Page:EB1911 - Volume 19.djvu/1009

 Modern surveying ships no longer make use of hempen lines with enormously heavy sinkers, such as were employed on the “Challenger,” but they sound instead with steel piano wire not more than to  of an inch in diameter and a detachable lead seldom weighing more than. The soundings are made by means of a special machine fitted with a brake so adjusted that the revolution of the drum is stopped automatically the instant the lead touches the bottom, and the depth can then be read directly from an indicator. The line is hauled in by a steam or electric winch, and the sounding-tube containing a sample of the bottom deposit is rapidly brought on board. The sounding machines most frequently employed are those of Admiral C. D. Sigsbee, U.S.N., of Lucas, which was perfected in the Telegraph Construction and Maintenance Company’s ships, and of the Prince of Monaco, constructed by Leblanc of Paris. All attempts to dispense with a lead and line and to measure the depth by determining the pressure at the bottom have hitherto failed when applied to depths greater than 200 fathoms; a new hydraulic manometer has been tried on board the German surveying ship “Planet.” A. Siemens has pointed out that a profile of the sea-bed can be delineated by taking account of the varying strain on a submarine cable while it is being laid, and the average depth of a section can thus be ascertained with some accuracy. All deep-sea measurements are subject to uncertainty because the sounding machine merely measures the length of wire which runs out before the lead touches bottom, and this agrees with the depth only when the wire is perpendicular throughout its run. It is improbable, however, that the smooth and slender wire is much influenced by currents, and the best deep-sea soundings may be taken as accurate to within 5 fathoms.

Relief of the Ocean Floor.—Recent soundings have shown that the floor of the ocean on the whole lies some 2 or 3 m. beneath the surface, and O. Krümmel has calculated the mean depth to be 2010 fathoms (12,060 ft.), while the mean elevation of the surface of the continents above sea-level is only 2300 ft. Viewed from the floor of the ocean the continental block would thus appear as a great plateau rising to a height of 14,360 ft. Nevertheless, the greatest depths of the ocean below sea-level and the greatest heights of the land above it are of the same order of magnitude, the summit of Mount Everest rising to 29,000 ft. above the sea-level, while the Nero Deep near Guam sinks to 31,600 ft. (5268 fathoms) below sea-level. Of course the area at great heights is very much less than the area at corresponding depths. Above the height of 15,000 ft. there are 800,000 sq. km. (310,000 sq. m.), and below the depth of 15,000 ft. there are 120,000,000 sq. km. (46,300,000 sq. m.); above the height of 20,000 ft. there are on the whole surface of the earth only 33,000 sq. km. (12,800 sq. m.), while below the depth of 20,000 ft. there are no less than 5,400,000 sq. km. (2,100,000 sq. m.). According to Krümmel’s calculation the areas of the ocean beyond various depths are as follows:—

On the whole the floor of the ocean is very smooth in its contours, and great stretches can almost be called level. Modern orometry has introduced the calculation of the mean angle of the slope of a given uneven surface provided that maps can be prepared showing equidistant contour lines. If the distance between the contour lines is ℎ and the length of the individual contour lines 𝑙, the sum of their lengths (𝑙), and A the area

of the surface under investigation, then the mean angle of slope is obtained from the equation

Calculating from sheet A I of the Prince of Monaco’s Atlas of Ocean Depths, Krümmel obtained a mean angle of slope of 0° 27′ 44″ or an average fall of 1 in 124 for the North Atlantic between 0° and 47° N., the enclosed seas being left out of account. In the same way a mean angle of slope of approximately half a degree was found for the Adriatic and the Black Sea. Large angles of slope may, however, occur on the flanks of oceanic islands and the continental borders. On the submarine slopes leading up to isolated volcanic islands angles of 15° to 20° are not uncommon, at St Helena the slopes run up to 38° and even 40°, at Tristan d’Acunha to 33°. E. Hull found a mean angle of slope of 13° to 14° for the edge of the continental shelf off the west coast of Europe, and off Cape Toriñana (43° 4′ N.) as much as 34°. Where the French telegraph cable between Brest and New York passes from the continental shelf of the Bay of Biscay to the depths of the Atlantic the angle of slope is from 30° to 41°. Such gradients are of a truly mountainous character, the angle of slope from the Eibsee to the Zugspitze is 30°, and that from Alpiglen station to the summit of the Eiger is 42°. Particularly steep slopes are found in the case of submarine domes, usually incomplete volcanic cones, and there have been cases in which after such a dome has been discovered by the soundings of a surveying ship it could not be found again as its whole area was so small and the deep floor of the ocean from which it rose so flat that an error of 2 or 3 m. in the position of the ship would prevent any irregularity of the bottom from appearing. While such steep mountain walls are found in the bed of the ocean it must be remembered that they are very exceptional, and except where there are great dislocations of the submarine crust or volcanic outbursts the forms of the ocean floor are incomparably gentler in their outlines than those of the continents. Being protected by the water from the rapid subaerial erosion which sharpens the features of the land, and subjected to the regular accumulation of deposits, the whole ocean floor has assumed some approach to uniformity. Still there are everywhere gentle inequalities on the smoothest ocean floor which give to its greater features a distinct relief.

In spite of the increase of deep-sea soundings in the last few decades, they are still very irregularly distributed in the open ocean, and the attempt to draw isobaths (lines of equal depth) on a chart of the world is burdened with many difficulties which can only be evaded by the widest generalizations. Bearing this caution in mind the existing bathymetrical charts, amongst which that of the prince of Monaco stands first, give a very fair idea of the great features of the bed of the oceans. A definite terminology for the larger forms of sub-oceanic relief was put forward by the International Geographical Congress at Berlin in 1899 and adopted by that at Washington in 1904. Equivalent terms, which are not necessarily identical or literal translations, were adopted for the English, French and German languages, the equivalence being closest and most systematic between the English and German terms.

The larger forms designated by special generic terms include the following. The continental shelf is the gentle slope which extends from the edge of the land to a depth usually about 100, though in some cases as much as 300 fathoms, and is there demarcated by an abrupt increase in the steepness of the slope to ocean depths. In the deep sea two types of feature are recognized under the general names of depression and elevation. The depression is distinguished according to form and slope as (1) a basin when of a roughly round outline, (2) a trough when wide and elongated, or (3) a trench when narrow and elongated lying along the edge of a continent. The extension of a basin or trough stretching towards the continent is termed an embayment when relatively wide and a gully when narrow. The elevation includes (1) the gently swelling rise which separates