Ocean and Oceanography

Deep-sea Soundings.

The hand-lead attached to a line di vided into fathoms was a well-known aid to navigation even in high antiquity, and its use is mentioned in Herodotus (ii. 5) and Acts (xxvii. 29). The earliest deep-sea sounding on record is that of Capt. Phipps on Sept. 4, 1773, in the Norwegian sea, in 65° N. 3° E., on his return from his expedition to Spitsbergen. He spliced together all the sounding-lines on board, and with a weight of 1501b. attached he found bottom in 683 fathoms and secured a sample of fine soft blue mud. He detected the moment of the lead touching the bottom by the sudden slackening in the rate at which the line ran out. The honour of first sounding really oceanic depths belongs to Sir James Clark Ross, who made some excellent measurements in very deep water using expensive kilo metre-long stout hemp lines. Captain Matthew Fontaine Maury, U.S.N., introduced a ball of strong twine attached to a cannon shot, which ran it out rapidly; when the bottom was reached the twine was cut and the depth deduced from the length of string left in the ball on board. The time of touching bottom was judged by timing each oo-fathom mark and noting the sudden increase in the time interval when the shot reached the bottom. In 1854, J. M. Brooke, a midshipman 'of the United States Navy, invented a method of sounding by means of an automatically released heavy weight.

Modern surveying ships sound with steel piano wire not more than o.6 to 0.9mm. in diameter and a detachable lead seldom weighing more than 701b. (See SOUNDING.) Since 1920 the acoustic method has been used to measure the ocean depths. The first results are satisfactory and the so-called "echo-data" are increasing rapidly in number and value. When the sound wave is engendered in the water, one must measure the time (in seconds) which the wave takes to reach the sea bottom and the time the echo takes to get back from there to the ship. The speed of the sound increases with the temperature, with the salt content and with the pressure, and varies between 1,400 and 1,620 metres per second. Generally the results of echo observations and wire loosening are in fairly close agreement ; but formerly one was not sure whether the wire dropped vertically or whether it was deflected by currents and thus registered too great a depth.

Relief of the Ocean Floor.

Recent soundings have shown that the floor of the ocean on the whole lies some 3 or 4 km. beneath the surface and E. Kossinna has, 1921, calculated the mean depth to be 3,80o metres, while the mean elevation of the surface of the continents above sea-level is only 84o metres. Viewed from the floor of the ocean the continental block would thus appear as a great plateau rising to a height of 4,64o metres. The greatest depths of the ocean below sea-level and the greatest heights of the land above it are not of the same order of magni tude, the summit of Mt. Everest rising to 8,800 metres above the sea-level, while the Philippine Trench near Mindanao sinks to Io,800 metres (5,900 fathoms) below sea-level. Of course the area at great heights is very much less than the area at cor responding depths. According to Kossinna's calculation the areas of the ocean beyond various depths are as follows :— Considering these areas and those of continental lands of various elevations, it is possible to classify the whole into three groups.

( 1) the area of the continents and their shelves down to the 200 metre submarine contour, this makes up 34.7% of the earth's surface; (2) The area between the 200 metre and 3,00o metre submarine contour, 10.7% of the earth's surface; (3) The area below the 3,00o metre submarine contour, 54.6% of the earth's surface. (Fig. I.) If land and water were evenly balanced on the earth the most widespread depth figure for the ocean should be about 2,400 metres, but the actual facts are surprisingly dif ferent. Several students, including A. Wegener, interpret matters by supposing that the continental masses are of material different from that of the deep-sea floor. The first, which they call sial, rich in silica, has a specific gravity of 2.6. The second, which they call sima, is formed of basic material and has a specific gravity of 2.9. They think that the continental masses float in the deep-sea floor rocks, much as icebergs float in the sea. This interpreta tion makes the deep-sea floor something quite different from the continents and is, at any rate, a useful working hypothesis. On the whole the floor of the ocean is not so smooth in its con tours as had been believed until recently. According to the new "echo data" the apparent great flat stretches show marked varia tions of relief. 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 1, the sum of their lengths (1), and A the area of the surface under investigation, then the mean angle of slope is obtained from the equation Calculating from sheets of the prince of Monaco's Atlas of Ocean Depths, Kriimmel obtained a mean angle of slope of o° 27' 44" or an average fall of 1 in 124 for the north Atlantic between o° and N., the enclosed seas being left out of account. 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 381° and even at Tristan d'Acunha to 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 Torinana (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 W. Such gradients are of a truly mountainous character; the angle of slope from the Eibsee to the Zugspitze is 30°. Particularly steep slopes are found in the case of sub marine 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 than an error of 4 or 5 km. in the position of the ship would prevent any irregularity of the bottom from appearing.