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HOME UNIVERSITY LIBRARY OF MODERN KNOWLEDGE

No. 76

Editors :

HERBERT FISHER, M.A., F.B.A. Prof, GILBERT MURRAY, Litt.D.,

LL.D., F.B.A. Prof. J. ARTHUR THOMSON, M.A. Prof. WILLIAM T. BREWSTER, M.A.

THE HOME UNIVEKSITY LIBKAKY OF MODEKIT KN^OWLEDGE

i6mo cloth, 50 cents net, by mail 56 cents HISTORY AND GEOGRAPHY.

Already Published

THE DAWN OF HISTORY ... By J. L. Myres

ROME By W. Warde Fowlm

THE PAPACY AND MODERN

TIMES By William Barry

MEDIEVAL EUROPE By H. W. C. Davis

THE FRENCH REVOLUTION . By Hilaire Belloc,

NAPOLEON By H. A. L. FisHEit

CANADA By A. G. Bradley

THE COLONIAL PERIOD ... By Charles M. Andrews FROM JEFFERSON TO LINCOLN By William MacDonald

THE CIVIL WAR By Frederic L. Paxson

RECONSTRUCTION AND UNION

(1865-1912) By Paul L. Ha WORTH

THE HISTORY OF ENGLAND . By A. F. Pollard HISTORY OF OUR TIME (i885-

1911) By G. P. GoocH-

POLAR EXPLORATION (with maps)By W. S. Bruce THE OPENING UP OF AFRICA By Sir H. H. Johnstok THE CIVILIZATION OF CHINA By H. A. Giles PEOPLES AND PROBLEMS OF

INDIA By Sir T. W. Holderness

A SHORT HISTORY OF WAR

AND PEACE ByG. H. Perris

MODERN GEOGRAPHY By Marion Newbigin

MASTER MARINERS By J. R. Spears

THE OCEAN By Sir John Murray

LATIN AMERICA By W. R. Shepherd

GERMANY OF TO-DAY .... By Chas. Tower

Future Issues

A SHORT HISTORY OF EUROPE By Herbert Fisher

ANCIENT GREECE By Gilbert Murray

A SHORT HISTORY OF RUSSIA By Paul Milyoukov

FRANCE OF TO-DAY By M. Albert Thomas

THE GROWTH OF EUROPE . . By G. A. J. Cole

THE REFORMATION By Principal Lindsay

PREHISTORIC BRITAIN .... By R. Munro

ANCIENT EGYPT By F. L. Griffith

THE ANCIENT EAST By D. G. Hogarth

MODERN TURKEY By D, G. Hogarth

THE BYZANTINE EMPIRE . . By N. H. Baynes HISTORY OF SCOTLAND . . . By R. S. Rait

LONDON By Sir Laurence Gommi

ALPINE EXPLORATION ... By Arnold Lunn

Woods Hole Oceanographic Institutioa

THE OCEAN

A GENERAL ACCOUNT OF THE SCIENCE OF THE SEA

BY

SIR JOHN MURRAY

K.C.B., F.R.S., LL.D., D.SC, PH.D.

NATURALIST H. M. S. ** CHALLENGER," 1872-1876

"MICHAEL SARS " NORTH ATLANTIC

EXPEDITION, I 910

Editor of " Challenger " Reporti ; Joint Author of " The Depths of the Ocean^'' etc.

OFFICE OF THE RESEARCH LIBRARIAN

NEW YORK . HENRY HOLT AND COMPANY

LONDON WILLIAMS AND NORGATE

DEDICATED

TO MY ASSISTANTS IN THE "CHALLENGER OFFICE" DURING THE PAST THIRTY- SEVEN YEARS

CONTENTS

OHAFTIR PAGE

I HiSTOBiCAL Notes : Methods and Insteu-

MENTs OF Deep -Sea Research . . 7

II The Depth of the Ocean ... 23

in The Waters of the Ocean: Sauntty,

Gases

45

IV

The Waters OP THE Ocean : Temperature 65

V

The Waters op the Ocean: Compressi BiLiTY, Pressure, Colour, Viscosity Penetration of Light, Tides, Waves Seiches

.' 94

VI

Oceanic Circulation , . .

, 116

vn

Life in the Ocean : Plants .

, 133

VIII

Life in the Ocean : Animals

157

IX

Marine Deposits

194

X

The Geospheres

226

Glossary

244

Conversion of Measures: Tables

249

Bibliography

250

Index

253

Explanation of the Plates .

255

Plates: Instruments, Ocean MapSj

Organisms, etc.

THE OCEAN

CHAPTER I

HISTORICAL NOTES : METHODS AND INSTRU- MENTS OF DEEP-SEA RESEARCH

Historical. Many of the phenomena exhibited at the surface of the sea were regarded with terror by primitive man, and poets have sung the praises of that hero who first shaped a hollow canoe out of a fallen tree and thus initiated shipbuilding and the navigation of the open ocean.

The early Greeks had a practical knowledge only of the enclosed Mediterranean, which they called Thalassa, but they had also some knowledge of what was called the great River of the Ocean beyond the Pillars of Hercules, as well as of the Arabian Gulf, which was called the Erythraean Sea. They are said to have derived their information concerning this great outer ocean from the Phoenicians. Necho, an early Egyptian king, is reported to have ordered his Phoenician sailors to sail down the east coast of Africa, 7

8 THE OCEAN

and to return to the Mediterranean by the Atlantic. Whether this voyage was accom- phshed or not, the Phoenicians appear to have reached the southern hemisphere, for they reported that at their most southerly point they had the sun on their right hand a statement that could hardly have been invented, and was of course true if they rounded Africa.

It was a great event in the history of oceanography and of the world when, in the fourth century before our era, the Greek, Pytheas, burst into the Atlantic with his ships, and sailed as far north as the coasts of Great Britain. It was a similarly great event when Hippalus, about the first century before our era, discovered the monsoon winds of the Indian Ocean, for after that coast routes were abandoned, and voyages of six months' duration across the open ocean were made to the coasts of India.

The thirty years between 1492 and 1522 are ever memorable for the great advance made in our knowledge of the surface of the earth. Within this period Columbus sailed across the Atlantic to America, Da Gama rounded the Cape of Good Hope and reached India, and the survivors of Magellan's expedition in one of his ships completed the first circum- navigation of the globe a whole hemisphere

METHODS OF RESEARCH 9

was added to the charts of the known world almost at a single bound.

It is interesting to note that Magellan, when crossing the Pacific in 1521, attempted to sound the open ocean ; his short line failed to reach the bottom, and he naively concluded that he had discovered the deepest part of the ocean.

The early voyages in the sixteenth and seventeenth centuries led to a vast increase of knowledge of the superficial extent of the various oceans, their currents, tides, winds, temperature, and salinity. True deep-sea soundings were taken by Captain Cook during the second half of the eighteenth century, by Captain Phipps in the Arctic in 1773, by Sir John Ross in the Arctic in 1818, and by Sir James Clark Ross in the Antarctic in 1840, all these, except when in very deep water, being fairly accurate.

The proposal to lay an electric cable between Europe and America gave a great impetus to oceanographical investigations, and led to great improvements in the apparatus for taking deep-sea soundings. About 1850 Brooke introduced a method of detaching the heavy weight used to carry down the line and tube ; on striking the bottom the weight was left there, and the tube with its sample of bottom-deposit was hauled up to the

10 THE OCEAN

surface. After this time deep-sea soundings became much more frequent and more accurate.

About the year 1840 Edward Forbes, as a result of researches in the ^gean Sea, came to the conclusion that both plants and animals ceased to exist in the ocean beyond a certain depth, the zero of vegetable life being at a less depth than that of animal life. The zero of animal life he placed at about 300 fathoms, and his views were very largely applied by naturalists to the whole ocean. Investiga- tions subsequently undertaken by Michael Sars, Wyville Thomson, W. B. Carpenter and others on the Atlantic coasts of Europe showed that animal life existed at a depth of one or two miles.

These and similar researches led to the despatch of the great " Challenger " Expedi- tion by the British Government in the year 1872 for the express purpose of examining the physical and biological conditions of the great ocean basins. The " Challenger " had on board a staff of scientific observers, who during a circumnavigation of the world last- ing for three and a half years made continu- ous observations, on the depth, temperature, salinity, currents, animal and vegetable life, and deposits, at all depths throughout the great oceans. The results of this expedition

METHODS OF RESEARCH 11

were published by the British Government in fifty quarto volumes, and these have formed the starting point for all subsequent deep-sea investigations, and laid down the broad general foundations of the modern science of oceanography.^

During the past thirty years nearly every civilised nation "has sent forth expeditions to undertake deep-sea researches, and during

*The term Thalassography has been used, largely in the United States, to express the science which treats of the ocean. The term Oceanography is, however, likely to prevail. The Greeks appear to have used the word Thalassa almost exclusively for the Mediterranean, whereas the almost mythical " oceanus " of the ancients corresponds to the ocean basins of the modem geographer. In recent times I believe the word Oceanography was introduced by myself about 1880, but I find from Murray's English Dictionary that the word *' oceanographie " was used in French in 1584, but did not then survive.

The words Oceanography and Oceanology are not " mongrel " words ; on the contrary, they are both absolutely correct formations, on such analogies as geography, topography, and theology, demonology, anthro- pology, zoology. The Greek dictionary knows such a word as thalassographos, but not oceanographos. But to insist on this point would be the merest pedantry, for even now it is not of the Atlantic, Pacific, and Arctic Seas that we speak, but of the oceans bearing those names. Sutherland Black says : "By thalassography the Greek dictionary chiefly means the description of the Mediter- ranean. A very myopic pedant might raise some scruple over -graphy on the ground that a mythographer is a * writer ' of myths, and a logographer a ' writer * of prose ; but then a topographer is not a writer of places, but a describer of them ; so also with geographer,"

12 THE OCEAN

the past twelve years an International Com- mission has been engaged in the scientific exploration and study of the North Sea and Norwegian Sea. In addition marine labora- tories have been established in many parts of the world for the purpose of carrying on the systematic examination of the ocean and its inhabitants. The general result has been that all the methods and instruments used in deep-sea work have been much improved. It is impossible in this little book to follow all the developments that have taken place in this direction, but a short statement concerning the methods and instruments now generally in use may be instructive and interesting.

Methods and Instruments. In making obser- vations at the surface of the sea all the ordinary methods and instruments in use among meteorologists and physicists are available, but whenever we commence to explore the deeper waters of the ocean other methods must be adopted and other instruments invented. While functioning beneath the surface the instruments are removed from direct observation, and various contrivances must be used to control their action. The difficulties which have to be overcome add very greatly to the interest of all deep-sea investigations. Thermometers for deep-sea work must be protected from pressure on the

METHODS OF RESEARCH 13

bulb containing the mercury. A reversing thermometer may be used, but a contrivance must be arranged for reversing it after it has taken the temperature of the water at a given depth. Water-bottles must be lowered empty or open at both ends, and closed at any given depth from which a sample may be desired. It is the same with the appliances for ascer- taining the intensity and quality of sunlight at a given depth : the apparatus with the sensitive plates must be sent down closed, then opened to expose the plates, then closed again before being hauled to the surface.

A great many of these operations are accom- plished by sending messengers small metal weights down the line to release springs or to open catches. The time taken for these messengers to run down a line of two or three miles is very great, but with experience much time may be saved by letting the messengers go while the line is still running out. Another contrivance is an attached propeller, which, on being pulled a few fathoms towards the surface, revolves and releases a spring, thus closing a bottle or reversing a thermometer.

Sounding. During the " Challenger " Expe- dition some experiments were made with wire sounding lines, but fine hemp lines were always used in the regular work, and in very deep soundings the time when each

14 THE OCEAN

100-fathoms mark went over the ship's side was carefully noted. When there was a sudden change in the rate at which the line ran out, it was known that the bottom had been reached, and the depth was then recorded. Deep soundings, even in 4000 fathoms, care- fully taken in this way are believed to be correct to within 25 fathoms.

A marked advance was made when the late Lord Kelvin introduced piano wire for sounding purposes. In the Lucas sounding- machine (see Plate I.), which at the present time is the one most in use, the moment when the weight strikes the bottom is automatically indicated on the machine, and the running- out of the wire is stopped. Soundings taken with this machine are believed to be correct to within one fathom, even in deep water. A sounding in 2000 fathoms now requires about 45 minutes from start to finish, z.e., from the time the lead is let go till it is hauled on deck with a sample of the bottom-deposit, and one in 3000 fathoms about 75 minutes. The " Challenger's " deepest sounding in 4475 fathoms occupied about 2 J hours. The single-strand piano wire used for sounding is only about one-twentieth of an inch in diameter; a three-strand wire of the same diameter is also used, while a seven-strand wire with a diameter of one-six,teenth of an

METHODS OF RESEARCH 15

inch has recently come into use in the Navy. The stranded wire used with water-bottles and thermometers is about one-eighth of an inch in diameter.

Trawling and Dredging. During the " Chal- lenger " Expedition strong hempen lines were used. Sometimes a hempen line eight miles in length was out astern of the ship when trawling in 3000 fathoms. A very great advance was subsequently made when the late Alexander Agassiz introduced wire ropes, about one-third of an inch in diameter, for these operations. The use of piano wire for sounding and steel rope for trawling and dredging is now almost universal. The dredges used on board the " Challenger " (see Plate I.) were 3 to 5 feet in width, and the trawls had beams 10 to 17 feet in length, the smaller size being used in very deep water. On board the " Challenger " a trawling in 3000 fathoms used often to take from 12 to 14 hours. The trawls and dredges are now of many different designs from the smallest oyster dredge to the large otter trawl with a span of 50 feet. This latter appliance was used successfully by the " Michael Sars " Expedition of 1910 in 3000 fathoms (see Plate I.).

Tow-Nets and Vertical Nets.- For capturing the animals and plants swimming or floating at the surface and in intermediate waters

16 THE OCEAN

various forms of tow-nets have been devised. Those used during the " Challenger " and other early expeditions were simply long bags of muslin or bunting or silk, tapering from the metal hoop forming the mouth, about a foot in diameter, to the opposite closed end of the net. They were towed horizontally, with a weight fixed on the line a short distance in front of the opening, or vertically by being tied to the sounding or dredging line (see Plate I. ). Subsequently much larger nets and pelagic trawls have been employed, and many attempts have been made to construct nets and traps that could be opened and closed at any desired depth. The material of which such nets is now made is the same as that used by millers in separating the various grades of flour ; it is a fine silk, made with meshes of various sizes, the finest make having about 6000-6500 meshes to the square centimetre. The meshes are pentagonal in shape, and the web is so constructed that they are not easily distorted.

During the recent " Michael Sars " Expedi- tion various nets, both open and closing, and pelagic trawls were used with varying success, but the best results were obtained by towing simultaneously for long distances a number of pelagic appliances (sometimes as many as ten) attached to two lines at certain definite

METHODS OF RESEARCH 17

intervals, so that the approximate depth of each haul was known (see Plate I.). The results obtained were then controlled by using vertical closing nets.

Centrifuge. Recent pelagic investigations have shown that a great many marine organ- isms are so small that they pass through the meshes of even the finest silk nets. These minute organisms may be studied by centri- fuging samples of sea-water taken by the water-bottle from various depths. The " Michael Sars " used a large centrifuge driven by one of the small steam winches on board. This had six glasses, and as much as 1200 cubic centimetres of sea-water could be centrifuged at one time. It made 700 to 800 revolutions in one minute, and after eight minutes the organisms were all collected at the bottom of the glasses. The clear water was then poured away, and the deposit after being rinsed was put into a smaller glass with a tapering bottom, where it was subjected to the action of a small hand-centrifuge, such as is used by physiologists. In this way all the contents of say 300 cc. of sea-water were col- lected in one drop, which was examined in a counting chamber beneath the microscope.

Deep-Sea Thermometers. Six's maximum and minimum thermometer was one of the earlier forms used in taking deep-sea tempera-

18 THE OCEAN

tures, and as modified by Miller and Casella was the one mostly in use during the " Chal- lenger " and other expeditions of the same or an earlier period. This thermometer is satisfactory for work in the open ocean, where the temperature as a rule decreases gradually from surface to bottom. Towards the polar regions and in some enclosed seas, however, layers of different temperatures may lie one above another, and such conditions would not be disclosed by the Six form of thermo- meter ; it has, therefore, been replaced to a large extent by another form, which may be reversed, and the temperature thereby regis- tered, at any desired depth.

The reversing thermometer (see Plate I.) is sent down with the bulb lowermost ; there is a narrowing of the tube just above the bulb, and the length of the column of mercury above the constriction depends upon the temperature. On being reversed the mercury is broken off at the constriction, the bulb being now upper- most, and the colunm of mercury that was above the constriction falls down, the tempera- ture at the moment of reversal being read off in the reversed position. The reversing thermometer has been modified and improved by Negretti and Zambra and by Richter, so that at the present day it is a very efficient instrument. Occasionally an error may be

METHODS OF RESEARCH 19

introduced by the faulty formation of the constriction, which may lead to the mercury not always being broken off at the same point.

Now that the greatest possible accuracy is called for in deep-sea work, it is not unusual to send down two reversing thermometers side by side in order to obtain corroboration as to the actual temperature at the depth of reversal.

Water-bottles. Recently a reversing water- bottle has come into use, designed to carry one or two reversing thermometers, so that a temperature-reading and a water-sample are secured at one and the same time (see Plate I.)- Another method of attaining this object is by means of insulacing water- bottles, which protect the contained samples from temperature-changes while being hauled up to the surface, so that the temperature of a sample immediately on being brought on board may be supposed to indicate practically the temperature at the depth where the apparatus was closed. An instrument of this km^ is the Pettersson-Nansen insulating water-bottle.

For obtaining a water- sample from the surface an ordinary bucket may be used, and in quite shallow water a stoppered bottle may be sent down attached to the line in such

20 THE OCEAN

a way that a jerk pulls out the stopper. In deeper water a stopcock water-bottle was employed during the " Challenger " Expedi- tion for intermediate samples and a slip water-bottle for bottom samples.

Photometer. For investigating the penetra- tion of sunlight into the sea and the intensity and composition of that light at different depths, various forms of photometric appara- tus have from time to time been employed. The latest is that designed by B. Helland- Hansen (see Plate I.) and used by him success- fully during the recent cruise of the " Michael Sars " in the North Atlantic. It consists of a framework, on which two cubes, fitting one inside the other, slide up and down, the smaller cube having five openings, one on each of the four sides and one on the top, for the exposure of photographic plates with or without colour filters. The cubes are carried into the dark room, and the plates are placed in the smaller cube, which is then enclosed inside the larger cube and thus effectually protected from the light. The two cubes are suspended in the upper part of the frame, and the apparatus is lowered to the desired depth, when a messenger is sent down the line to release the smaller cube, which falls to the bottom of the frame, thus exposing the plates. After the necessary interval a second

METHODS OF RESEARCH 21

messenger is sent down to release the larger cube, which falls and covers the smaller one, thus ending the exposure. The apparatus is hauled up and the cubes taken into the dark room for the development and fixing of the plates.

Current Measurements. For measuring the velocity and direction of currents in the sea many devices have been employed. Informa- tion regarding surface currents may be obtained !rom the drift of floating objects, such as drift-bottles, wreckage, icebergs, vessels frozen in the ice of polar regions, and movements of water-masses at the surface and under it may often be traced by studying their physical and chemical properties (tem- perature, salinity, dissolved gases). For the direct measurement of undercurrents it is necessary to use a current meter of somewhat complicated construction. The latest form is that designed by V. W. Ekman.

Hydrometers. The direct determination of density may be made by means of the hydro- meter, a glass cylinder which floats in water. Densities so found are re-calculated by means of tables to a standard temperature. Owing to the uniform composition of sea-salts a definite density at a definite temperature corresponds rigidly to a definite salinity. Hence, by referring to tables the salinity of

22 THE OCEAN

sea-water can be found from its density at standard temperature.

Densimeter, For the determination of the relative densities of sea-water samples, a differential densimeter was recently intro- duced by J. J. Manley. It is somewhat compli- cated in design, and was at first intended for use in a laboratory on shore. The apparatus has been modified so as to adapt it for use on board ship, and a series of observations has been carried out by N. P. Campbell during a voyage to Ceylon on board one of the Orient mail steamers, apparently with good results.

For methods of determining salinity see Chapter III.

CHAPTER II

THE DEPTH OF THE OCEAN

Archdeacon J. H. Pratt, the mathema- tician, in discussing the pendulum investi- gations among the Himalaya mountains, appears to have believed that the great Pacific Ocean could only be explained on the assumption of " some excess of matter in the solid parts of the earth between the Pacific Ocean and the earth's centre, which retains the water in its place, otherwise the ocean would flow away to other parts of the earth." According to this view the existence of the oceans in their present positions is referred to an excess of density in the sub-oceanic portions of the globe. In one sense this is confirmed by recent geodetic and gravity observations, but in all probability the sub-oceanic heavi- ness does not extend to depths greater than thirty miles. The great centrosphere of the earth is inferred to be more or less homo- geneous in structure and composition, and also as rigid as steel, whatever the nature 23

24 THE OCEAN

and condition of the materials of which it is composed. On the other hand, it is known that the rocky crust is heterogeneous in composition to the depth of several miles.

The gravitational attraction of emerged land causes a heaping-up of ocean waters along continental shores. In consequence of this it has been stated that the waters of the Mid-Indian Ocean are lowered fully 1500 feet by the attraction of the elevated regions of the Himalayas. More recent calculations, however, show that the total deformation of the ocean's surface from that which would be assumed by a true spheroid of revolution is not likely to be more than 300 or 400 feet. Other causes, such as difference of barometric pressure, the action of winds, temperature, evaporation, precipitation of rain, and the inflow of rivers, all tend to alter the level of the ocean, the surface of which is really a very complicated one, and must at some points be further removed from the geometrical centre of the earth than at others. It is to this complicated surface of the ocean (or of the geoid) that all measurements of height and depth are referred.

Areas at different Depths. The land-surface of the globe covers about 57 millions and the water-surface about 140 millions of English square miles : in other words 29

DEPTH OF THE OCEAN 25

per cent, of the surface of the globe consists of land and 71 per cent, is covered by water. The depth-soundings which have up to the present time been taken in the ocean are ex- tremely numerous, more especially in the shal- low water close to land. It is not practicable to estimate the number of soundings in depths less than 1000 fathoms (6000 feet), but the numbers recorded from depths exceeding 1000 fathoms in the annual blue-books published by the British Admiralty between 1888 and 1910 are as follows :

1000-2000 fathoms

6000 soundings.

2000-3000

3250

3000-4000

300

4000-5000

17

Over 5000

3

Total exceeding 1000 fathoms 9570

To this must be added a very large number of soundings taken by other ships not re- corded at the Admiralty. The Prince of Monaco, the Berlin Institut fiir Meereskunde, and the writer have published maps showing practically all the deep-sea soundings known up to the present time.

From these data the estimated areas within the different zones of depth are given in the following table :

26

THE OCEAN

Depth.

Estimated

area in

English

square miles.

Percentage of total Ocean- Floor.

Between 0 and 1000 f ms. 1000 2000 2000 3000 3000 4000 4000 5000

Exceeding 5000 fathoms

22,000,000

27,000,000

81,000,000

9,800,000

195,000 1

6,000]

16 19

58

7

[less than 1]

140,000,000

100

It will be seen from an examination of this table that only 16 per cent, of the ocean- floor lies between the shore-line and 1000 fathoms (6000 feet), so that 84 per cent, lies deeper than 1000 fathoms ; again, only about 7 per cent, lies deeper than 3000 fathoms (18,000 feet), so that 77 per cent, lies between the depths of 1000 and 3000 fathoms, while more than one-half of the entire floor of the ocean (58 per cent.) is covered by depths between 2000 and 3000 fathoms. It is probable that these estimates are not far from the truth, for it is remarkable how little the contour-lines have been altered by the great number of deep soundings that have been recorded during the last ten or twelve years.

DEPTH OF THE OCEAN 27

Deeps. There is a special interest attached to the greater depths of the ocean, so the writer some years ago called all those areas where the depth exceeds 3000 fathoms (18,000 feet) " deeps," and gave them distinctive names, generally after exploring ships, navi- gators and scientific men, thus following the practice of naming mountain peaks. Fifty- seven such " deeps," based upon some 500 soundings, are now known : 32 in the Pacific, 18 in the Atlantic, 5 in the Indian Ocean, and 2 in the Southern Ocean. Their positions are shown in Plate II. They vary greatly in outline : in some cases they are very irregular or basin-like, but often they are trough or trench-shaped and relatively close to continent- al land. Some, like the Valdivia Deep in the Southern Ocean and the Murray Deep in the North Pacific, are believed to cover a large area, while a few are very small, being based on single isolated soundings. The total area covered by these deeps is altogether only about 7 per cent, of the ocean-floor. Occasionally a cone-like elevation with a depth on its summit of less than 3000 fathoms rises from the centre of a deep.

Deepest Soundings. Two deeps in the Atlantic and seven in the Pacific have depths exceeding 4000 fathoms, 46 soundings in depths greater than 4000 fathoms having

28 THE OCEAN

been recorded up to the present time. Only 8 soundings are recorded in depths exceeding 5000 fathoms (30,000 feet). Three of these occur in the Aldrich Deep in the South Pacific near the Friendly and Kermadec Islands, where the deepest cast is 5155 fathoms. Four occur in the North- West Pacific in the Challenger Deep, the deepest cast being 5269 fathoms, while the greatest known depth, of 5348 fathoms, was recorded in the Swire Deep, off Mindanao, by the German ship " Planet " in 1912. This greatest known depth in the ocean is 32,089 feet, or 409 feet more than six English miles. If the highest known moun- tain (Mount Everest in the Himalayas, 29,002 feet) could be placed in this area of the Pacific, its summit would be covered by the waters of the ocean to a depth of 3087 feet.

It is only in the Pacific that depths exceed- ing 5000 fathoms are known. The deepest sounding in the Atlantic is 4662 fathoms (north of the West Indies), and in the Indian Ocean the greatest depth is 3828 fathoms (south of Java). It is unlikely that the greatest depth is yet known, but when we remember the small number of soundings in depths greater than 5000 fathoms, and the relatively small area which the deeps of the ocean occupy, it seems most improbable that depths greater than 6000 fathoms six geo-

DEPTH OF THE OCEAN 29

graphical miles will ever be recorded in the ocean. The extreme variation in the irre- gularities of the external surface of the earth's crust, so far as is known at the present day, is 61,091 feet, or over eleven and a half English miles that is between the top of Mount Everest and the bottom of the Swire Deep.

Submarine Elevations. With the progress of our knowledge regarding the depth of the ocean the number of isolated submarine cones made known from time to time has been greatly increased. Some of these are deeply submerged, while others rear their summits so near to the surface as to become dangerous to navigation. With the excep- tion of a few situated near continental shores, they are probably all of volcanic origin, although some of them are now covered with a white mantle of calcareous ooze or coral growths. Many of the coral atolls and other oceanic islands are merely the summits of such volcanic cones. In the vicinity of these sub- marine cones the sea-floor may be very irreg- ular, and in one or two cases there is evidence of precipitous cliffs on the submerged slopes, but, generally speaking, what we know of ocean soundings leads us to suppose that the sub-oceanic slopes are extremely gentle. The steepest gradients usually occur on the continental slope in depths of 100 to 1700

30 THE OCEAN

fathoms, and on some coasts there appear to be submerged terraces, but as a rule even these submarine slopes do not exceed the gradients found on our roads and railways, while the ocean-floor throughout the abyssal region mr«y be regarded as consisting of vast gently undulating plains, interrupted here and there by elevations, some of which rise above the sea-surface to form oceanic islands, as, for instance, Bermuda in the Atlantic, Christmas Island in the Indian Ocean and many Pacific islands.

Sub-Oceanic Regions. The mean depth of the ocean is estimated at 2080 fathoms (12,480 feet) and the mean height of the land at 375 fathoms (2250 feet). If we now suppose the higher portions of the continental eleva- tions to be cut away and filled into the oceanic depressions, the depth of the ocean around the whole world would then be about 1700 fathoms (say 10,000 feet). This depth of 1700 fathoms has been called the mean sphere level. The region lying deeper than 1700 fathoms may be regarded as the great abyssal region or abyssal plain, covering an area exceeding 100 millions of square miles, or more than one-half of the earth's surface. Turning now to the continental shores, we find the continental shelf extending out from the shore-line to an average depth of 100

DEPTH OF THE OCEAN

31

fathoms, covering an area of about 10 millions of square miles; many deep submerged gullies which cross this shelf appear to be the continuations of existing river-channels, and are regarded as evidence that the con- tinental shelf represents a former extension

4500 4000 J500 3000 2S00 2000 1500 1000 500 0

soo

1000 1500

2000 2500 3000 3500 4000 4500 5000 5500

Fia.

Area Percentage

OtolOOfms. ISmiU.saail«. 8 100 -SCO 500-1000 10 I000-20OO- 4 OW-2000- _2, 57

Area Percentage

OtolOOfms. lOBMH.Kjrote- 5n

I0O-5OO - 7 - SOO-IOOO- 5 I00O-2000- 27 . 2000-100O- 81 3000- JO.- 140-

"wl etf

30Z

wz

6QX

702 so;; 9qz looz

1. Diagrammatic section showing the average contour of the lithosphere, based upon the percentages of the areas between the contour- lines above and below sea-level.

of the continents. The connecting con- tinental slope, between the 100-fathoms line and the depth of the mean sphere level, covers an area of about 30 millions of square miles, or only three times that occupied by the continental shelf, although the interval of depth is 1600 fathoms as compared with 100 fathoms. The junction between the

32 THE OCEAN

continental slope and the continental shelf has been called the continental edge, and varies in depth and in distance from the shore in different parts of the world, though on an average it may be put at about 100 fathoms and about 50 miles off coasts bordering the great ocean basins (see Fig. 1, which is based upon the percentages of the areas at definite intervals of depth below sea-level and of height above sea-level).

The foregoing paragraphs, which apply to general considerations concerning the depth of the great ocean basins as a whole, may be now supplemented by some information concerning each ocean, the enclosed seas connected therewith, and the submarine barriers separating them from the main ocean- basin.

Depth of the Atlantic Ocean. The dominant feature of the relief of the North Atlantic Ocean is a low submarine ridge, called the Dolphin Rise, running from north to south almost exactly in the central line of the basin, with a series of deeps on either side. After a break at the equator this central ridge, called the Challenger Ridge, is continued through the South Atlantic beyond lat. 50° S. The entire ridge takes the form of an open S in accordance with the trend of the coasts, and is covered by an average depth of about

DEPTH OF THE OCEAN 33

1700 fathoms. Towards the northern end the ridge widens out and the water shallows to the plateau on which the Azores are situated ; farther north it merges into the " Telegraph Plateau," which extends across nearly the whole ocean from Ireland to Newfoundland.

Another remarkable feature of the North Atlantic is the series of submerged cones or oceanic shoals made known off the north-west coast of Africa between the Canary Islands and the Spanish peninsula, of which we may mention : the " Coral Patch " in lat. 34° 57' N., long. 11°57'W., covered by 362 fathoms; the " Dacia Bank " in lat. 31° 9' N., long. 13°34'W., covered by 47 fathoms; the " Seine Bank " in lat. 33° 47' N., long. 14° 1' W., covered by 81 fathoms ; the " Concepcion Bank" in lat. 30° N. and long. 13° W., covered by 88 fathoms ; the " Josephine Bank " in lat. 37° N., long. 14° W., covered by 82 fathoms ; the " Gettysburg Bank " in lat. 36° N., long. 12° W., covered by 34 fathoms.

Precipitous slopes have been found round some of these banks : thus on the western edge of the " Coral Patch " the sinker dis- tinctly struck bottom in 550 fathoms, tumbled over and continued to sink, struck in 620 fathoms, again tumbled over, and finally found a resting-place in 835 fathoms. When o

34 THE OCEAN

it came up it had a large brownish-black streak where it had evidently struck obliquely on manganese peroxide. On the Dacia Bank a mark-buoy happened to be let go just on the edge of the bank in 175 fathoms. On trying to lift the moorings the buoy-rope carried away, and it was found to have been chafed through about 100 fathoms from the surface. The currents had evidently been rubbing it against the cliff during the two days that it was down.

The North Atlantic contains a relatively small number of islands, the principal of which are the British Isles, Newfoundland, the West Indies, the Azores, the Canaries, and the Cape Verde Islands.

The Arctic Ocean has a maximum depth of 2200 fathoms. It is cut off from the Norwegian Sea, in which the depth also exceeds 2000 fathoms, by the shallow water between the North Cape and Spitzbergen, and there are indications of a deeply submerged ridge also between Spitzbergen and Greenland. The Norwegian Sea is separated from the North Atlantic by what may be regarded as a continuous ridge running from Greenland to the British Islands plateau, of which Iceland and the Faroe Islands are emerged portions ; that part of the ridge between Shetland and Faroe has been called the Wyville Thomson

DEPTH OF THE OCEAN 35

Ridge, that between Faroe and Iceland the Faroe-Iceland Ridge, and that between Ice- land and Greenland the Iceland-Greenland Ridge.

The North Sea is shallow, all less than 100 fathoms