National Geographic Magazine/Volume 1/Number 2/Geography of the Sea
REPORT—GEOGRAPHY OF THE SEA.
By George L. Dyer.
In presenting to the National Geographic Society this first annual summary of work accomplished in the domain of the Geography of the Sea, I find it impossible satisfactorily to limit the range of subjects that may be assigned to it. The great ocean is so large a factor in the operations of Nature, that the attempt to describe one of its features speedily involves the consideration of others lying more or less in that shadowy region which may be claimed with equal force by other sections of the Society. It is to be understood, therefore, that the following account merely touches upon several of the characteristics of the oceanic waters, and is not in any sense an attempt to treat them all.
This being the first report to the Society it has been thought advisable to give a brief outline of the progress made in our knowledge of the sea since 1749, when Ellis reported depths of 650 and 891 fathoms off the north-west coast of Africa. Even at that time an apparatus was employed to lift water from different depths in order to ascertain its temperature. It does not appear that this achievement gave impetus to further efforts in this direction, for, except some comparatively small depths and a few temperatures recorded by Cook and Forster in their voyage around the world in 1772-75, and in 1773 by Phipps in the Arctic, at the close of the last century there was but little known of the physical conditions of the sea.
At the beginning of the present century, however, more activity was shown by several governments, and expeditions sent out by France, England and Russia, in various directions, began to lay the foundation of the science of Oceanography.
Exploration of little known regions was the main purpose of most of these expeditions, but attention was paid also to the observation and investigation of oceanic conditions, so that accounts of soundings, temperatures of sea water at various depths, its salinity and specific gravity, the drift of currents, etc., form part of their records.
The first to give us a glimpse of the character of the bottom at great depths was Sir John Ross, the famous Arctic explorer. While sounding in Ponds Inlet, Baffin Bay, in 1819, by means of an ingeniously constructed contrivance called a deep sea clam, he succeeded in detaching and bringing up portions of the bottom from depths as great as 1,000 fathoms. The fact that this mud contained living organisms was the first proof of life at depths where it was thought impossible for it to exist. The truth of this discovery, however, was not generally accepted, many eminent men of science on both sides of the Atlantic contending for and against it, and the question was not finally settled until long afterward, in 1860, when, by the raising of a broken telegraph cable in the Mediterranean, unimpeachable evidence of the existence of life at the greatest depths in that sea was obtained. The science, however, remained in its infancy until about 1850, when Maury originated his system of collecting observations from all parts of the globe, and by his indomitable energy aroused the interest of the whole civilized world in the investigation of the physical phenomena of the sea.
Through Maury's efforts the United States Government issued an invitation for a maritime conference, which was held in Brussels in 1853 and attended by representatives of the governments of Belgium, Denmark, France, Great Britain, Netherlands, Norway, Portugal, Russia, Sweden and the United States. The main object of the conference, to devise a uniform system of meteorological observations and records, was accomplished. According to the agreement, ships' logs were to have columns for recording observations of the following subjects: latitude, longitude, magnetic variation, direction and velocity of currents, direction and force of wind, serenity of the sky, fog, rain, snow and hail, state of the sea, specific gravity and temperature of the water at the surface and at different depths. It was also proposed that deep-sea soundings should be taken on all favorable occasions, and that all other phenomena, such as hurricanes, typhoons, tornadoes, waterspouts, whirlwinds, tide-rips, red fog, showers of dust, shooting stars, halos, rainbows, aurora borealis, meteors, etc., should be carefully described, and tidal observations made when practicable.
The practical results of this conference were great. The systematic and uniform collection of data by men of all nations is going on uninterruptedly to-day, and is furnishing the means for the solution of many of the problems relating to the Geography of the Sea.
An epoch in the progress of this science is marked by the appearance of Maury's Wind and Current Charts, his Physical Geography of the Sea, and his Sailing Directions, which contain the record of the first deep soundings taken by United States vessels; and to the United States, through Maury's efforts, belongs the honor of having inaugurated the first regular cruise for the purpose of sounding in great depths.
Under the instructions of Maury the U. S. brig Dolphin, commanded by Lieutenant Lee, and subsequently by Lieutenant Berryman, was detailed in 1851-3 to search for reported dangers in the Atlantic, and to sound regularly at intervals of 200 miles going and returning. The Dolphin was provided with Midshipman Brooke's sounding apparatus and with it succeeded in obtaining specimens of the bottom from depths of 2,000 fathoms. About the same period the U. S. ships Albany, Plymouth, Congress, John Adams, Susquehanna, St. Louis and Saranac also made soundings in various localities, and to the U. S. S. Portsmouth, in 1853, belongs the honor of having reported the first really deep-sea sounding obtained in the Pacific, 2,850 fathoms, in about 39° 40′ N., and 139° 26′ W.
The practicability of this work was thus fully demonstrated, and, although some of the earlier results, through defective appliances and lack of experience, were not entirely trustworthy, its character and success will always be a tribute to American enterprise and ingenuity.
With the advent of the submarine telegraph the investigation of the depth and configuration of the ocean bed became of vital importance, and the work of sounding for that purpose was taken up with activity; one of the first voyages in the interest of these projects was that of the U. S. S. Arctic, under the command of Lieut. O. H. Berryman, in 1856, between St. Johns, Newfoundland, and Valentia, Ireland.
The civil war naturally put a stop to these operations by United States ships. The U. S. schooner Fenimore Cooper was about the last engaged in this work, sounding in 1858-59 in the Pacific to 3,400 fathoms, and also reporting a sounding of 900 fathoms only ¾ of a mile west of Gaspar Rico Reef, in about 14°41' N. and 168° 56' E.
The work so well begun by the Americans was quickly taken up by other governments, and we find from that time to the present, the records of a large number of expeditions for diverse scientific observations in all parts of the world. Continued improvements in the appliances and instruments have made the results more precise than was possible in the earlier times, and, as the data accumulate, the bathymetric charts of the oceans are becoming more accurate. Not until this work is much further advanced, however, shall we be able to arrive at an estimate of the depths and weights of the oceans at all comparable to our knowledge of the heights and weights of the various great land masses above sea level.
Other important results of these expeditions have been the verification of many reported elevations of the ocean bed formerly considered doubtful, the discovery of new ones, and proof of the non-existence of others, which had been reported as dangers to navigation.
The Geography of the Sea reached a decidedly more advanced stage by the inception of several great scientific expeditions, of which that of the Lightning, in 1868, to the Hebrides and Faroe Islands, under the superintendence of Professors Carpenter and Wyville Thompson, was the forerunner. This was followed by the three years' cruise of the Challenger (Br.) in 1873-75, the Tuscarora (Am.) in 1874, and the Gazelle (Ger.) in 1875, by those despatched under the authority of the U. S. Coast Survey and of the U. S. Fish Commission, and others of lesser importance, sent out under the auspices of European governments, and by private individuals. All of these have contributed in an eminent degree to the progress of the science by giving us a better understanding of the physical and biological conditions of the sea at all depths. Special mention must be made of the splendid work that is being done continually by the expeditions sent out by the U. S. Fish Commission. This branch of the United States service, originally established for the investigation of the causes of the decrease in the supply of useful food fishes and of the various factors entering into that problem, in pursuance of these objects has been prosecuting a detailed inquiry, embracing deep-sea soundings and dredging, observation of temperatures at different depths, transparency, density and chemical composition of sea-water, investigation of surface and under currents, etc.; in other words, making a complete exploration of the physical, natural and economic features of the sea, besides collecting a large number of specimens of natural history. The expeditions sent out by this Commission have brought to light from the deep beds of the ocean an extraordinary variety of animal life, previously unknown to science. Few vessels have furnished a greater number of deep-sea soundings than the F. C. S. Albatross. This steamer has explored fishing grounds on the east and west coasts of the continent; and since the beginning of last year has made a cruise from the North to the South Atlantic along the east coast of South America, through Magellan Strait, and northward along the west coast to Panama and the Galapagos Islands, and thence to San Francisco and Alaska; the scenes of her latest operations have been the plateau between the Alaskan coast and Unalaska and the banks off San Diego, California.
A large share in the progressive state of the science of the Geography of the Sea must also be credited to the systematic collection of marine observations by the Hydrographic Offices and other institutions all over the world. This forms the stock from which, as I have already indicated, must be drawn, through intelligent reduction and deduction, a better knowledge of the intricate laws governing the various phenomena of the sea and air.
Oceanic Circulation.
The existence of currents in certain localities was known at a very early date, and navigators in their voyages to the new world soon discovered the Gulf Stream and other currents of the Atlantic. The first current charts were published more than two hundred years ago. Theories were soon advanced to explain the causes, one group of scientific men attributing the origin of currents to differences of level produced by an unequal distribution of atmospheric pressure over the oceans, another set connecting the tidal phenomena with the cause of ocean currents, and still another finding in the rotation of the earth a sufficient reason for their existence. The polar origin of the cold deep water found in low latitudes has long been considered probable, and has given rise to a theory of a general oceanic circulation in a vertical and horizontal direction, produced by differences of temperature and density. Recent theoretical investigations, however, seem to indicate that these causes alone are incapable of producing currents, and, to-day, the theory that the winds are mainly responsible for all current movements very largely predominates. Benjamin Franklin was probably the first who recognized in the trade winds the cause of the westerly set in the tropics, and Rennel soon after made the division of drift and stream currents. The objections which have appeared against the wind theory have been met with the reply that the present state of oceanic movements is the result of the work done by the winds in countless thousands of years.
Current phenomena is briefly summarized as follows by one of the latest authorities on the subject:
- The greater portion of the current movement of the ocean must be regarded as a drift, produced by the prevailing winds, whose mean direction and force are the measures for the mean set and velocity of the current.
- Another group of currents, and in fact a fraction of all currents, consists of compensating or supply streams, created by the necessity of replacing the drifted water in the windward portion of the drift region.
- A third group results from drifts deflected by the configuration of the coasts; these which are denominated free currents, quickly pass into compensating streams.
- The deflecting force of the rotation of the earth is considered as of subordinate importance, but may have some influence on currents that are wholly or in part compensating or free.
Late investigations of the Gulf Stream by the U. S. Coast Survey give interesting facts in regard to that notable current.
A satisfactory explanation of the cause of the stream has not yet been found, but many believe, with Franklin, that the powerful trade drift entering the Gulf of Mexico through the broad channel between Yucatan and Cuba presses the water as a strong current through Florida Strait, where the stream is turned to the northward along the coast. Since 1850 American naval officers have added greatly to our knowledge of the characteristics of this stream, particularly within the last decade, during which notable investigations have been carried on by Commanders Bartlett and Sigsbee and Lieut. Pillsbury, U. S. N., under the direction of the U. S. Coast Survey, and by Lieutenant Commander Tanner, U. S. N., in the Fish Commission steamer Albatross.
Of special importance are the valuable and interesting results in regard to tidal action in the stream obtained by Lieut. Pillsbury, U. S. N., in the Coast Survey steamer Blake, from observations begun by him in 1885 at the narrowest part of Florida Strait, between Fowey Rocks and Gun Cay (Bah.), and continued since between Rebecca Shoal and Cuba, and between Yucatan and Cape San Antonio (Cuba), and off Cape Hatteras.
During the past year Lieut. Pillsbury extended the field of operations to the passages between the islands encircling the Caribbean Sea, and in order to study the Atlantic flow outside the limits of the trade drift a station was to have been occupied about 700 miles to the north-east of Barbados; this, however, was unfortunately prevented by bad weather.
The deductions from the observations in Florida Strait showed very clearly a daily and a monthly variation in the velocity of the stream, the former having a range of 2½ knots, and reaching a maximum on the average about 9h 9m before and 3h 37m after the moon's upper transit, and the monthly variation reaching its maximum about two days after the maximum declination of the moon. The variations in this section were found greater on the western than on the eastern side of the strait, and the axis of the stream, or position of strongest surface flow, was located by Lieutenant Pillsbury 11½ miles east of Fowey Rocks, and, farther north, about 17 miles east of Jupiter Light. The average surface current at this section was 33⁄5 knots, the maximum 5¼ knots, and the minimum 1¾ knots per hour. The results also indicate that when the current is at its maximum the surface flow is faster than at any depth below it, but when at its minimum the velocity at a depth of 15 fathoms or even down to 65 fathoms is greater than at the surface, and that there is at times a current running south along the bottom in all parts of the stream except on the extreme eastern side.
The results of the investigations in 1887 and 1888 have not yet been published, but from information kindly furnished by the authorities of the Coast Survey, I am able to give a brief outline of the more prominent facts ascertained.
In the section between Rebecca Shoal and Cuba the daily variation in velocity was found as prominent as in Florida Strait, the mean time of eight maxima corresponding to 9h 18m before, and that of three maxima to 3h 25m after the moon's transit. The axis of the stream in this section was found near the center of the current prism, and the flow was easterly and inclined on either side toward the axis. The axis seemed to occupy a higher level than other parts of the stream, and this appears to be borne out by the fact that about half the number of the current bottles thrown out in Florida Strait on the west side of the axis were recovered along the east coast of Florida, while of those thrown out east of the axis not a single one was heard from. As a rule it was found that the stronger the current the more constant the direction and the deeper the stratum. Remarkable fluctuations in the flow near the axis were noted, the velocity increasing sometimes one knot in ten or fifteen minutes, and then as suddenly decreasing again. Lieutenant Pillsbury attributes this, however, to a serpentine movement of the maximum flow, which would sometimes strike the station occupied by the Blake. The edge of the stream was found at about 30 miles south of Rebecca Shoal light-house.
Between Yucatan and Cape San Antonio the stream was found flowing about north, and the line of maximum velocity corresponds on the average to 10h before and to 2h 20m after the moon's transit. The excessive variations were like those in Florida Strait, on the west side of the stream, and the maximum velocity of 6¼ knots was found about 5 miles off the 100-fathom line of Yucatan Bank. The eastern edge of the stream lies about 20 miles west of Cape San Antonio, and between this edge and the island, eddy currents exist. At the time the easternmost station in this section was first occupied, the declination of the moon was low and the set of the surface current north-easterly. At a high south declination of the moon the surface current was found south-easterly in direction, and east or south-east below the surface. The normal flow below the surface was in each case from the Gulf into the Caribbean Sea, and this makes it probable that the station was situated inshore of the average limit of the stream. On Cape San Antonio Bank the currents are tidal, flood running northward and ebb southward. On the Yucatan Bank the currents were also tidal, but as the edge of the bank is approached the stronger flow of the Gulf Stream predominates. The monthly variation in velocity, which was found clearly defined at the first two sections occupied, appeared at this section to be obliterated by anomalies not existing at the former.
Off Cape Hatteras the Blake accomplished the remarkable feat of remaining at anchor in 1,852 fathoms, and this with a surface current of over 4 knots. Two stations were occupied, and similar variations in velocity were observed as at the other stations. The notable feature at this station was the discovery of tidal action beneath the Gulf Stream, the currents at 200 fathoms depth changing their direction very regularly, the average current flowing about S. S. E. ½ E. for 7 hours and N. N. W. ½ W. for a little over 5 hours.
The first section investigated in 1888 was in the equatorial drift between Tobago and Barbados, where seven stations were occupied. The axis of the stream was found west of the middle, or nearer the South American shore, and the average direction was towards the north. At none of the stations did the current set in the direction of the wind, although the trades were blowing at all times with a force of from 2 to 7. The daily variation was also here very pronounced, the average time of maximum flow occurring about 5h 56m after the moon's transit. At 65 and 130 fathoms depth the current, at three of the stations occupied, was north-westerly; at one south-easterly. The velocity at 130 fathoms was greater than at 65 fathoms, and greater at the surface than at 15 and 30 fathoms.
At all of the three stations between Grenada and Trinidad tidal action was observed, with deflections due to local influences. The passage between Santa Lucia and St. Vincent appears to be in the line of the equatorial stream. At each of the five stations in this passage tidal action was pronounced, the currents setting in and out of the Caribbean Sea at some depth. The daily variation in this passage reaches a maximum at about 6h 3m after the moon's transit, and a minimum when the moon is on the meridian. The currents entering the Caribbean Sea through this passage are but 100 fathoms in depth, but there is probably an almost equal volume flowing out below that depth.
Between the Windward Islands the currents flow generally westward, but tidal action is everywhere apparent.
To the east of Desirade the currents at all observed depths have a northerly direction, fluctuating between about N. E. by E. to N. W. by N.
In the eastern part of the Anegada Passage the surface current flows into the Caribbean Sea in directions varying between S. S. W. and S. E., but the submarine current down to 130 fathoms flows in a direction lying between north and east.
In the more western part of the passage the currents are more complex, apparently on account of the greater variations in depth in the vicinity of the station occupied.
In the Mona Passage no regular currents were perceptible. Between Mona and Puerto Rico the currents observed set out of the Caribbean Sea, varying in direction from about W. by N. Geography of the Sea. 145 to E. N. E., except at 65 fathoms depth, where there appeared to be an inward flow. On the western side of the passage, near Santo Domingo, the direction of the currents was between S. S. E. and S. W. by W. But few observations could be taken on account of unfavorable weather.
In the Windward Passage, on the western side the currents from the surface down to 130 fathoms set in the directions lying in the S. E. quadrant, and at 200 fathoms the direction changed to W. by S. On the eastern side the surface current varied between E. N. E. and E. S. E., with about ½ knot velocity. Variations in the direction similar in extent characterized also the subsurface currents in the middle and on the eastern side of the passage.
The average of the observations at these three stations gives but a small volume of water passing in either direction.
In the old Bahama Channel, at the station north of Cayo Romano (island off the north coast of Cuba) the currents at and near the surface set south of east; at 65 fathoms, however, the direction varies from about N. W. to E. The deeper current of great volume flowed continually to the north of west with a velocity of over 1½ knots at depths of 130 and 200 fathoms.
Outside the Bahamas, to the north of Great Abaco, a slight current flows about N. W. on the surface and down to 30 fathoms; at 65 fathoms depth the direction changes to a point more westerly, and at 130 fathoms to a point more easterly than the set of the surface current. The maximum in the daily variation at this station occurs about 12h after the moon's transit.
The observations so far as completed by Lieutenant Pillsbury furnish the most valuable data we have at present concerning the Gulf Stream, and it is hoped that further investigation and the analytical treatment of these observations will clearly develop the dynamic laws involved and lead us to a correct theory of current phenomena in general.
Tidal Phenomena.
The causes for many of the inequalities in the tidal elements observed at different places have not yet been satisfactorily explained. The phenomena are dependent on many purely terrestrial conditions. While we are able to ascertain with tolerable accuracy from certain constants, derived from observation, the times and heights of the tides, the problem to compute theoretically the tides of an ideal ocean of known depth and configuration remains still unsolved. According to Ferrel our present knowledge of tidal phenomena is comparable to that possessed 2,000 years ago of the science of astronomy.
Temperature of the Sea.
The temperature of sea water had already been observed by Ellis, in 1749, in the Atlantic, and subsequent expeditions have furnished a great number of temperature observations in various seas and for various depths. The diversity of instruments and of methods employed by the earlier observers, and the faulty methods of recording, have made the uniform reduction of many of these observations difficult or impossible. The most complete and valuable collection of these older observations up to 1868, with an account of the instruments and methods used by each observer, was published by Prestwich, in 1876, in the Philosophical Transactions, Vol. 165.
With the advent of the great scientific expeditions, which were supplied with modern and refined instruments, our knowledge of the thermal conditions of the sea has progressed immensely, and we are now able to construct charts of all the oceans, showing the distribution of the isotherms with considerable accuracy.
The annual average surface temperature has been found higher in the Indian Ocean than in either the Atlantic or Pacific; the North Atlantic is slightly warmer than the North Pacific, but the South Pacific is warmer than the South Atlantic; this holds generally good also for the temperatures between surface and bottom.
The temperature generally decreases more or less rapidly from the surface down to about 500 fathoms, at which depth it is quite uniformly between 39° and 40° F. From that depth it decreases slowly towards the bottom: in the Polar seas to between 27° and 28° F.; in the middle and higher latitudes of the northern hemisphere and at depths of 2,000 to 3,000 fathoms, to between 34° and 36° F.; at the equator and in southern latitudes it remains in the neighborhood of 32° F.
The low temperatures at the bottom are thought to be due to a steady but slow circulation of water from the Polar seas towards the equator, and, where the circulation is most free and unobstructed, as in the South Atlantic, South Pacific and Indian Ocean, the bottom temperature is slightly lower than in the North Atlantic and North Pacific, both of which are connected with the Polar Sea by comparatively narrow and hallow straits.
The theory of this circulation from the Polar seas is greatly strengthened by the facts appearing from the investigation of the bathymetric isotherms in inclosed seas, i. e., seas which are separated from the deep oceans by submarine barriers. In such seas the temperature decreases slowly from the surface down to the depth of the barrier, and from there on remains constant to the bottom.
The influence of currents on the surface temperature is very marked, cold currents bending the isothermal lines towards the equator, and warm currents bending them towards the poles. The seasonal changes in surface temperatures are considerable, being the least in the tropical zones.
In the Atlantic Ocean the maximum surface temperature lies near the coast of South America, between Para and Cayenne, and another maximum occurs near the west coast of Africa, between Freetown and Cape Coast Castle.
The Pacific Ocean shows the peculiarity that the surface temperatures on the western side are lower than those on the eastern side. Between 45° N. and 45° S. the temperature does not fall below 50°, but between those parallels and the poles it remains most always below that figure.
The warmest water is found in the Red Sea where the surface temperature has been recorded as high as 90°. North of the equator the mean annual temperature is considerably above 80°, but south of it, to about the parallel of 25°, it varies from 80° to 70°.
Chemical Composition, Salinity and Density of Sea Water.
In this branch of inquiry great progress has been made, and sea water is now known to contain at least 32 elementary bodies. Its chief constituents are found to consist of the chlorides and sulphates of sodium, magnesium, potassium and calcium. It also contains air and carbonic acid.
The salinity and density of sea water have been investigated very thoroughly, particularly in the Atlantic. As the salinity of the sea water is an index of its density, changes in the former naturally affect the latter. The salinity has been found generally to decrease in the neighborhood of coasts, where rivers discharge their water into the sea, and it is a maximum in the trade zones, and a minimum in the equatorial rain belt. The salinity is affected by the degree of evaporation and by the frequency of rainfall, and is now recognized as an important factor in the biologic conditions of the sea.
Of the three great oceans, the Atlantic, with a salinity of 3.69 per cent., shows a slight preponderance over that of the Pacific and Indian Ocean, whose average salinity is 3.68 and 3.67, respectively.
In the trade belts the great evaporation augments the salinity, and hence, also, the density, and in the polar zones the formation of ice brings about the same result, though in a lesser degree. In the equatorial calm region the frequent rainfall diminishes salinity and density through the dilution of the salt water. Density and salinity are thus in a certain degree subject to seasonal changes.
In the Atlantic the density increases in general from the higher latitudes towards the equator, but the maxima are separated by a zone of lesser density. The maximum in the North Atlantic ocean is found between the Azores, the Canaries and the Cape Verde Islands, and the minimum between the equator and 15° N.
In the South Atlantic two maxima occur, one to the north of Trinidad, and the other near St. Helena and between that island and Ascension.
Taking pure water at 4° C. for unity, the maximum density in the Atlantic is 1.0275 and in the Pacific, 1.0270.
In the North Pacific the maximum density occurs between 30° and 31° N., and the minimum in about 7½° N., in the equatorial counter current, where it was found as low as 1.02485.
In the South Pacific, which has a slightly greater density than the North Pacific, the maximum has been found in the vicinity of the Society Islands.
The density of the waters of the Indian Ocean is not yet as well known as that of the Atlantic and Pacific, but the results ascertained indicate a lesser density in its northern part, with a maximum in the region between 20° and 36° S. and long. 60° to 80° E.
In the vicinity of Java and Sumatra, probably on account of the extreme humidity of the atmosphere and of frequent rainfall, the density has been found as low as 1.0250.
In regard to the density of the water at various depths, it has been ascertained that as a general rule it decreases from the surface down to about 1,000 fathoms, after which it increases again slowly to the bottom. In the equatorial calm regions, however, where the heavy rains dilute the surface water, the density decreases from the surface down to between 50 and 100 fathoms, after which it follows the law found for other parts of the ocean. The bottom densities of the South Atlantic and Pacific have been found about alike, varying only from 1.02570 to 1.02590; those of the North Atlantic, however, show a greater value, varying from 1.02616 to 1.02632.
Greatest Depths of the Oceans.
Atlantic.―Rejecting some of the earliest soundings as trustworthy, the greatest known depth in the North Atlantic is to the north of the island of Puerto Rico, in about latitude 19° 39′ N., longitude 66° 26' W., found by the C. S. S. Blake, Lieut. Commander Brownson, U. S. N., in 1882-83, 4,561 fathoms.
The deepest known spot in the South Atlantic is 3,284 fathoms, in about latitude 19° 55' S., longitude 24° 50' W., sounded by the U. S. S. Essex, Commander Schley, in 1878.
The general run of the soundings indicates that greater depressions exist nearer the western than in the eastern or middle part of the Atlantic, North and South.
Pacific.―In the North Pacific the greatest depression has been found by the U. S. S. Tuscarora, Commander Geo. E. Belknap, U. S. N., in 1874, 4,655 fathoms, in latitude 44° 55' N., longitude 152° 26' E. The next deepest sounding in the North Pacific was located by the Challenger in 1875, 4,475 fathoms, in latitude 11° 24' N., longitude 143° 16' E. As in the Atlantic, the greater depths appear to exist in the western part and particularly off the coasts of Japan.
In the South Pacific the greatest depths were supposed, up to a recent period, to be in the eastern part. Within the last two years, however, the British surveying vessel Egeria has discovered greater depressions in the western part of the South Pacific, one spot sounding 4,430 fathoms in latitude 24° 37' S., longitude 175° 08' W., and another, 12 miles farther south, 4,298 fathoms.
Indian Ocean.―In this ocean the greatest depths appear to exist to the north and west of the Australian continent, where there are more than 3,000 fathoms in a number of widely separated spots, indicating a depressed area of considerable extent.
In the most southerly part of the Indian Ocean, or rather in the Antarctic region, the Challenger obtained, in 1874, a maximum depth of 1,673 fathoms, in latitude 65° 42' S., longitude 79° 49′ E.
Arctic Ocean.―The greatest depth was sounded by the Sofia in 1868, 2,650 fathoms, in latitude 78° 05' N., longitude 2° 30' W. In the minor seas the maximum depths so far as ascertained are:
Caribbean Sea . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . |
3,452 fms., south of Great Cayman. | |
Gulf of Mexico . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . |
2,119 fms.“ (Sigsbee Deep). | |
Mediterranean . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . |
2,170 fms.“ | |
North Sea . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . |
375 fms.“ | |
Baltic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . |
178 fms.“ | |
China Sea . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . |
2,100 fms.“ | |
Coral Sea . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . |
2,650 fms.“ | |
Sulu Sea . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . |
2,550 fms.“ | |
Celebes Sea . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . |
2,600 fms.“ | |
Banda Sea . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . |
2,800 fms.“ | |
January, 1889.