Thus Captain Wilkes, commander of the U. S. Exploring Expedition, found the height of the waves near Orange Harbour, where they rose higher and more regular than at any other time during the cruise, to be thirty-two feet (depression and altitude), and their apparent progressive motion about twenty-six and a half miles in an hour.
Sir James Ross calculated the height of the waves on a strongly agitated sea at twenty-two feet, and, according to the French naturalists who sailed in the frigate "La Venus," on her voyage round the world, the highest waves they met with never exceeded that measure.
Thus, according to the joint testimony of the most eminent nautical authorities, the waves in the open sea never attain the mountain-height ascribed to them by the exuberant fancy of poets or exaggerating travellers. But when the tempest surge beats against steep crags or rocky coasts it rises to a much more considerable height. The lighthouse of Bell Rock, though 112 feet high, is literally buried in foam and spray to the very top during ground-swells, even when there is no wind. On the 20th November, 1827, the spray rose to the height of 117 feet above the foundation or low-water mark, which, deducting eleven feet for the tide that day, leaves 106 feet for the height of the wave. The strength of that remarkable edifice may be estimated from the fact, that the power of such a giant billow is equivalent to a pressure of three tons per square foot.
In the Shetland Islands, which are continually exposed to the full fury of the Atlantic surge (for no land intervenes between their western shores and America), every year witnesses the removal of huge blocks of stone from their native beds by the terrific action of the waves. "In the winter of 1802," says Dr. Hibbert, in his description of that northern archipelago, "a tabular-shaped mass, eight feet two inches by seven feet, was dislodged from its bed and removed to a distance of from eighty to ninety feet. I measured the recent bed from which a block had been carried away the preceding winter (A.D. 1818), and found it to be seventeen feet and a half by seven feet, and the depth two feet eight inches. The removed mass had been borne to a distance of thirty feet, when it was shivered into thirteen or more lesser fragments, some of which were carried still farther from 30 to 120 feet. A block nine feet two inches by six feet and a half, and four feet thick, was hurried up the acclivity to a distance of 150 feet."
The great storm of 1824, which carried away part of the breakwater at Plymouth, lifted huge masses of rock, from two to five tons in weight, from the bottom of the weatherside and rolled them fairly to the top of the pile. One block of limestone weighing seven tons was washed round the western extremity of the breakwater, and swept to a distance of 150 feet. In 1807, during the erection of the Bell Rock lighthouse, six large blocks of granite which had been landed on the reef were removed by the force of the sea and thrown over a rising ledge to the distance of twelve or fifteen paces, and an anchor weighing about twenty-two hundredweight was cast upon the surface of the rock.
With such examples before our eyes, we cannot wonder that in the course of centuries all shores exposed to the full shock of the waves, lashing against them with every returning tide, should gradually be wasted and worn away. One kind of stone stands the brunt of the elements longer than another, but ultimately even the hardest rock must yield to the rage of the billows, which when provoked by wintry gales, batter against them with all the force of artillery.
Thus, all along our coasts we find innumerable instances of their destructive power. Tynemouth Castle now overhangs the sea, although formerly separated from it by a strip of land, and in the old maps of Yorkshire we find spots, now sand-banks in the sea, marked as the ancient sites of the towns and villages of Auburn, Hartburn, and Hyde. The cliffs of Norfolk and Suffolk are subject to incessant and rapid decay. At Sherringham, Sir Charles Lyell ascertained, in 1829, some facts which throw light on the rate at which the sea gains upon the land. There was then a depth of twenty feet (sufficient to float a frigate) at one point in the harbour of that port, where only forty-eight years ago there stood a cliff fifty feet high with houses upon it! "If once in half a century," remarks the great geologist, "an equal amount of change were produced suddenly by the momentary shock of an earthquake, history would be filled with records of such wonderful revolutions of the earth's surface; but if the conversion of high land into deep sea be gradual, it excites only local attention." On the same coast, the ancient villages of Shipden, Wimpwell, and Eccles have disappeared, several manors and large portions of neighbouring parishes having gradually been swallowed up; nor has there been any intermission, from time immemorial, in the ravages of the sea along a line of coast twenty miles in length in which these places stood. Dunwich, once the most considerable sea-port on the coast of Suffolk, is now but a small village with about one hundred inhabitants. From the time of Edward the Confessor, the ocean has devoured, piece after piece, a monastery, seven churches, the high road, the town-hall, the gaol, and many other buildings. In the sixteenth century not one-fourth of the ancient town was left standing, yet, the inhabitants retreating inland, the name has been preserved,—
"Stat magni nominis umbra,"—
as has been the case with many other ports, when their ancient site has been blotted out.
The Isle of Sheppey is subject to such rapid decay, that the church at Minster, now near the coast, is said to have been in the middle of the island fifty years ago, and it has been conjectured that at the present rate of destruction, the whole isle will be annihilated before the end of the century.