accumulation of its waters in the Gulf of Mexico—
the relief of the pressure caused by this accumulation, through
the formation in both cases of a powerful current which forces
its way through the northern end of the barrier of islands and
joins the branch of the equatorial current which has been moving
northwards outside this barrier—finally, the subdivision of both
equatorial currents, after their encounter with the polar currents,
into branches, some of which continue their course into the polar
seas, while others bend round, and, gradually cooling in contact
with the currents from the north, flow down the western coasts
of the opposite continents in order to resume once more their
course in the character of equatorial currents, form two parallel
series of occurrences, the resemblance between which is too
close to be the result of mere accident. An exception to this
comparison may be found in the return southwards of a portion
of the North Pacific Equatorial Current through the Western
Carolines and the Pelew Islands into the Sea of Papua in con-
junction with the polar under-current. It is in this latter current
that we must seek the cause of the remarkably rapid decrease of
temperature in the stratum between 100 and 200 fathoms which
forms another prominent feature of this section. The polar

From the Admiralty Islands to Fapan. rg,

current, after its encounter with the equatorial current between
Station 234 and Station 229, continues its course as an under-
current through the Sea of Magallanes. The decrease of
temperature in the stratum below 1oo fathoms, where the two
currents, one flowing south, the other north, are in) contact:
amounts to about 15° C. (27° F.) in less than a hundred fathoms.
A portion of the Arctic current passes down between the Pelew
Islands and the Philippines, and we trace its presence in the
high level of the isotherms of 5° and 2°.5 C. between Station 218
and Station 214.

A comparison of the isotherms between the latter stations
with those of the Sea of Celebes, of the Molucca Passage, the
Arafura Sea, and the observations made by the “ Gazelle”
between North-West Australia and Timor, leaves little doubt but
that the Arctic current, after sending a branch into the Sea of
Celebes, flows as an under-current through the above-described
deep channel or “fault” between the plateau of the Indian
Archipelago and the Papua-Australian plateau, for along
the whole length of this channel we find the isotherms of
iE @.pand.2.5°€. at about the same depth, the former in
an average depth of 500 fathoms, the latter in an average
depth of goo fathoms. A branch of this current probably flows
through the Straits of Manipa, past Amboina, into the Banda
Sea, and out of the latter past Timor into the Indian Ocean.
The isotherms of the China Sea show that the Arctic current
also finds its way into that basin. Another branch of the Arctic
under-current turns eastward, and, in conjunction with the southern
branch of the North Pacific Equatorial Current flowing from the
Sea of Magallanes into the Sea of Papua, is the probable cause
of the sinking of the isotherms of 5°C. and 2°.5 C. between
Station 224 and Station 218, or between the Papuan plateau
from Humboldt Bay to the Admiralty Islands and the Caroline
Islands (Plates 16 and 17, and Curve Fig. 10).

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Frrom Yokohama to Station 253. Ts

Between Station 225 and 229 we observe the usual fan-like
arrangement of the isotherms, caused by the sinking of the
heavier equatorial water through the lighter strata of the polar
current (Plates 9 and 19). The Kuro-Siwo, running at Station
234 and Station 235, at a short distance from the south coast of
Nipon, flows, like the Gulf Stream, over and between the cold
waters of the Arctic current, contending with the latter for the
alternate possession of the romantic bays and inlets of the south
coast of Nipon, Sikok, and Kiusiu. A branch of the Kuro-Siwo
penetrates through the Straits of Korea into the Sea of Japan.

The usual alternation of streaks of warm and of cold water
which characterise the scene of the meeting between equatorial
and polar currents was observed by the “ Challenger” during the
last three days of her cruise to Japan. Between midnight of
the 8th and the morning of the rith April, 1875, the ship
crossed three streaks of cold water of a surface-temperature of
17° C.,, divided from each other by warm streaks of a temperature
of 20°C. The most northern streak entered the Bay of Yoko-
hama, falling to a temperature of 13.3 C. at the latter port.
During the stay of the expedition in Japan, while the water of
the Kuro-Siwo outside ranged from 20° to 23° C., the surface-
temperature of the bays and inlets of the south coast of Nipon
varied between 15 and 17° C.

SECTION FROM YOKOHAMA TO STATION 253 (Plate 18, Table
XII.).—This section crosses the North Pacific Ocean between
the parallels of lat. 34° N. and lat. 38° N., from the coast of
Japan to the meridian of the Sandwich or Hawaiian Islands.
Its western portion exhibits the relations between the equa-
torial and the polar currents eastward of Nipon. After traversing
the belt of cold water which fringes the east and south coast of this
island, we enter, at Station 237, the Kuro-Siwo at the point where
it joins the main stream of the North Pacific Equatorial Current,
which flows outside the line of islands that separate the northern

116 Temperature Sections Surveyed.

part of the Sea of Magallanes from the North Pacific basin. Like
the Gulf Stream, the Kuro-Siwo imposes its name upon its
more powerful though less conspicuous parent. The axis of the
current is at Station 238, where it depresses the isotherm of 2°.5
C. from its average North Pacific level at 700 fathoms down to
below tooo fathoms. This axis corresponds with the axis of
the 4000-fathom channel, which, as formerly described, stretches
northward along the coast of Nipon and Yezo. The breadth of
the warm current, measured from its western limit off the
Japanese coast to beyond Station 239, is over 400 miles. At
Station 240 we find ourselves in the middle of a great polar
current which flows down between Station 239 and Station 241
in a south-westerly direction, and reduces the temperature of the
water to a depth of more than 600 fathoms. This is the same
current whose course we have been tracing through the Sea of
Magallanes, past the Pelew Islands, into the Molucca Passage,
and through the Indian Archipelago into the Indian Ocean. It
probably divides itself into two branches, one entering the Sea
of Magallanes north of the Bonin Islands, and between the
Bonin and the Mariana Islands (Stations 228-231), and con-
tinuing its south-westerly course towards the Philippines, the
other turning down outside these islands into the 3000-fathom
basin situated north of the Carolines.

The isotherms of the stations to the eastward of Station 240
indicate the existence of alternate warm and cold currents—the
former, branches of the equatorial current flowing first eastward,
then turning southward, across the parallel of lat. 4o° N.; the
latter, cold currents from the sea of Okhotsk and the Behring
Sea. There are warm currents at Stations 241, 243, 246, and
250, cold currents at Station 242, between 244 and 245, and at
Station 248. A cold current seems to flow down on each side
of the projecting north-western extremity of the Hawaiian
plateau at Station 246. From Stations 248 to 253, after cross-

Frrom Station 253 to Station 288. i Gy;

ing the meridian of long. 180°, we pass into the thermal area
of the North-Eastern Pacific.

SECTION FROM STATION 253, ALONG THE MERIDIAN oF Hono-
LULU AND TAHITI, TO STATION 288 (Plate 19, Table XIII.).—
Embracing nearly 80 degrees of latitude, and extending along a
track of considerably over 5000 nautical miles divided into 35
stations, this section, surveyed in the third year of the “ Chal-
lenger” cruise round the world, is a lasting monument of the
skill and perseverance of the officers and men of the old English
frigate.

A minute examination of the section could only lead to an
unnecessary repetition of much that has been said in connection
with the other sections. With the assistance of the sketch given
in previous chapters of the leading phenomena of oceanic circu-
lation, it will not be difficult to arrive at the principal facts
connected with the distribution of temperature in the Pacific
Ocean, viz.: The warm surface-stratum between the parallels of
lag 30; IN} and! lat 40; 5, the “cold wall” between the 35th
and goth parallels, and the gradual warming of the intermediate
strata indicated by the spreading out of the isotherms from the
equatorial belt towards the 35th parallel.

A comparison of the Atlantic section (Plate 9) with the
Pacific section (Plate 19) brings out the principal contrast
between the two oceans. While the North Atlantic basin is
considerably warmer than the South Atlantic basin, we observe
the contrary in the Pacific Ocean; or it would be more correct to
say that the South Pacific is warmer than the South Atlantic,
and the North Pacific colder than the North Atlantic, since the
two sections do not afford a fair comparison between north and
south in the two oceans. The differences observed between the
Atlantic and the Pacific are due chiefly to the great difference
between their respective areas. Owing partly to the projection
of the South American coast at Cape S. Roque into the com-

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Frrom Station 253 to Station 288. 119

paratively narrow channel between the latter cape and Cape
Palmas in Africa, and partly to the fact that the northern limit of
the south-east trades is placed for the greater part of the year
north of the equator, a considerable portion of the South Atlantic
Equatorial Current is diverted along the north-east coast of South
America into the Northern Atlantic. On the other hand, nearly
the whole of the South Pacific Equatorial Current remains on its
own side of the equator, hence the western half of the South
Pacific is much warmer than the western half of the South
Atlantic. Although the North Atlantic receives such an impor-
tant contribution from the South Atlantic, the accumulation of
warm water in its western half is much inferior to that in the
western half of the North Pacific ; yet, on account of the restricted
area of the Atlantic, its eastern half, both north and south of the
equator, receives a greater quantity of warm water in the shape
of equatorial return-currents than the eastern half of the Pacific,
and the eastern half of the former is therefore much warmer than
that of the latter.

Another remarkable phenomenon in the circulation of the
oceanic waters, and one the explanation of which has exercised
many minds, is to be found in the equatorial counter-currents
of the Atlantic, Indian, and Pacific Oceans. In all the three
oceanic basins this current is identified with the equatorial belt
of calms, and is known to flow from west to east, while the
equatorial currents on both sides of it flow from east to west.
Its presence in the belt of calms, to which it is exclusively
confined, is no doubt not accidental, and leads to the conclusion
that the absence of all permanent atmospheric currents must
be a circumstance favourable to the formation of a current with
a tendency, as in the present case, to flow in an easterly direc-
tion. At the same time, we are led to examine whether there is
anything in the temperature-conditions of this belt and of the
zones immediately adjoining which may cause the formation of a

120 Temperature Sections Surveyed.

current flowing from west to east. The zones in the immediate
vicinity of the belt of calms are occupied by the equatorial cur-
rents, the surface-stratum of which, under the influence of the
trade-winds, moves from east to west. At the same temperature,
or at nearly equal temperatures, the surface-stratum of these
currents must be of less specific gravity than the surface-water of
the belt of calms, and this difference would cause the water of
the equatorial currents to flow over at the limits of the belt of
calms where the trade-winds are no longer felt. At the same
time, this difference of specific gravity would be greatest near the
eastern end of the belt of calms, where the currents arriving
from higher latitudes turning to westward assume for the first
time the character of equatorial currents; and least towards the
western end, where the surface-water of these currents, after
flowing for some time under an equatorial sun, has already
increased in specific gravity. This circumstance would give the
water flowing over into the belt of calms a permanent tendency
to flow eastward, which, through accumulation of effect along the
whole length of the belt, would establish and maintain a per-
manent current flowing from west to east in the equatorial belt
of calms. It may be remembered that, in the absence of atmo-
spheric currents and of differences of temperature, the specific
gravity of the water is the sole arbitrator in the arrangement of
strata—a principle which, as we have seen, applies on a much
larger scale to the changes which take place in the belt of calms
of the parallel of lat. 30° N. and S., and in the polar areas of
calms.

Actual observation has established the fact of the overflowing
of the water of the equatorial currents into the belt of calms, and,
as we might expect, chiefly along the southern limit of the belt.
The counter-current, arrested by the continents which stretch
across its course, in its turn overflows north and south, and
rejoins the equatorial currents on each side of it.

From Station 288 to Magellan Straits. 121

A more simple explanation of this counter-current may be
found in the fact that the equatorial currents, as they flow on
each side of the belt of calms, remove the water from the
eastern and accumulate it at the western side of the basin, and
that the counter-current tends to restore the equilibrium thus
constantly disturbed.

SECTION FROM STATION 288 TO VALPARAISO AND MAGELLAN
Straits (Plate 20, Table XIV.).—This section may be compared
with the soundings between Tristan d’ Acunha and the Cape of
Good Hope (Plate 11). It extends for the greater part along
the parallel of lat. 4o° S. between the meridians of Pitcairn
Island and Juan Fernandez, and coincides with the limit
between the thermal areas of the South Pacific and of the
Southern Ocean. With rapidly-decreasing temperatures in the
surface-stratum between lat. 30° and 4o’ S., we arrive, at Station
288 on the goth parallel with a surface-temperature of 12°.5 C.,
and the isotherm of 10° C., at a depth of 95 fathoms. Proceeding
eastward, we observe a sudden rise of the latter isotherm from
95 fathoms to 45 fathoms at Station 288, and to 40 fathoms at
Station 290, gradually falling to 90 fathoms at Station 294,
indicating a cold surface-current from the south between Stations
289 and 292. ‘The increase of temperature further eastward
between the surface and 100 fathoms, combined with the lessen-
ing latitude of the stations, shows that we enter more and more
into the warmer water of the equatorial return-current which flows
over the plateau of Juan Fernandez (Plate 5), especially pércep-
tible at Station 296; while the sinking down of the isotherm of
2°.5 C. between Stations 292 and 296 attests the presence of a
warm under-current, which, flowing along the southern slope of
the plateau, turns southward towards the Antarctic shores of
Graham Land. Part.of this current is probably carried through
the Strait of Cape Hoorn. On the other hand, the rise of the
isotherms of 2.5, 5,-and 10 C. at Stations 297, 301, 302, and

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308, shows that on approaching the coast of Patagonia we enter
the southern branch of the Antarctic current, which, bending
round, flows southward along that coast and through the Strait
of Cape Hoorn. Its northern branch sweeps over the plateau of
Juan Fernandez, and follows the coast of South America up to
the equator (Plate 5).

The Bay of Valparaiso must present contrasts of temperature
similar to those observed in False Bay near the Cape of Good
Hope. A westerly wind would bring the warm water of the
equatorial return-current into the bay, while a southerly wind
would fill the latter with the cold water of the Antarctic current.
The latter name is perhaps not a correct designation of the great
surface-current which, flowing from west to east through the
Southern Ocean, makes the circuit of the world between the
parallels of lat. 40: and 60° S. and forms the “cold wall,’ on
encountering which all equatorial return-currents are split up
into currents running eastward along this wall, and into currents
flowing as warm under-currents into the Antarctic region.

CHAP Tee ke ay
THE BED OF THE OCEAN.

Changes in the Distribution of Land and Water—Formation of Sub-oceanic Strata—
Formation of Central Oceanic Plateaux—Formation of Areas of Elevation and of
Areas of Depression—Formation and Transformation of Continents—Forma-
tion of Mountain Ranges and Submarine Ridges. _

CHANGES IN THE DiIsTRIBUTION OF Lanp and WarTeER.—It
was mentioned in an earlier chapter that the ordinary conception
of the relative distribution of land and water over the surface of
the earth may be replaced or rather supplemented by one which
more adequately embodies the results of modern research, and
according to-which the surface of the solid earth-crust may be
considered as composed of hills and hollows, areas of elevation
and areas of depression—the former not necessarily constituting
dry land, the latter not always occupied by water. It was also
shown how the data furnished by recent sounding operations
afford additional evidence of the observation—not made for the
first time, since it has attracted the attention of every student of
comparative geography—that the principal land-masses, more or
less combined into one great area of elevation, gravitate towards
the North Pole as their common centre; while the different
oceanic basins, constituting one great area of depression,
gather round the South Pole as their centre. If this obser-
vation conveys any information beyond the familiar fact
that there is more land in the northern and more water in the
southern hemisphere, it means that the slow but unceasing
changes which take place in the distribution of land and water
obey a general tendency to accumulate land in the northern and
water in the southern hemisphere. There are numerous indica-

Fig. 13.
Diagram showing Decrease of Diamater of
Rotation, from the Equator to the Poles._

Axis of
Rotalion

Fig. 14.
Fall of | Mile in 5 Miles.

=.

Sea Surface

or a depth of 2000 fms._20. Naut. Miles from the Shore

Fall of | Mile in 20 Miles

Sca Surface

ora depth of 3000 Fms._60 Naut. Miles from the Shore.

Changes tn Distribution of Land and Water. 125

tions of a similar tendency to transfer land and water from east
to west, so that a combination of both tendencies would result
in a general movement of land from south-east to north-west,
and of water from north-east to south-west.

The investigation of the problem suggested by this general
movement of land and water, if it really exists, seems to belong
more to the domain of the astronomer than of the student of
physical geography, since the transfer of great masses of solid
and fluid matter could not, apparently, take place without affect-
ing the distribution of terrestrial gravity, the pogition of the axis
of rotation, &c. However, instead of invoking cosmic agencies
which sometimes escape the grasp of the most accomplished
mathematician, it may be possible to discover causes whose
action is more within reach of direct observation, and which
may afford a sufficient explanation of the phenomenon above
alluded to.

At the outset it appears, from a comparison of the
height of the protuberances or of the depth of the hollows
which compose the surface of the solid earth-crust with their
lateral extension, that even a slight elevation or depression of
portions of that surface, insignificant in amount when contrasted
with the diameter of our planet, may produce a considerable
change in the distribution of land and water. According to the
soundings taken in every part of the ocean, an elevation or depres-
sion amounting to roo fathoms, the eighty-thousandth part of
the earth’s diameter, would completely change the outlines of the
dry land as they are at present laid down in our charts. Great
Britain, for example, would either form part of the Continent of
Europe, or be reduced to a cluster of small islands rising out of
the sea at a great distance from the French coast, formed by
the slopes of the Ardennes, the Vosges, and the mountains of
Auvergne. It so happens that both events have occurred
in the past. The effect which such a change of level must have

126 The Bed of the Ocean.

upon oceanic and atmospheric currents, upon climate, and upon
the whole fauna and flora of the region where it takes place,
may be readily appreciated.

The average height of the dry land above the level of the
sea has been calculated to amount to less than 200 fathoms,
while the average depth of the ocean is probably over 2000
fathoms ; so that, if we deduct the mountain ranges and elevated
plateaux which largely contribute to the above average, a great
portion of the dry land must be less than 100 fathoms above the
level of the sea, A depression of 100 fathoms, while it would
cause almost all dry land to disappear—all but the most elevated
regions—would reduce the depth of the ocean by only one-
twentieth. In connection with this subject, it is necessary to guard
against an impression produced by recent discoveries of exten-
sive areas of great depth in the vicinity of the land, and encouraged
by the small scale on which the results of sounding operations
have to be presented to the eye. The comparatively rapid
increase of depth, so frequently observed beyond the 100-fathom
line, has suggested the idea that the continents of the old and
new world rise abruptly from the bottom of the sea and form
high plateaux, whose steep sides descend within a short distance
of the shore into depths of two or three and occasionally four
or five miles. It is but natural that a distance of five, ten, or
twenty miles should appear very short when compared with the
wide expanse of an oceanic basin; but it will become evident,
to any one who will take the trouble to put down on paper the
proportion between distance and depth, that a depth of 1000
fathoms, or of one mile, at a distance of five miles from the
shore, by no means forms what is generally understood by
a “steep incline,” as the angle is little over 11°. A depth
of one mile at a distance of ten miles is a comparatively rare
occurrence, and in most cases where the soundings seem to
increase with more than usual rapidity to depths of 2000 and

Changes in Distribution of Land and Water. 127

3000 fathoms, the distance from the shore at which they are
found is seldom less than forty or sixty miles—that is to say, a
descent of one mile in twenty miles, or an angle of about 3°
(Fig. 14). On measuring the inclines of several islands of
volcanic origin, such as Pico in the Azores, Ascension Island,
Marion Island, and the island of Hawaii, as these appear on
sketches made during the cruise of H.M.S. “Challenger,” the
angle is found to decrease from an average of 30° at the crater
or craters, to 15 and 10 upon the intermediate slopes, while the
final incline dips into the sea at an angle of from to’ to 6°—that
is to say, a fall of one mile in ten miles, which.a few miles from
the shore is reduced to 3°, or a fall of one mile in twenty miles.
Yet those islands have the aspect of rising abruptly from the
level of the sea, and depths of over 2000 fathoms are obtained
within a few hours’ sail from their shores.

The purpose of the above remarks is to point out that
continents are but the most elevated areas of wide and low
undulations, and that these differ in no respect from the sub-
marine plateaux discovered by recent exploring expeditions,
except in having partially risen above the surface of the ocean.
We also see that, on account of the low angle of the inclines,
a comparatively slight alteration either in the level of the
land or in the level of the sea may produce a considerable
change in the distribution of land and water, and that the
rise and fall of these undulations rarely exceed five miles in
a distance of 100 miles, and are generally much below this
proportion.

The comparatively rapid increase of depth beyond the 1o0-
fathom line was a phenomenon of sufficiently frequent occurrence
to attract the attention of those engaged in the recent sounding
operations, and can hardly be considered as accidental. It has
probably some connection with the limits of the alterations of
level which have taken place during the most recent geological

128 The Bed of the Ocean.

period, and which apparently do not range beyond the 100-
fathom line.

FORMATION OF SuB-OCEANIC STRATA.—Oscar Peschel, in his
remarkable essay on Mew Problems in Comparative Geography,
has already expressed the opinion that the continents are older
than the mountain ranges we find upon them; that the latter
have been raised up along the coast-lines of the former,
and that their elevation appears to be due to lateral pressure.
He also remarks that most of these coast-ranges are backed
on the land side by high plateaux. A study of the results of
recent deep-sea exploration will lead to the same conclusions.

If there be little doubt that the currents of the ocean are
the carriers and distributors of temperature throughout the
vast depths of the seas which cover so large a portion of the
surface of our planet, it is equally clear that water in every
shape, from the smallest stream to the great oceanic rivers, is
the principal solvent, carrier, and distributor of the solid matter
which composes the only portion of the earth-crust with which
we are acquainted. The solid particles thus held in suspense
are deposited according to their weight and bulk—the heavier
ones first and nearest to the place whence they came, whilst
the lighter ones are carried to a greater distance. Those
which are light and yet bulky remain in suspense for some
time: if lighter than water, they will never reach the bottom;
if a little heavier, they will do so only after a lapse of time,
longer or shorter according as the conditions are more or less
favourable. Chief amongst these conditions is the velocity
of the current of water which acts as the carrier of solid matter ;
the greater that velocity, the greater is the weight of the solid
particles held in suspense, and the greater is the distance
to which they are carried, and vice versa.

The matter distributed by oceanic currents is mainly com-
posed of inorganic detritus, the result of sub-aérial and sub-

Formation of Sub-oceantc Strata. 129

marine denudation, of organic remains derived from plants and
animals, and of substances held in solution, such as salts, gases,
&c. In accordance with the above-mentioned conditions, we
may expect the solid particles to form deposits varying in
quantity and quality in proportion to the distance to which they
have been carried, and to the greater or lesser velocity of the
currents which occupy the area in which they have been deposited.
- This conclusion is borne out by facts which have come to
light in the course of the recent researches into the nature and
composition of the deposits found at the bottom of the sea.

_ The samples brought up from the bottom in the tube of the
sounding apparatus reveal a marked difference between deposits
formed near the land and deposits accumulated in the more central
parts of an oceanic basin. This difference is sufficiently great
to render it possible—as soon as we shall possess a complete
analysis of the specimens already collected—to decide whether
a certain sample of the sea-bottom, the origin of which may be
doubtful, belongs to a stratum deposited in a deep sea or in a
shallow sea, near the margin or near the centre of an oceanic
basin.

It is evident that a large proportion of the detritus derived
from sub-aérial’ and submarine denudation, including all the
heavier and at the same time more voluminous particles, will be
deposited within a short distance from the margin; that the com-
position of this marginal deposit will depend upon, and vary with,
the materials which make up the surface-strata of the adjoining
land; and that the distance to which it extends from the shore
will be influenced by the presence or absence of shore-currents,
or of rivers emptying themselves into the sea. Thus the breadth
of the marginal deposits may amount to several hundred miles at
the mouth of great rivers, such as the Amazon, the Rio de larP lata
the Mississippi, &c., while it may be reduced to a few miles in places
where the shore is swept by*powerful currents. Hence, under

I 2

130 The Bed of the Ocean.

certain conditions, large submarine plateaux may be formed
in connection with the land; their rate of accumulation will be
comparatively rapid, and they will have a tendency to alter the
configuration of the basin as well as the direction of its
currents. |

The lighter and finer particles are carried to a greater
distance from the margin, and deposited in the more central
parts of the basin. Their rate of accumulation will be much
slower; so that an oceanic as well as an inland basin has a
tendency to fill up from the margin towards the centre, and
may end in being completely filled up, unless this gradual
accumulation is kept in check through the action of currents
which remove a portion of the deposits and transfer it elsewhere.
This transference is the general rule, for as the bed of a current
becomes more and more restricted, its velocity increases in the
same proportion, and with it its power to remove part of the
deposits. The distance to which the lighter and finer particles
are carried by oceanic currents before they arrive at their final
resting-place may amount to several thousand miles, and this
is probably the cause of the remarkable uniformity which has been
observed in the character and composition of the deposits formed
not only over vast areas of the same basin, but also in the
different oceanic basins, as compared with the variety which
exists in the composition of marginal deposits.

_ Hence we may infer that a stratum of a nearly uniform
character, which is found to extend over wide areas, must have
been deposited at the bottom and towards the centre of an
oceanic basin ; while strata of lesser extent, and offering a greater
variety in their composition, must have been marginal deposits.

ForMATION OF CENTRAL Oceanic PLATEAUX.—The lighter
and finer particles distributed by oceanic currents’ may be
divided, according to their origin, into inorganic and organic
particles. The former are, as a'rule, much heavier in comparison

Formation of Central Oceanic Plateaux. * 131

with their volume than the latter, and will therefore be deposited
sooner than organic particles, which can only fall to the bottom
and form strata under peculiarly favourable conditions. The
most favourable of these conditions is the absence or nearly
-complete absence of currents; and this conclusion is remarkably
confirmed by observation. A large proportion, if not by far the
largest proportion, of the particles suspended in the waters
of the ocean consists of the bodies of the myriads of animal
and plant organisms which there live and die, and no doubt
derive their sustenance from the still finer organic and inorganic
particles dissolved in the surrounding fluid. A teaspoonful of salt
water examined under the microscope reveals the presence of
hundreds and thousands of these minute organisms. Their
distribution in the ocean, no less than that of the larger animals,
depends, among other conditions, upon the nature and abun-
dance of the food they require; and hence we can distinguish
between a marginal and a central oceanic fauna, and between
a surface and a bottom fauna. Although the dredge has brought
to light sufficient proofs of the presence of animal life at great
depths—two or three miles from the surface—yet a considerable
diminution has been observed beyond these limits, ending with
an almost complete absence of living organisms as we attain a
depth of 4000 fathoms. Most of the minute organisms seem to
have their home in the upper strata, being especially abundant
at or near the surface, and their bodies reach the bottom: only
after death, and after having floated for a considerable time with
the currents. During this time they undergo a process of
decomposition which reduces them to a mere skeleton, and the
latter, being heavier in proportion to its bulk than the living
body, ultimately sinks to the bottom.

The deposit of these’ light remains of organic life can
only take place, as already mentioned, over areas of minimum
circulation, and these areas are confined to the centre of

132 The Bed of the Ocean.

oceanic basins and to what we have termed the critical
latitudes. We may therefore expect the formation of deposits
composed mainly of organic particles in the centre of oceanic
basins and in the critical latitudes, where, as is the case
with atmospheric currents, we find areas of calms. This
conclusion is in harmony with observed facts. The central
plateaux of the North Atlantic and of the South Atlantic, the
wide plateaux between the latter, the Indian and the South
Pacific Ocean, and the Southern Ocean, are all found to be
covered with a stratum composed of the remains of the minute
organisms which live in the ocean. On the contrary;
the bottom of the areas of depression consists principally of
inorganic particles in the shape of extremely fine and very
tenacious clays, varying in colour from grey to yellow and red,
and occasionally deepening to a chocolate colour.

Awaiting a more complete analysis of the specimens of
bottom brought up by the sounding-tube in the course of the
“Challenger” expedition, the marginal deposits have been des-
cribed as mud, sand, stones, rock, shells, &c., the deposits in the
areas of depression as red clay or grey ooze, and those in the areas
of elevation as globigerina ooze. The clays are composed of
very fine mineral particles mixed up with a small percentage
of organic remains. As the depth decreases, this percentage
is found to increase, until, at depths of less than 2000 fathoms,
the deposit is almost exclusively composed of the skeletons
and fragments of skeletons of the minute forms of animal life
which inhabit the ocean.

The previous arguments, which may be modified by future and
more detailed research, lead to several conclusions of some
importance to the geologist. As the discoveries made by the
expeditions on board H.M.S. “ Lightning” and “ Porcupine,”
in the area between the Fzroe Islands and Scotland, have shown
that an Arctic and a southern fauna may exist side by side |

Formation of Central Oceanic Plateaux. £38

at a distance of a few miles from each other, so the results of
the “ Challenger” expedition tend to prove that the paucity or total
absence of organic remains ina geological stratum ts no evidence of
ats relative antiquity. The difference which is observed between
the deposits found in areas of depression and those accumulated in
areas of elevation, shows that a comparatively rapid accumulation
of organic remains may take place in one portion of an oceanic
basin, contemporancously with the slower deposit of a formation
which is almost or nearly destitute of organic remains in another
portion of the same basin. This remark may be extended to
the remains of the higher forms of animal life. Some astonish-
ment was created on board the “ Challenger” that the dredge,
after having been dragged over miles and miles of the bottom
of the sea, and up and down almost every oceanic basin, should
never bring up any bones of fish or whale, or any remains of
other large animals which inhabit the sea, or whose bodies
may have been carried down to the sea; for, with the exception
of a few shark’s teeth and some ear-bones of whales, no portion
of one of the more highly organised animals was ever found in
the dredge or in the bag of the trawl, always excepting those
forms which we had learned to associate with the bottom of the
sea, and which have also been found in abundance in the strata
of former geological periods. What becomes of the wrecks
innumerable and of the bones of the multitudes who have, in the
service of their country and their race, found an honourable
grave in the depths of the sea? No portion of a ship or any
other article of human manufacture, no human bones, ever came
to the surface; and though a satisfactory explanation of this
curious fact may yet be found, it shows that we should hesitate
before accepting the absence of these remains as conclusive
evidence of the antiquity of a geological stratum, or of the
non-existence of higher organisms, including man, in former

periods.

134 The Bed of the Ocean.

FORMATION OF AREAS OF ELEVATION AND OF AREAS OF
Derpression.—Assuming the previous deductions to be correct,
we can now conceive the formation of areas of elevation and
areas of depression, in consequence of the unequal distribution
of solid matter by oceanic currents, even in the absence of pre-
existing dry land, and antecedent to all other phenomena of
elevation and depression due to volcanic or other agencies. If
we suppose the whole surface of our planet covered with
water, the more rapid accumulation of solid matter in areas
where there is little or no current, and its slow deposit in
areas where strong currents prevail, would after a time divide the
bed of the ocean into plateaux and depressions. It is remark-
able that, while the direction of the principal mountain ranges is
so frequently from north to south, the direction of the lines which
divide the great river systems of our continents is generally from
east to west, which would imply that the longitudinal axis or central
ridge of the original plateaux, previous to the rising up of the
mountain ranges, was from east to west (Plate 4a). The con-
version of submarine plateaux into dry land would be effected
by the gradual rising of the areas of elevation through con-
tinuous accumulation of solid matter, simultaneously with the
deepening of the areas of depression through the removal of
deposits by currents, whose velocity must increase as their area
becomes more restricted. It may also be the result of a diminu-
tion in the total quantity of water contained in the ocean, or of
a retreat of the ocean, for there seems to be no argument to
prove that this quantity must be constant, or that the level of
the ocean must always be exactly at the same distance from the
centre of terrestrial gravity. On the contrary, if we separate the
centre of gravity of the whole mass of oceanic waters from the
centre of gravity of the solid portion of our planet, the former
may be subject to certain fluctuations, as the latter must be
affected by changes in the arrangement of the solid earth-crust.

Areas of Elevation and Depression. als

Both may be said to move round their common centre of gravity,
which is that of the whole planet, and, in consequence, we may
conceive a gravitation of the whole mass of the ocean in one direc-
tion—for example, in favour of the southern hemisphere, which
would result in the sinking of the level of the ocean—z.c., the crea-
tion of dry land in the northern hemisphere. The great plains of
the present continents, such as the plain which stretches from
the English Channel to the coast of Siberia, and the great
North American plain, have all the appearance of having
been converted into dry land, not through the action of a
subterraneous agency which lifted them above the level of the
sea, of which action they bear little or no trace, but in con-
sequence of the retreat of the ocean; and the great rivers which
now wind their course through these plains have carved out
their bed, not through strata previously deposited by themselves
in a former geological epoch, but through strata deposited at the
bottom of former oceanic basins and great inland seas.

Or, supposing the quantity of oceanic waters to be constant,
the bed of the ocean may be either deepened or rendered more
shallow, or its total area made wider or narrower, in consequence
of submarine denudation, the formation of new seas, the accumu-
lation of fresh deposits, and the uplifting or depression of wide
areas by subterraneous forces, Any such alteration in the con-
tour of its bed, of the occurrence of which in the past and in the
present there is ample evidence, may either raise or lower the
level of its surface, and it has already been shown how pro-
foundly the distribution of land and water would be affected by
a change of level amounting to the merest fraction of the total
depth of the ocean.

FORMATION AND TRANSFORMATION OF CONTINENTS.—If we
suppose that at one time the ocean covered the whole surface of
the earth, the plateaux accumulated in consequence of the
unequal distribution of solid matter by the thermal oceanic cur-

136 The Bed of the Ocean.

rents would occupy the critical latitudes, and their direction
would be from east to west, or parallel with the equator. We
should have an equatorial plateau separated by zones of depres-
sion from plateaux occupying the parallels between lat. 30° and
50 N.and S., which again would be divided by zones of depres-
sion from the plateaux of the polar regions, and the surface of
our planet would have the appearance of being divided into
more or less parallel strips composed of alternate areas of eleva-
tion and depression.

The elevation of ridges parallel with the axis of these
plateaux, and due to what at present is termed volcanic or sub-
terraneous agency, would at once cause a change in the system
of oceanic circulation, and consequently in the distribution of
solid matter. As they rose up from the surface of the plateaux
directly in the path of the currents, the latter were compelled to
flow along the side of the ridge opposed to them, and the result
was.a denudation of the plateau on one side of the ridge, while
the accumulation of strata’ continued on the other side.
We have here a possible explanation of the fact that we
generally find an area of depression on one side of a mountain
range—z.e., that the latter forms or has formed at one time a
coast range with a high plateau on the opposite side. On one
side of the ridge we have a comparatively steep incline caused
by the denudation of the plateau, on the opposite side the low
and wide-spreading incline of the original plateau.

The continued action of the currents would ultimately result
in the cutting through at right angles of the original plateaux,
and in the formation of new plateaux following the direction of
the meridian, while the ridges subsequently raised up on their
surface would follow the same direction, stretching from
north to south. The surface of our planet would now present
the appearance of Arzmary areas of elevation running parallel
with the equator, with their ridges or mountain ranges

formation and Transformation of Continents. 137

stretching from east to west, backed up by plateaux on their
polar or equatorial slopes, according as the denudation has been
effected by equatorial or polar currents; and of secondary areas
of elevation, following the direction of the meridian, with their
mountain ranges running north and south, backed up by plateaux
on their eastern or western slopes. In the case of the secondary
areas of elevation, their direction along the meridian exposes
them to denudation on both sides by equatorial and polar cur-
rents, hence the triangular shape of the present continents with
their apex pointed towards the South Pole. Hence also the
observed transfer of land from east to west. Both equatorial
and polar currents are more powerful along the east coast than
along the west coast of the continents, and the deposit of solid
matter is in consequence least on the western side of an oceanic
basin, greater on its eastern side, and greatest in its centre.
This agrees with observed facts, for the western part of an
oceanic basin is as a rule deeper than the eastern, while the
plateaux are found in the centre.

Primary areas of elevation are exposed to denudation by
equatorial currents upon their equatorial slopes, and by polar
‘currents upon their polar slopes. The former currents being
more powerful than the latter, the plateaux predominate upon
the polar slopes, as we find it to be the case in the present con-
tinents; but the combined action of both equatorial and polar
currents ultimately tends to break through the primary areas of
elevation in the direction of the meridian, and to cut them up
into separate continents. The latter would then present a com-
bination of primary and secondary areas of elevation, with their
respective watersheds and mountain ranges running at right
angles to each other (Plate 4a).

ForMATION OF MounTAIn RANGES AND SUBMARINE RIDGES.—
The application of the previous remarks to the configuration of
the continents at present existing on the surface of the globe is

138 The Bed of the Ocean.

too obvious to require further elucidation. There remains yet
another question which the historian of our planet may be ex-
pected to answer, viz., the probable cause of the formation of
ridges or mountain ranges, and of the creation of centres of vol-
canic activity.

Starting with Humboldt’s and Sir Charles Lyell’s definition
of volcanic action as “ the influence exerted by the heated interior
of the earth on its external covering,” we are led to inquire—
What is the origin of this internal heat? The answer usually
given is, that it proceeds from a primarily heated and fluid
nucleus, to the gradual cooling of which we must attribute the
formation of the solid external covering called the earth’s crust.
Observation has proved that the temperature of the earth-crust
increases from the surface downwards, but the greatest depth at
which it has been ascertained in mines and artesian wells does
not exceed 360 fathoms (where it is found to remain constant at
75 F., or about 24° C.). On the other hand, the existence of a
heated and fluid nucleus has been shown by recent calculations
to be open to grave doubts, if not altogether impossible.

If in the absence of this cause of internal heat we proceed to
look for another, we may possibly find it in an element which
has been found invariably associated with volcanic action, and in
a cause of heat the effects of which come under daily obser-
vation. This element is the ocean, and the cause of heat,
pressure—namely, the pressure of superincumbent strata, both
fluid and solid. Pressure, as an important factor in the struc-
tural development of the earth-crust, has not escaped the atten-
tion of the geologist, but the enormous pressure which the water
contained in an oceanic basin must exert upon the bottom and
the sides of the basin—a pressure roughly calculated to amount
to one ton to the square inch for every mile of depth—has not
been sufficiently insisted upon as an adequate cause of heat in
the solid strata gradually accumulating at the bottom of the sea,

Mountain Ranges and Submarine Ridges. 139

and consequently as the primary cause of the various phenomena
which are observed in connection with geological formations,
such as stratification, cleavage, metamorphosis, and the final
melting and eruption of strata in the form of fluid or semi-fluid
matter.

We have seen that the deposits found at the bottom of the
sea are different in their composition, according to the distance
and the depth at which they are laid down. We may therefore
expect that they will be affected differently by the heat
developed under the pressure of the superincumbent ocean.
If we attribute to pressure the observed increase of about
1° C. for every 20 fathoms in sub-aérial strata, we may expect
a much greater rate of increase in strata subject to the
enormous pressure of the ocean, and we may conceive the
possibility of the existence of strata in a fluid or semi-fluid
form at various depths below the bottom of the sea. The
earth’s crust would in that case be composed of strata of
different degrees of solidity or fluidity, and the matter of the
more fluid strata would, under the continuous influence of
pressure, have a tendency to escape in a lateral direction. Now
this lateral pressure will manifest itself at the point of least
resistance—namely, upon the limit of an oceanic basin
where the vertical pressure of the superincumbent ocean ceases
altogether, or is sufficiently reduced to give way to the lateral
pressure. The result will be an upheaval of the overlying strata
along the margin of the oceanic basin or along the axis of a
submarine plateau, and the formation of a mountain range or of
a submarine ridge, both of which may or may not assume the
character of an axis of volcanic eruption.

In this manner it may be explained why areas of elevation
are older than the mountain ranges we find upon them, why
mountain ranges are thrown up along sea-coasts—an almost
certain evidence of the existence of lateral pressure exerted

140 The Bed of the Ocean.

from the centre of the oceanic basin towards its margin—
and also why the axis of a submarine plateau is generally
found to coincide with an axis of volcanic eruption and a line
of volcanic islands. In accordance with this view, we may
conclude that where there are several ranges running parallel
with the coast, the one nearest the coast will be of more recent
origin than those further inland.

Several other phenomena, the explanation of which has until
now been a matter of controversy, might be quoted in support
of the oceanic origin of the dry land, but their discussion belongs
more to the domain of geology than to that of physical
geography. It is a significant fact that the results of recent
microscopic examination of the materials which compose the
different geological formations has led to a partial revival of a
favourite theory of the early geologists—namely, the theory
of the aqueous origin of rocks in opposition to the theory
of their volcanic origin. |

As the air of the atmosphere and the water of the ocean are
distributed and renewed by a system of combined horizontal and
vertical circulation, so the solid matter which composes the
earth-crust is distributed and accumulated through the agency
of oceanic currents, and also of atmospheric currents, but chiefly
of the former, thus undergoing an unceasing process of disinteg-
ration and reformation. To borrow an expression from the
studio, water is the vefzcle in which Nature dissolves her colours ;
while the ocean with his broad brush lays down the ground-
tints of her pictures, the atmosphere with a more delicate pencil
puts in the finishing touches.

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