On many flat coasts the drift-sand has raised dunes, wearying the eye by their monotonous uniformity; on others, where these natural bulwarks are wanting, artificial embankments, or dykes protect the lowlands against the encroachments of the sea, or else the latter forms vast salt-marshes and lagunes. On some coasts these submerged or half-drowned lands have been transformed by the industry of man into fertile meadows and fields, of which the Dutch Netherlands afford the most celebrated example; while in other countries, such as Egypt, large tracts of land once cultivated have been lost to the sea, in consequence of long misrule and tyranny.
How deep is the sea? How is its bottom formed? Does life still exist in its abyssal depths? These mysteries of ocean, which no doubt floated indistinctly before the mind of many an inquisitive mariner and philosopher of ancient times, have only recently been subjected to a more accurate investigation. Their solution is of the highest importance, both to the physical geographer, whose knowledge must necessarily remain incomplete until he can fully trace the deep-sea path of oceanic currents, and to the zoologist, to whom it affords a wider insight into the laws which govern the development of the innumerable forms of life with which our globe is peopled.
The ordinary system of sounding by means of a weight attached to a graduated line, and "armed" at its lower end with a thick coating of soft tallow, so as to bring up evidence of its having reached the bottom in a sample of mud, shells, sand, gravel, or ooze, answers perfectly well for comparatively shallow water, and for the ordinary purposes of navigation, but it breaks down for depths much over 1000 fathoms. The weight is not sufficient to carry the line rapidly and vertically to the bottom; and if a heavier weight be used, ordinary sounding line is unable to draw up its own weight along with that of the lead from great depths, and gives way, so that by this means no information can be gained as to the nature of the sea-bottom. To obviate this difficulty, several ingenious instruments have been invented, such as the "Bull-dog" sounding machine, which is so contrived that on touching the bottom the weight becomes detached, while at the same time a pair of scoops, closing upon one another scissorwise on a hinge, and permanently attached to the sounding-line, retain and are able to bring up a sample of the bottom.
With the aid of steam, dredging has also been successfully carried down to 2,435 fathoms, so that the ocean bed may become in time as well known to us as the bed of the Mersey or the Thames.
Both sounding and dredging at great depths are, however, difficult and laborious tasks, which can only be performed under very favourable circumstances, and require a vessel specially fitted at considerable expense.
Many of the early deep soundings in the Atlantic, which reported the astonishing depths of 46,000 or even 50,000 feet, are now known to have been greatly exaggerated. In some cases bights of the line seem to be carried along by submarine currents, and in others it is found that the line has been running out by its own weight only, and coiling itself in a tangled mass directly over the lead. These sources of error vitiate very deep soundings; and consequently, in the last chart of the North Atlantic, published on the authority of Rear-Admiral Richards in November 1870, none are entered beyond 4000 fathoms, and very few beyond 3000.
"The general result," says Professor Wyville Thomson,[A] "to which we are led by the careful and systematic deep-sea soundings which have been undertaken of late years is that the depth of the sea is not so great as was at one time supposed, and does not appear to average more than 2000 fathoms (12,000 feet), about equal to the mean height of the elevated table-lands of Asia.
[A] "The Depths of the Sea," p. 228.
"The thin shell of water which covers so much of the face of the earth occupies all the broad general depressions in its crust, and it is only limited by the more abrupt prominences which project above its surface, as masses of land with their crowning plateaux and mountain ranges. The Atlantic Ocean covers 30,000,000 of square miles, and the Arctic Sea 3,000,000, and taken together they almost exactly equal the united areas of Europe, Asia, and Africa—the whole of the Old World—and yet there seem to be few depressions on its bed to a greater depth than 15,000 or 20,000 feet—a little more than the height of Mont Blanc; and, except in the neighbourhood of the shores, there is only one very marked mass of mountains, the volcanic group of the Açores."