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THE WAVES OF THE OCEAN.

Waves and the Mode of their Formation.—Height and Velocity of Storm-Waves, on the High Seas, according to the Calculations of Scoresby, Arago, Sir James Ross, and Wilkes.—Their Height and Power on Coasts.—Their Destructive Effects along the British Shore.—Dunwich.—Reculver.—Shakspeare's Cliff.

After having admired the sea in the grandeur of its expanse, and the profundity of its depths, I shall, in this and the two following chapters, examine in what manner the perpetual circulation of its waters is maintained.

H.M.S. "Resolute" lying to in the North Atlantic.

"The movements of the sea," says Humboldt, "are of a three-fold description: partly irregular and transitory, depending upon the winds, and occasioning waves; partly regular and periodical, resulting from the attraction of the sun and the moon (ebb and flood); and partly permanent, though of unequal strength and rapidity at different periods (oceanic currents)."

Who has ever sojourned on the coast, or crossed the seas, and has not been delighted by the aspect of the waves, so graceful when a light breeze curls the surface of the waters, so sublime when a raging storm disturbs the depths of the ocean?

But it is easier to admire the beauty of a wave than clearly to explain its nature, so as to convey an accurate or sufficiently general conception of its formation to the reader's mind. Those who are placed for the first time on a stormy sea, discover with wonder that the large waves which they see rushing along with a velocity of many miles an hour do not carry the floating body along with them, but seem to pass under the bottom of the ship with scarcely a perceptible effect in carrying the vessel out of its course.

In like manner, the observer near the shore perceives that floating pieces of wood are not carried towards the shore with the rapidity of the waves, but are left nearly in the same place after the wave has passed them as before. Nay, if the tide be ebbing, the waves may even be observed rushing with great velocity towards the shore, while the body of water is actually receding, and any object floating in it is carried in the opposite direction to the waves out to sea.

What, then, is wave-motion as distinct from water-motion? The force of the wind, pushing a given mass of water out of its place into another, dislodges the original occupant, which is again pushed forward on the occupant of the next place, and so on. As the water-particles crowd upon one another, in the act of going out of their old places into the new, the crowd forms a temporary heap visible on the surface of the fluid, and as each successive mass is displacing the one before it, the undulation or oscillatory movement spreads farther and farther over the waters. Wave-motion is, in fact, the transference of motion without the transference of matter: of form without the substance, of force without the agent.

The strongest storm cannot suddenly raise high waves, they require time for their development. Fancy the wind blowing over an even sea, and it will set water-particles in motion all over the surface, and thus give the first impulse to the formation of small waves. Numberless oscillations unite their efforts, and create visible elevations and depressions. Meanwhile, the wind is constantly setting new particles in motion; long before the first oscillations have lost their effect, countless others are perpetually arising, and thus the sum of the propelling powers is constantly increasing, and gradually raising mountain-waves, until their growth is finally limited by the counterbalancing power of the earth's attraction.

As the strength of the waves only gradually rises, it also loses itself only by degrees, and many hours after the tornado has ceased to rage, mighty billows continue to remind the mariner of its extinguished fury. The turmoil of waters awakened by the storm propagates itself hundreds of miles beyond the space where its howling voice was heard, and often, during the most tranquil weather, the agitated sea proclaims the distant war of the elements.

The velocity of waves depends not only on the power of the impulse, but also on the depth of the subjacent waters, as I have already mentioned in the preceding chapter.

For this reason, as increased velocity augments the power of the impulse, the waves in the Atlantic or Pacific, the mean depth of which may be estimated at 12,000 or 18,000 feet, attain a much greater height than in the comparatively shallow North Sea.

The breaking of the waves against the shore arises from their velocity diminishing with their depth. As the small flat wave rolls up the beach, its front part, retarded by the friction of the ground, is soon overtaken by its back, moving in swifter progression, and thus arises its graceful swelling, the toppling of its snow-white crest, and finally its pleasant prattle among the shingles of the strand. This is one of those pictures of nature which Homer describes with such inimitable truth in various places of his immortal poems: he paints with admirable colours the slow rising of the advancing wave, how it bends forward with a graceful curve, and, crowning itself with a diadem of foam, spreads like a white veil over the beach, leaving sea-weeds and shells behind, as it rustles back again into the sea.

The height which waves may attain on the open sea has been accurately investigated by the late Rev. Dr. Scoresby, during two passages across the Atlantic in 1847 and 1848.

"In the afternoon of March 5th, 1848," says that eminent philosopher, "I stood during a hard gale upon the cuddy-roof or saloon deck of the 'Hibernia:' a height, with the addition of that of the eye, of 23 feet 3 inches above the line of flotation (the ship's course being similar to that of the waves). I am not aware that I ever saw the sea more terribly magnificent; the great majority of the rolling masses of water was more than 24 feet high, (including depression as well as altitude, or reckoning above the mean-level, more than 12 feet). I then went to the larboard paddle-box, about 7 feet higher (30 feet 2 inches up to the eye), and found that one half of the waves rose above the level of the view obtained.

"Frequently I observed long ranges (200 yards), which rose so high above the visible horizon, as to form an angle estimated at two or three degrees when the distance of the wave's summit was about 100 yards from the observer. This would add near 13 feet to the level of the eye, and at least one in half-a-dozen waves attained this altitude. Sometimes peaks or crests of breaking seas would shoot upward, at least 10 or 15 feet higher.

"The average wave was, I believe, fully equal to that of my sight on the paddle-box, or more than 15 feet, and the mean highest waves, not including the broken or acuminated crests, rose about 43 feet above the level of the hollow occupied at the moment by the ship. It was a grand storm-scene, and nothing could exceed the pictorial effect of the partial sunbeams breaking through the heavy masses of clouds." From the time taken by a regular wave to pass from stern to stem, Dr. Scoresby calculated its velocity at 2875 feet in each minute, or 32·67 English statute miles in an hour. The mean length of the wave-ridges, was from a quarter to a third of a mile.

To those who might be inclined to doubt the accuracy of these measurements, the remark may suffice that our celebrated countryman had been for years engaged in the northern whale-fishery, where he had ample opportunities for practising his eye in measuring distances. Besides, the conclusions of many other trustworthy observers coincide with the evaluations of Dr. Scoresby.

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.

Another remarkable instance of the destructive action of the tidal surge is that of Reculver, on the Kentish coast, an important military station in the time of the Romans, now nothing but a ruin and a name. So late as the reign of Henry VIII., Reculver was still a mile distant from the sea; but, in 1780, the encroaching waves had already reached the site of the ancient camp, the walls of which, cemented as they were into one solid mass by the unrivalled masonry of the Romans, continued for several years after they were undermined to overhang the sea. In 1804, part of the churchyard with the adjoining houses was washed away, and then the ancient church with its two lofty spires, a well-known landmark, was dismantled and abandoned as a place of worship.

Shakspeare's Cliff at Dover has also suffered greatly from the waves, and continually diminishes in height, the slope of the hill being towards the land. About the year 1810, there was an immense landslip from this cliff, by which Dover was shaken as if by an earthquake, and a still greater one in 1772.

Thus the fame of the poet is likely to outlive for many centuries the proud rock, the memory of which will always be entwined with his immortal verse:—

"How fearful,

And dizzy 'tis to cast one's eyes so low!

The crows, and choughs, that wing the midway air,

Show scarce so gross as beetles: half way down

Hangs one that gathers samphire; dreadful trade!

Methinks, he seems no bigger than his head.

The fishermen, that walk upon the beach,

Appear like mice; and yon tall anchoring bark,

Diminish'd to her cock; her cock, a buoy

Almost too small for sight. The murmuring surge,

That on th' unnumber'd idle pebbles chafes,

Cannot be heard so high."

The peninsulas of Purbeck and Portland, the cliffs of Devonshire and Cornwall, the coasts of Pembroke and Cardigan, the stormy Hebrides, Shetland and Orcadia, all tell similar tales of destruction, a mere summary of which would swell into a volume.

During the most violent gales the bottom of the sea is said by different authors to be disturbed to a depth of 300, 350, or even 500 feet, and Sir Henry de la Bêche remarks that when the depth is fifteen fathoms, the water is very evidently discoloured by the action of the waves on the mud and sand of the bottom. But in the deep caves of ocean all is tranquil, all is still, and the most dreadful hurricanes that rage over the surface leave those mysterious recesses undisturbed.