Popular Science Monthly/Volume 43/May 1893/Growth of our Knowledge of the Deep Sea
|GROWTH OF OUR KNOWLEDGE OF THE DEEP SEA.|
BEFORE the time of the project for the Atlantic telegraph cable in 1854, there seemed to be no practical value attached to a knowledge of the depths of the sea, and, beyond a few doubtful results obtained for purely scientific purposes, nothing was clearly known of bathymetry, or of the geology of the sea bottom. The advent of submarine cables gave rise to the necessity for an accurate knowledge of the bed of the ocean where they were laid, and lent a stimulus to all forms of deep-sea investigation. But although our extensive and accurate knowledge of the deep sea is of so late an origin, the beginnings of deep-sea research date far back into antiquity. The ancients can not be said to have had any definite conceptions of the deep sea. Experienced mariners, like the Phœnicians and Carthaginians, must necessarily have possessed some knowledge of the depths of the waters with which they were familiar, but this knowledge, whatever its extent, has now passed away. To the writings of Aristotle, who lived during the fourth century b. c., are credited the first bathymetric data. He states that the Black Sea has whirlpools so deep that the lead has never reached the bottom; that the Black Sea is deeper than the Sea of Azov, that the Ægean is deeper than the Black Sea, and that the Tyrrhenian and Sardinian Seas are deeper than all the others. The first record of a deep-sea sounding should be credited to Posidonius, who stated, about a century b. c., that the sea about Sardinia had been sounded to a depth of one thousand fathoms. No account is given of the manner in which the sounding was taken, and we have no information as to the methods employed by the ancients in these bathymetric measurements.
The opinions of the learned with respect to the greatest depth of the sea, in the first and second centuries a. d., may be gleaned from the writings of Plutarch and Cleomedes, the first of whom says, "The geometers think that no mountain exceeds ten stadia [about one geographic mile] in height, and no sea ten stadia in depth." And the second: "Those who doubt the sphericity of the earth on account of the hollows of the sea and the elevation of the mountains, are mistaken. There does not, in fact, exist a mountain higher than fifteen stadia, and that is also the depth of the ocean."
There was no important addition to our knowledge of the deep sea during the middle ages, and no definite attempt to provide effective means for deep-sea sounding appears to have been made until Nicolaus Causanus, who lived in the first half of the fifteenth century, invented an apparatus consisting of a hollow sphere, to which a weight was attached by means of a hook, intended to carry the sphere down through the water with a certain velocity. On touching the ground the weight became detached and the sphere ascended alone. The depth was calculated from the time the sphere was under water. This apparatus was afterward modified by Plücher and Alberti, and, in the seventeenth century, by Hooke, who substituted a piece of light wood well varnished over for the hollow sphere. Hooke's instrument was no doubt fairly accurate in shallow water, but useless in great depths, where the enormous pressure waterlogged the wood and, by materially increasing its density, greatly diminished the speed with which it rose from the bottom. When used in currents the float was carried away and the record lost.
During the period when the voyages of Columbus, Vasco da Gama, and Magellan added a hemisphere to the chart of the world and forever established the fundamental principles of all scientific geography, navigators had sounding lines of one hundred and two hundred fathoms in length, and, although they eagerly studied the oceanic phenomena revealed at the surface, the deep sea did not engage their attention. Kircher, in his Mundus Subterraneus, gives the ideas as to the depths of the sea that were accepted in the first half of the seventeenth century, stating that "in the same manner as the highest mountains are grouped in the center of the land, so also should the greatest depths be found in the middle of the largest oceans; near the coasts with but slight elevations the depth will gradually diminish toward the shore. I say coasts with but slight elevations, for, if the shores are surrounded by high rocks, then greater depths are found. This is proved by experience on the shores of Norway, Iceland, and the islands of Flanders."
Several soundings were taken in deep water during the eighteenth century, but they were not of much value. The first at all reliable were made by Sir John Ross during his well-known arctic expedition in 1818. He brought up six pounds of mud from 1,050 fathoms in Baffin Bay, and obtained correct soundings in 1,000 fathoms in Possession Bay, finding worms and other animals in the mud procured. Sir James Clark Ross, during his antarctic expedition from 1839 to 1843, obtained satisfactory soundings of 2,425 and 2,677 fathoms in the South Atlantic, with a hempen cord. He also dredged successfully in depths of 400 fathoms.
Meanwhile, about the middle of the eighteenth century, the first definite ideas about the formation of the bottom soil began to be advanced, although there had been speculations on the formation of alluvial layers since the time of Herodotus. In 1725 Marsilli made a few observations on the bathymetric knowledge then possessed concerning the nature of the bottom of the sea. He admitted that the basin of the sea was excavated "at the time of the creation out of the same stone which we see in the strata of the earth, with the same interstices of clay to bind them together," and pointed out that we should not judge of the nature of the bottom of the basins by the materials which seamen bring up in their soundings. The dredgings almost always indicate a muddy bottom, and very rarely a rocky one, because the latter is covered with slime, sand, and sandy, earthy, and calcareous concretions, and organic matter. These substances, he said, conceal the real bottom of the sea, and have been brought there by the action of the water. Lastly, by way of explanation, he compared the bed of the sea to the inside of an old wine cask, which seems to be made of dregs of tartar although it is really of wood.
Donati's studies on the bottom of the Adriatic Sea led him to announce, about the middle of the eighteenth century, that it is hardly different from the surface of the land, and is but a prolongation of the superposed strata in the neighboring continent, the strata themselves being in the same order. The bottom of this sea is, according to him, covered with a layer formed by crustaceans, testaceans, and polyps, mixed with sand, and to a great extent petrified. This crust may be seven or eight feet deep, and he attributed to this deposit, bound together with the remains of organisms and sedimentary mineral matter, the rising of the bottom of the sea, and the encroachment of the water on the coasts.
In 1836 Ehrenberg produced the first of a long series of publications relating to microscopic organisms which distinguished him as a naturalist of rare sagacity. He devoted the whole of his life to the study of microscopic organisms, to the examination of materials brought up from deep-sea soundings, and to all questions appertaining to the sea. Having discovered that the siliceous strata known as tripoli, found in various parts of the globe, are but accumulations of the skeletons of diatoms, sponges, and radiolaria, and having found living diatoms and radiolaria on the surface of the Baltic of the same species as those found in the Tertiary deposits of Sicily, and having shown that in the diatom layers of Bilin in Bohemia the siliceous deposit had, under the influence of infiltrated water, been transformed into compact opaline masses, he concluded that rocks like those which play so important a part in the terrestrial crust are still being formed on the bottom of the sea.
The investigation of the distribution of marine animals according to the depths of the sea may be said to have commenced in 1840 with Forbes's studies in the Mediterranean. He maintained that the dredgings showed the existence of distinct regions at successive depths, having each a special association of species; and remarks that the species found at the greatest depths are also found on the coast of England—concluding, therefore, that such species have a wider geographical distribution. He divided the whole range of depth occupied by marine animals into eight zones, in which animal life gradually diminished with increase of depth, until a zero was reached at about three hundred fathoms. He also supposed that plants, like animals, disappeared at a certain depth, the zero of vegetable life being at a less depth than that of animal life.
It has already been mentioned that probably the first reliable deep-sea soundings ever made were by Sir John Ross in 1818. To him is due the invention of the so-called deep-sea clam, by means of which specimens of the bottom were for the first time brought up from great depths in any quantity. This instrument was in the form of a pair of spoon-forceps, kept apart while descending, but closed by a falling weight on striking the bottom. Two separate casts were usually made, one to ascertain the depth and the other to bring up a specimen of the bottom soil.
For the development of accurate knowledge of the depths of the sea the world will ever be indebted to the genius of Midshipman Brooke, of the United States Navy, who made the first great improvement in deep-sea sounding in 1854 by inventing a machine in which, applying Causanus's idea of disengaging a weight attached to the sounding line, the sinker was detached on striking the bottom and left behind when the tube was drawn up. The arrangement of the parts is shown in the accompanying figure. When the tube B strikes the bottom, the lines A A slack and allow the arms C C to be pulled down by the weight D. When these arms have reached the positions indicated by the dotted lines, the slings supporting the weight have slipped off, and the tube can be hauled up, bringing within it a specimen of the bottom. This implement has been improved from time to time by various officers of our own and foreign navies by changing the manner of slinging and detaching the sinker, and by adding valves to the upper and lower ends of the tube to prevent the specimen from being washed out during the rapid ascent which has been rendered possible by the use of wire sounding line and steam hoisting engines; but in all the essential features it is the same as the most successful modern sounding apparatus. The impulse given to deep-sea sounding by Brooke was seconded by the successful adaptation of pianoforte wire to use as a sounding line, in 1872, by Sir William Thomson; and within recent years soundings have been taken far and wide in all the seas by national vessels during their cruises, by vessels engaged in laying submarine cables, and by various specially organized expeditions, among which that known as the Challenger Expedition, sent out by the Government of Great Britain during the period from 1873 to 1876, stands pre-eminent. As a result of this work many of the questions which perplexed the naturalists of the middle of the present century have now been cleared away.
Many of the specimens of the bottom that were brought up in the early days of deep-sea sounding were studied through the microscopes of Ehrenberg, of Berlin, and Bailey, of West Point. Maury, who believed that there are no currents and no life at the bottom of the sea, wrote: "They all tell the same story. They teach us that the quiet of the grave reigns everywhere in the profound depths of the ocean; that the repose there is beyond the reach of wind; it is so perfect that none of the powers of earth, save only the earthquake and volcano can disturb it. The specimens of deep-sea soundings are as pure and as free from the sand of the sea as the snowflake that falls when it is calm upon the lea is from the dust of the earth. Indeed, these soundings suggest the idea that the sea, like the snow cloud with its flakes in a calm, is always letting fall upon its bed showers of these microscopic shells; and we may readily imagine that the 'sunless wrecks' which strew its bottom are, in the process of ages, hid under this fleecy covering, presenting the rounded appearance which is seen over the body of a traveler who has perished in the snowstorm. The ocean, especially within and near the tropics, swarms with life. The remains of its myriads of moving things are conveyed by currents, and scattered and lodged in the course of time all over its bottom. The process, continued for ages, has covered the depths of the ocean as with a mantle, consisting of organisms as delicate as the macled frost and as light as the undrifted snowflake. of the mountain."
Maury was right in respect to the covering of the bed of the deep sea, for, as a result of all our researches, it is found that in waters removed from the land and more than fourteen hundred fathoms in depth there is an almost unbroken layer of pteropod, globigerina, diatom, and radiolarian oozes, and red clay which occupies nearly 115,000,000 of the 143,000,000 square miles of the water surface of the globe. But he was wrong in asserting that low temperature, pressure, and the absence of light preclude the possibility of life in very deep water.
Ehrenberg held the opposite opinion with regard to the conditions of life at the bottom of the sea, as may be seen from the following extract from a letter which he wrote to Maury in 1857: "The other argument for life in the deep which I have established is the surprising quantity of new forms which are wanting in other parts of the sea. If the bottom were nothing but the sediment of the troubled sea, like the fall of snow in the air, and if the biolithic curves of the bottom were nothing else than the product of the currents of the sea which heap up the flakes, similarly to the glaciers, there would necessarily be much less of unknown and peculiar forms in the depths. The surface and the borders of the sea are much more productive and much more extended than the depths; hence the forms peculiar to the depths should not be perceived. The great quantity of peculiar forms and of soft bodies existing in the innumerable carapaces, accompanied by the observation of the number of unknowns, increasing with the depth—these are the arguments which seem to me to hold firmly to the opinion of stationary life at the bottom of the deep sea."
It would appear to have been definitely established by the researches of the last fifty years that life in some of its many forms is universally distributed throughout the ocean. Not only in the shallower waters near coasts, but even in the greater depths of all oceans, animal life is exceedingly abundant. A trawling in a depth of over a mile yielded two hundred specimens of animals belonging to seventy-nine species and fifty-five genera. A trawling in a depth of about three miles yielded over fifty specimens belonging to twenty-seven species and twenty-five genera. Even in depths of four miles fishes and animals belonging to all the chief invertebrate groups have been procured, and in a sample of ooze from nearly five miles and a quarter there was evidence to the naturalists of the Challenger that living creatures could exist at that depth.
Recent oceanographic researches have also established beyond doubt that while in great depths the water is not subjected to the influence of superficial movements like waves, tides, and swift currents, there is an extremely slow movement, in striking contrast with the agitation of the surface water. Although the movement at the bottom is so slow that the ordinary means of measuring currents can not be applied accurately to them, the thermometer furnishes an indirect means of ascertaining their existence. Water is a very bad conductor of heat, and consequently a body of water at a given temperature passing into a region where the temperature conditions are different retains for a long time, and without much change, its original temperature. To illustrate: The bottom temperature near Fernando do Noronha, almost under the equator, is 0·2° C., or close upon the freezing point; it is obvious that this temperature was not acquired at the equator, where the mean annual temperature of the surface layer of the water is 21° C, and the mean normal temperature of the crust of the earth not lower than 8° C. The water must therefore have come from a place where the conditions were such as to give it a freezing temperature; and not only must it have come from such a place, but the supply must be continually renewed, however slowly, for otherwise its temperature would gradually rise by conduction and mixture. Across the whole of the North Atlantic the bottom temperature is considerably higher, so that the cold water can not be coming from that direction; on the other hand, we can trace a band of water at a like temperature at nearly the same depth continuously to the Antarctic Sea, where the conditions are normally such as to impart to it this low temperature. There seems, therefore, to be no doubt that there is a current from the antarctic to the equator along the bottom of the South Atlantic.
From the millions of reliable deep-sea soundings that have been made during the last forty years the more general features of the bathymetric chart of the world have been firmly established; and the ancient idea, derived chiefly from a supposed