Pleasant Ways in Science

by Richard A. Proctor

At Punta de Eobos two vessels were lost, and fourteen ships more or less damaged, by the wave. Antofagasta, Mexillones, Tocopilla, and Cobigo, on the Bolivian coast, suffered simultaneously from the earthquake and the sea-wave. The sea completely swept the business portion of Antofagasta during four hours. Here a singular phenomenon was noticed. For some time the atmosphere was illuminated with a ruddy glow. It was supposed that this light came from the volcano of San Pedro de Atacama, a few leagues inland from Antofagasta. A somewhat similar phenomenon was noticed at Tacna during the earthquake of August, 1868. About three hours after the earthquake an intensely brilliant light made its appearance above the neighbouring mountains. It lasted fully half an hour, and was ascribed to the eruption of some as yet unknown volcano.

As to the height of the great wave along this part of the shore-line of South America, the accounts vary. According to those which are best authenticated, it would seem as though the wave exceeded considerably in height that which flowed along the Peruvian, Bolivian, and Chilian shores in August, 1868. At Huaniles the wave was estimated at sixty feet in height, at Mexillones, where the wave, as it passed southwards, ran into Mexillones Bay, it reached a height of sixty-five feet. Two-thirds of the town were completely obliterated, wharves, railway stations, distilleries, etc., all swallowed up by the sea.

The shipping along the Peruvian and Bolivian coast suffered terribly. The list of vessels lost or badly injured at Pabellon de Pica alone, reads like the list of a fleet.

I have been particular in thus describing the effects produced by the earthquake and sea-wave on the shores of South America, in order that the reader may recognize in the disturbance produced there the real origin of the great wave which a few hours later reached the Sandwich Isles, 5000 miles away. Doubt has been entertained respecting the possibility of a wave, other than the tidal-wave, being transmitted right across the Pacific. Although in August, 1868, the course of the great wave which swept from some region near Peru, not only across the Pacific, but in all directions over the entire ocean, could be clearly traced, there were some who considered the connection between the oceanic phenomena and the Peruvian earthquake a mere coincidence. It is on this account perhaps chiefly that the evidence obtained in May, 1876, is most important. It is interesting, indeed, as showing how tremendous was the disturbance which the earth’s frame must then have undergone. It would have been possible, however, had we no other evidence, for some to have maintained that the wave which came in upon the shores of the Sandwich Isles a few hours after the earthquake and sea disturbance in South America was in reality an entirely independent phenomenon. But when we compare the events which happened in May, 1876, with those of August, 1868, and perceive their exact similarity, we can no longer reasonably entertain any doubt of the really stupendous fact that the throes of the earth in and near Peru are of sufficient energy to send oceanic waves right across the Pacific,—waves, too, of such enormous height at starting, that, after travelling with necessarily diminishing height the whole way to Hawaii, they still rose and fell through thirty-six feet The real significance of this amazing oceanic disturbance is exemplified by the wave circles which spread around the spot where a stone has fallen into a smooth lake. We know how, as the circles widen, the height of the wave grows less and less, until, at no great distance from the centre of disturbance, the wave can no longer be discerned, so slight is the slope of its advancing and following faces. How tremendous, then, must have been the upheaval of the bed of ocean by which wave-circles were sent across the Pacific, retaining, after travelling 5000 miles from the centre of disturbance, the height of a two-storied house! In 1868, indeed, we know that the wave travelled very much further, reaching the shores of Japan, of New Zealand, and of Australia, even if it did not make its way through the East Indian Archipelago to the Indian Ocean, as some observations seem to show. Although no news has been received which would justify us in believing that the wave of May, 1876, produced corresponding effects at such great distances from the centre of disturbance, it must be remembered that the dimensions of the wave when it reached the Sandwich Isles fell far short of those of the great wave of August 13–14, 1868.

It will be well to make a direct comparison between the waves of May, 1876, and August, 1868, in this respect, as also with regard to the rate at which they would seem to have traversed the distance between Peru and Hawaii. On this last point, however, it must be noted that we cannot form an exact opinion until we have ascertained the real region of Vulcanian disturbance on each occasion. It is possible that a careful comparison of times, and of the direction in which the wave front advanced upon different shores, might serve to show where this region lay. I should not be greatly surprised to learn that it was far from the continent of South America.

The great wave reached the Sandwich Isles between four and five on the morning of May 10, corresponding to about five hours later of Peruvian time. An oscillation only was first observed at Hilo, on the east coast of the great southern island of Hawaii, the wave itself not reaching the village till about a quarter before five. The greatest difference between the crest and trough of the wave was found to be thirty-six feet here; but at the opposite side of the island, in Kealakekua Bay (where Captain Cook was killed), amounted only to thirty feet. In other places the difference was much less, being in some only three feet, a circumstance doubtless due to interference, waves which have reached the same spot along different courses chancing so to arrive that the crest of one corresponded with the trough of the other, so that the resulting wave was only the difference of the two. We must explain, however, in the same way, the highest waves of thirty-six to forty feet, which were doubtless due to similar interference, crest agreeing with crest and trough with trough, so that the resulting wave was the sum of the two which had been divided, and had reached the same spot along different courses. It would follow that the higher of the two waves was about twenty-one feet high, the lower about eighteen feet high; but as some height would be lost in the encounter with the shore-line, wherever it lay, on which the waves divided, we may fairly assume that in the open ocean, before reaching the Sandwich group, the wave had a height of nearly thirty feet from trough to crest. We read, in accordance with this explanation, that “the regurgitations of the sea were violent and complex, and continued through the day.”

The wave, regarded as a whole, seems to have reached all the islands at the same time. Since this has not been contradicted by later accounts, we are compelled to conclude that the wave reached the group with its front parallel to the length of the group, so that it must have come (arriving as it did from the side towards which Hilo lies) from the north-east It was, then, not the direct wave from Peru, but the wave reflected from the shores of California, which produced the most marked effects. We can understand well, this being so, that the regurgitations of the sea were complex. Any one who has watched the inflow of waves on a beach so lying within an angle of the line that while one set of waves comes straight in from the sea, another thwart set comes from the shore forming the other side of the angle, will understand how such waves differ from a set of ordinary rollers. The crests of the two sets form a sort of network, ever changing as each set rolls on; and considering any one of the four-cornered meshes of this wave-net, the observer will notice that while the middle of the raised sides rises little above the surrounding level, because here the crests of one set cross the troughs of the other, the corners of each quadrangle are higher than they would be in either set taken separately, while the middle of the four-cornered space is correspondingly depressed. The reason is that at the corners of the wave-net crests join with crests to raise the water surface, while in the middle of the net (not the middle of the sides, but the middle of the space enclosed by the four sides) trough joins with trough to depress the water surface.[24]

We must take into account the circumstance that the wave which reached Hawaii in May, 1876, was probably reflected from the Californian coast, when we endeavour to determine the rate at which the sea disturbance was propagated across the Atlantic. The direct wave would have come sooner, and may have escaped notice because arriving in the night-time, as it would necessarily have done if a wave which travelled to California, and thence, after reflection, to the Sandwich group arrived there at a quarter before five in the morning following the Peruvian earthquake. We shall be better able to form an opinion on this point after considering what happened in August, 1868.

The earth-throe on that occasion was felt in Peru about five minutes past five on the evening of August 13. Twelve hours later, or shortly before midnight, August 13, Sandwich Island time (corresponding to 5 p.m., August 14, Peruvian time), the sea round the group of the Sandwich Isles rose in a surprising manner, “insomuch that many thought the islands were sinking, and would shortly subside altogether beneath the waves. Some of the smaller islands were for a time completely submerged. Before long, however, the sea fell again, and as it did so the observers found it impossible to resist the impression that the islands were rising bodily out of the water. For no less than three days this strange oscillation of the sea continued to be experienced, the most remarkable ebbs and floods being noticed at Honolulu, on the island of Woahoo.”

The distance between Honolulu and Arica is about 6300 statute miles; so that, if the wave travelled directly from the shores of Peru to the Sandwich Isles, it must have advanced at an average rate of about 525 miles an hour (about 450 knots an hour). This is nearly half the rate at which the earth’s surface near the equator is carried round by the earth’s rotation, or is about the rate at which parts in latitude 62 or 63 degrees north are carried round by rotation; so that the motion of the great wave in 1868 was fairly comparable with one of the movements which we are accustomed to regard as cosmical. I shall presently have something more to say on this point.