HOW THE CABLES ARE LAID ON FLOOR OF THE ATLANTIC.

Star (Christchurch), Issue 19036, 3 April 1930, Page 6

HOW THE CABLES ARE LAID ON FLOOR OF THE ATLANTIC.

To-Day's Signed Article

Specially written for the “Star” By Lawrence C. Goodman.

Eight cable repair ships tossing on the turbulent winter Atlantic, spending weeks mending the strands of wire broken in a few seconds by an underwater earthquake, emphasise the importance to mankind of the land that lies hidden beneath the world’s most travelled ocean. Almost nothing was known of the floor of the Atlantic before the work of laying the first cable was begun in 1857. Later, in the eighteen-seventies, came careful oceanographic surveys by British, German and American scientific expeditions, and gradually the world gained a knowledge of the unseen basins and valleys, the ridges and plateaus and mountain ranges that make up the floor of the Atlantic.

The practical work of the cable ships in connection with the laying of the twentyone cables that now cross the North Atlantic has filled in many of the details. First Efforts. When the first effort was made to connect Europe and America by cable in 1857 the most favourable underwater route was chosen largely by accident because it happened to lie under the narrowest portion of the North Atlantic, Later surveys disclosed the existence of an underwater plateau in this region between Newfoundland and Ireland. The depths over this upland range from a little over a quarter mile to two and a half miles. This seems deep enough, perhaps, to the layman, but a short distance to the south and to the northeast are depressions between three and four miles deep. The under-sea upland that stretches across from Newfoundland to Ireland was recognised to be so valuable for cable laying purposes that it was named “Telegraph Plateau.” Not only is it an asset because of the relatively shallow water and the resulting comparatively low pressure to which submerged cables are subjected; but the very nature of the material of the bottom is such as to help protect the cables. It is a soft ooze formed from the rotted shells and skeletons of innumerable small sea creatures, which serves as a cushion and a protective covering for the cables. “Telegraph Plateau” Supports Large Group. ** In the early days of the cable laying, when the technique of manufacturing the cables had not been developed and when they were neither so strong nor so well insulated as they are now, the “Telegraph Plateau” was invaluable. Now cable making has advanced to such a point that it is possible to make cables so sturdy and well insulated that they can be laid in water four or five miles deep. There the pressure amounts to several tons per square inch instead of the fifteen pounds per square inch that materials are

subjected to above the surface of the sea. When laying a cable at such depths the weight of the cable from ship to bottom causes a tremendous pull. It is still markedly cheaper to make cables for moderate depths and to lay and maintain them there. Fourteen of the twenty-one cables that now cross the North Atlantic make qse of the “Telegraph Plateau”; but the other seven have been plunged boldly into the deeper waters of the Northern American Basin, where the bottom lies three miles and more below the surface. These depths are encountered by the four cables that extend from Nova Scotia and Newfoundland to the Azores, by the two that extend directly from New York to the Azores, and by the one which strikes across the Atlantic from Cape Cod to Brest, France. An Earthquake Break. Just how an underwater earthquake breaks a cable is not clear. Theoretically a break either in the cable or its insulation might be caused by a sideslip, the opening of a crevasse, the dropping away of a portion of the bottom, the sudden rise of a section of the ocean bed, or by a landslide from some ocean peak or bluff. Probably only very severe earthquakes can break a cable. The average earth tremor of the land surface which rattles dishes and even throws down chimneys would not damage a somewhat elastic cable lying along the ground. Cables have a more or less definite life span, as have the rails of a railway. Even if . they escape breakage by earthquakes, ship anchors and the depredations of creatures of the sea, weak spots will be worn as a result of movement by currents or the insulating. material will disintegrate with age, permitting salt water to be forced to the copper, thus opening a door for the escape of electricity. Then the cable “dies.” The useful life of a cable is estimated to be from thirty to fifty years.

(Anglo-American N.S. Copyright.)