When a wave reaches shallow water it piles itself up to a height, as any one knows who has watched the waves coming in on the sea-shore, so that the height of a wave measured on the tide-gauge of a seaport is a good deal greater than that of the height of the wave when it is far out on the ocean. In fact, the mid-ocean height of the wave is likely to be inches while the in-shore wave is measured in feet. An illustration of this can be seen on the coast of Cornwall, where sometimes, on quite a calm day the sea that looks so still breaks on the shore in big rollers. We cannot tell exactly how high an earthquake wave may be in mid-ocean, but we know it cannot usually be very great, though it travels at great speeds—sometimes as much as five miles a minute, or three hundred miles an hour.

Thus we should not expect that ships far out at sea would often notice seaquakes unless the quake took place very near them. There are, however, some instances. Captain Gales, of the ship Florence Nightingale, reported that on January 25th, 1859, while near St. Paul's Rocks, not far from the Equator, "we felt a strong shock of an earthquake. It began with a rumbling sound like distant thunder and lasted about forty seconds. I was quite well acquainted with earthquakes, as I had experienced a good many on the west coast of America, but never had I felt so severe a one. Glass and dishes rattled so vigorously that I was surprised to find them uninjured. A good many objects fell down, and it was as if the ship were grounding on a reef." Another report from a locality not far from this speaks of a strange submarine noise not unlike distant thunder, or still more like the distant firing of heavy guns. At the same time there was a vibration of the ship as though the anchor had been let go.

The foregoing are representative of the large majority of the reports of seaquakes. The ship quivers, vibrates; loose objects clatter and tumble. There is a strange thunderous noise in the sea. The first impression is as if the ship were grinding upon the bottom, and there is an instinctive rush of the crew to the deck to see if the ship is not on a reef. In some instances there are some forcible disturbances. M. Vulet d'Aourst speaks of a seaquake so severe that "the Admiral feared the complete destruction of the corvette." Heavy objects, including cannon and their carriages, were thrown upon the deck. The ship itself seemed to be hurled upwards.

One of the explanations offered of a phenomenon such as the last described is that the vessel has been near a submarine volcanic eruption of great power. The places where some or most of the seaquakes have been observed have been charted, and certain districts of the ocean have been found to produce more of these disturbances than others. Among the first to be thus determined were two, located in the Atlantic Ocean, very near the Equator and nearly midway between Cape Palmas on the southeastern coast of Liberia and Cape St. Roque, Brazil. One of them is the St. Paul's Rocks district, of which mention has already been made. Another district from which seaquakes have been reported with exceptional frequency is the North Atlantic in the neighbourhood of the Azores. Between these islands and the coast of Portugal it may be remembered that the great quake originated which, on November 1st, 1775, destroyed Lisbon. The West Indian Deep, that profound basin of the Atlantic lying north of the Lesser Antilles and east of the Bahamas, where the Atlantic has its greatest depths and where its bottom has its greatest inequalities, is another district from which an unusual number of seaquakes have been reported. The usual explanation of their origin is that in these neighbourhoods, owing to the great pressure of water above them, there are continual slips and fractures of the sea bottom, like landslips on land, and that into the great cavities thus produced the water rushes, and thus sets up disturbances which show themselves on the surface like waves, very much in the same way that the water rushing through the escape of a bath produces small disturbances on the surface of the water in the bath. To satisfy the requirements of such a wave as rolled in upon the South American coast at Arica in 1868 would require the sudden drop of many hundred square miles of sea bottom—perhaps of several thousand square miles.

CHAPTER XIV

SOME FAMOUS EARTHQUAKES

Of all earthquakes perhaps the best known and remembered is that of Lisbon on November 1st, 1755, and volumes have been written about it. The first shocks of this earthquake came without other warning than a deep sound resembling thunder, which appeared to proceed from beneath the ground, and it was immediately followed by a quaking which threw down the entire city of Lisbon. In six minutes sixty thousand persons perished. The day was almost immediately turned into night, owing to the thickness of the dust from the ruined city. A few minutes afterwards fire sprang up among the ruins. The new Lisbon quay, which had been built entirely of marble, suddenly sank down into the bay with an immense crowd of people, who thronged to it for safety, and it is said that none of the bodies of the drowned were ever seen again. Following hard on the first shocks the sea retired from the land, carrying boats and other craft with it, only to return in a great wave, which completed the destruction in and about the city. This great sea wave, the mightiest that has ever been described in connection with an earthquake, is said to have washed not only the coasts of Portugal and Spain, but to have extended with destructive violence to other countries. At Kinsale, in Ireland, it was strong enough to whirl vessels about in the harbour and to pour into the market-place, and it was of great violence also at the island of Madeira. Portions of the sea-coast between Cape-da-Roca and Cape Carvociro fell away into the sea, and the damage was very great along the coast between Cape St. Vincent and the mouth of the Guadiana. The great Sierra da Estrella, on the west of the Tagus valley, was split and rent in a most remarkable manner, and threw down avalanches of rock into the valley.

The great earthquake which shook Calabria and North-Eastern Sicily in the year 1783 stands out in rather striking contrast with other disturbances of history, because it was carefully studied by a great number of skilled observers. Among them were Vivenzio, the court physician of the King of Naples, who has supplied us with a narrative of the events; Grimaldi, the Minister of War, who at the King's command visited the region and has left accurate measurements of the greater and lesser fissures associated with the earthquake; Pignaturo, a physician, who kept a record of the long-continuing shocks, together with an estimate of their intensities; the French geologist Dolomieu; and Sir William Hamilton, who was the British Ambassador at Naples. The Academy of Naples sent a special commission to the scene of the earthquake's destruction, and prepared a bulky report of great scientific value. Calabria is a country which has many times been racked with earthquakes; the disturbances being almost as conspicuous for number as in Japan. The areas shaken have not usually been great in extent, but as regards the geological changes and the loss to life by which they have been accompanied, they rank among the greatest in history.

The shocks of 1783, which cost thirty thousand lives, came without warning on February 5th, and in two minutes threw down the structures in hundreds of cities and villages scattered through Calabria and North-Eastern Sicily. The great central granite formation of Calabria, which was but slightly disturbed by the first shock, was more heavily shaken by those which followed; and it was noted by the early writers on this earthquake that the mountains had been a little raised in comparison with the neighbouring plains at their bases. The fact of the elevation of mountains by earthquakes or some other underground disturbance has been elsewhere noted. On November 19th, 1822, a great earthquake shook the Chilian coast for a distance of twelve hundred miles north and south. The greatest energy was shown about one hundred miles north of Valparaiso, where the coast was found to have risen suddenly from three to five feet for a distance which has never been accurately ascertained, but which is known to have exceeded thirty-five miles. In 1835 and in 1837 similar elevations of the coast were caused by earthquakes at Concepcion, about three hundred miles south of Valparaiso, and at Valdivia, about two hundred miles south of Concepcion. Charles Darwin, in the Voyage of the "Beagle," says: "I have convincing proofs that this part of the continent of South America has been elevated near the coast at least from four to five hundred feet, and in some parts from one thousand to thirteen hundred feet, since the epoch of living shells." Darwin finds his evidence in the raised beaches near the coast on which these shells abound. That this uplift has been going on by small and sudden movements, from a foot to ten feet at each shock, for more than two centuries is attested by good evidence. The coast in many places is proven to be from twenty to thirty feet higher to-day than it was in the middle of the seventeenth century. Sir Charles Lyell, in his Principles of Geology, gives a most interesting account of the sudden upheaval of a portion of a mountain range, with the accompaniment of a great earthquake, near Wellington, in New Zealand, in January, 1855. Both the North and South Islands of that colony have been affected by upliftings during the nineteenth century, and these movements have been attended by powerful and far-reaching earthquakes. The changes wrought by these movements on the shores and farther inland as well have been remarkable during the last hundred years.

Another example of the same kind of activity is seen in the occasional rise of islands from the sea; but to this we shall refer again, and for the present we may return to the Calabrian earthquake, which presented many curious and many characteristic features. During the earthquake the surface of the country heaved in great undulations, which were productive of a feeling of sea-sickness, and which, according to some observers, made the clouds appear to stand still, as they will sometimes seem to do from the deck of a tossing ship. The fissures which appeared in the ground were numbered by thousands, and sometimes the displacements of the earth amounted to as much as ten feet. Houses were lifted high up; in other places the land or the sea-floor sank several feet. Many of the fissures opened, spurted out sand or water, and then closed again; and some of the Calabrian plains after the earthquake were found to be dotted with circular hollows, on the average about the size of carriage wheels, which were like wells, but were sometimes filled with sand instead of water. These were afterwards found to be V-shaped. In addition to these hundreds of small cone-shaped hollows or wells there were other water basins more deserving the name of ponds or lakes. One of these in the neighbourhood of Seminara, to which the name of Lago di Tolfilo was given, was about a third of a mile in length, and was so copiously fed by the springs ranged in a fissure in its bottom that all attempts to drain it proved useless. Near Sitizam a valley was completely choked up by the landslip from opposite sides, and behind this new dam a lake was formed which was about two miles in length and one mile in breadth. Vivenzio states that fifty lakes arose at the time of the earthquake, and the Government surveyors, who included ponds, counted no fewer than 215. The first effect of the more violent shocks was generally to dry up the rivers. Immediately afterwards many of their beds were so blocked up over them that the rivers overflowed. From the rock of Scylla opposite to Charybdis, in the Straits of Messina, large sections of cliff were broken off, in one instance for a whole mile's length of coast. The sea and the neighbourhood was greatly disturbed; and soon after the fall of the cliffs of Scylla the sea rose to a height of twenty feet, and the wave rolling over the coast-line drowned 1500 people.