The conclusions of Commander Dayman confirmed in general those of Lieutenant Berryman, though in comparing the charts prepared by the two, we observe some differences which ought to be noticed. Both agree as to the general character of the bottom of the ocean along this latitude—that it is a vast plain, like the steppes of Siberia. Yet on the chart of Dayman the floor of the sea seems not such a dead level as on that of Berryman. (This may be partly owing to a difference of route, as Dayman passed a little to the north of the track of Berryman.) There are more unequal depths, which in the small space of a chart, appear like hills and valleys. Yet when we consider the wide distances passed over, these inequalities seem not greater than the undulations on our Western prairies. "This space," says Dayman, "has been named by Maury the telegraphic plateau, and although by multiplying the soundings upon it, we have depths ranging from fourteen hundred and fifty to twenty-four hundred fathoms, these are comparatively small inequalities in its surface, and present no new difficulty to the project of laying the cable across the ocean. Their importance vanishes when the extent of the space over which they are distributed (thirty degrees of longitude) is considered."
BED OF THE ATLANTIC, NORTH AND SOUTH, THROUGH THE CAPE DE VERDS, AZORES, AND TELEGRAPH PLATEAU.
According to Berryman and Dayman both, the ocean in its deepest part on this plateau, measured but two thousand and three or four hundred fathoms, or about fourteen thousand feet—a depth of but little over two and a half miles. This is not great, compared with the enormous depths in other parts of the Atlantic;[A] yet that it is something may be realized from the fact that if the Peak of Teneriffe were here "cast into the sea," it would sink out of sight, island, mountain and all, while even the lofty head of Mont Blanc would be lifted but a few hundred feet above the waves.
The only exception to this uniform depth, lies about two hundred miles off the coast of Ireland, where within a space of about a dozen miles, the depth sinks from five hundred and fifty to seventeen hundred and fifty fathoms! "In 14° 48' west," says Dayman, "we have five hundred and fifty fathoms rock, and in 15° 6' west we have seventeen hundred and fifty fathoms ooze. This is the greatest dip in the whole ocean."
"In little more than ten miles of distance a change of depth occurs, amounting to seventy-two hundred feet." This is indeed a tremendous plunge from the hard rock into the slime of the sea.
The same sharp declivity was noticed by Berryman, and has been observed in the several attempts to lay the cable. Thus in the second expedition of 1858, as the Agamemnon was approaching the coast of Ireland, we read in the report of her voyage: "About five o'clock in the evening, the steep submarine mountain which divides the telegraphic plateau from the Irish coast, was reached; and the sudden shallowing of the water had a very marked effect on the cable, causing the strain on, and the speed of it, to lessen every minute. A great deal of slack was paid out to allow for inequalities which might exist, though undiscovered by the sounding-line."
This submarine mountain was then regarded as the chief point of danger in the whole bed of the Atlantic, and as the principal source of anxiety in laying a cable across the ocean. Yet, after all, the ascent or descent of less than a mile and a half in ten miles, is not an impassable grade. More recent soundings reduce this still farther. Captain Hoskins, of the Royal Navy, afterwards made a more careful survey of this precipitous sea bottom, and with results much more favorable. The side of the mountain, it is now said, is not very much steeper than Holborn Hill in London, or Murray Hill in New York.[B] But the best answer to fears on this point, is the fact that in 1857, 1858, and 1865, the cable passed over it without difficulty. In 1857 the Niagara was a hundred miles farther to sea, when the cable broke. In 1865 the strain was not increased more than a hundred pounds. In the final expedition, that of 1866, this declivity was passed over without difficulty or danger.