This introduces the subject of depth—a matter which we have learned accurately only within a very few years. In the early days ropes alone were used for sounding, and these had to be of considerable size to bear the strain; but a mile or so of rope became too heavy to handle, and depths below that length remained unmeasured. Then a little machine was tried consisting of a heavy weight having attached to it, by a trigger, a wooden float. This was thrown overboard. It sank, and when it touched bottom the shock released the float. From the time that elapsed before the float reappeared the depth was estimated. This, however, was little better than guesswork; and accurate soundings exceeding one thousand fathoms were not obtained until an American naval officer began to use wire instead of rope. From this hint was developed elaborate machinery, operated by steam, using steel piano-wire, having automatic registers of the amount reeled out, and carried down by weights that were released when the bottom was struck, making it easier to recover the wire. To these weights (or rather to the wire just above them) were attached devices for clutching and bringing to the surface specimens of the bottom, self-closing jars to fetch water from the lowest layer, self-registering thermometers that recorded the temperatures at the greatest or at various intermediate depths, and other means of learning the character of the water, bottom-material, and animal life several miles below the surface, including methods of photographing by aid of a submerged electric light. Such investigations, carried on in ships suitably equipped, have been prosecuted by several governments, most notably by the expedition of the Challenger, a British surveying-ship which circumnavigated the globe during the years from 1872 to 1876.
SEA-CAVE
NEAR GIANT’S CAUSEWAY,
NORTH OF IRELAND.
This and many other expeditions have sounded in all parts of the world, and explored large tracts where the water uniformly exceeded three miles in depth. The United States ship Enterprise, after passing the Chatham Islands in her run from New Zealand to the Strait of Magellan, found the water everywhere more than thirteen thousand feet deep. Throughout her run from Montevideo to New York the water varied from twelve to eighteen thousand feet deep, and Captain Nares and Admiral Belknap found like depths over equally vast breadths elsewhere.
Yet even in these basins more profound pits and valleys exist. Several places are known near Japan and off Porto Rico exceeding five miles in depth; and an English officer sounded 29,400 feet in the southern Pacific Ocean, nineteen hundred miles east of Brisbane, without finding bottom.
The average depth of all the oceans is estimated at from twelve thousand to fifteen thousand feet. As, according to Humboldt, the average height of the lands of the globe is only about one thousand feet, it will be seen that all the land now above the water, and its foundations, could be shoveled into the ocean troughs and still leave water more than two miles in depth covering the whole planet.
The soundings and dredgings of which I have spoken enable us to make a tolerable map of the ocean beds and to describe their features. All the continents are bordered by a shelf reaching out under the shallow shore-water to a greater or less distance, and then dropping, usually with much abruptness, to the ocean trough. This shelf, perhaps originally a part of the primeval continent, bears most of the great islands near continents, such as Newfoundland, the West Indies, Great Britain and Ireland, Madagascar, the Aleutian, Japanese, and Philippine groups, the Malay Archipelago, and others. If you will look at a map that has marked upon it the line of one thousand fathoms’ depth along the shores of the various continents, you will find it reaching far out from the eastern shores of both Americas, the western and northern shores of Europe, the eastern shores of South Africa, prolonging India hundreds of miles, and embracing great spaces among the East Indies, while even the hundred-fathom line would connect many an island with the mainland or with some other island, as they actually have been connected in times gone by. The fact is, there is not a single proper mountain-peak rising out of deep water at any great distance from the margins of the continents. All the numerous islands of the wide oceans are either coral reefs or the summits of volcanic cones.
Upon this shelf, and for the most part within two hundred miles of the coast, are deposited all of the materials torn from the land by the sea or brought down by rivers or glaciers, excepting the very finest, which currents may float somewhat farther out, and also excepting the rocks that icebergs carry away and drop in mid-ocean; but this is not a great amount, for most icebergs strand on the shallows off Newfoundland or in Bering Sea.
Almost nothing from the shores, therefore, reaches the central depths of the open oceans, whose beds are in substantially the same condition that they were in at the beginning, except for two things—volcanic upheavals in some places, and the remains of animal life everywhere. The former exception is a very important one, since it is now known, according to Professor Shaler, that volcanoes, by their eruptions, send more dust and broken materials to the seas than the rivers and shores combined.
