The researches of Mr. Russell on the swiftness of the tide-wave, showing that the rapidity of its progress increases with the depth of the waters over which it passes, afford us another means, besides the sounding line, of determining approximately the distance of the sea-bottom from its surface. According to this method, the depth of the Channel between Plymouth and Boulogne has been calculated at 180 feet; and the enormous rapidity of the flood wave over the great open seas (300 miles an hour and more) gives us for the mean depth of the Atlantic 14,400 feet, and for that of the Pacific 19,500.
Natural philosophers have endeavoured to calculate the quantity of the waters contained within the vast bosom of the ocean; but as we are still very far from accurately knowing the mean depth of the sea, such estimates are evidently based upon a very unsubstantial foundation.
So much at least is certain, that the volume of the waters of the ocean as much surpasses all conception, as the number of their inhabitants, or of the sands that line their shores.
[Illustration: Torso Rock, near Point Deas Thomson, in the Arctic Ocean.]
The boundaries of the ocean are not invariable; while in some parts it encroaches upon the land, in others it retreats from the expanding coast. In many places we find the sea perpetually gnawing and undermining cliffs and rocks; and sometimes swelling with sudden rage, it devours a broad expanse of plain, and changes fertile meads into a dreary waste of waters. The Goodwin Sands, notorious for the loss of many a noble vessel, were once a large tract of low ground belonging to Earl Goodwin, father of Harold, the last of our Saxon kings; and being afterwards enjoyed by the monastery of St. Augustine at Canterbury, the whole surface was drowned by the abbot's neglect to repair the wall which defended it from the sea. In spite of the endeavours of the Dutch to protect their flat land by dykes against the inundatory waters, the storm-flood has more than once burst through these artificial boundaries, and converted large districts into inland seas.
But the spaces which in this manner the dry land has gradually or suddenly lost, or still loses, to the chafing ocean are largely compensated for in other places, by the vast accumulations of mud and sand, which so many rivers continually carry along with them into the sea. Thus at the mouths of the Nile, of the Ganges, and of the Mississippi, large alluvial plains have been deposited, which now form some of the most fruitful portions of the globe. The whole Delta of Egypt, Bengal, and Louisiana, have thus gradually emerged from the waters.