(From Light Science in Leisure Hours.) By
R.A. PROCTOR.
August 13th, 1868, one of the most terrible calamities which has ever visited a people befell the unfortunate inhabitants of Peru. In that land earthquakes are nearly as common as rain storms are with us; and shocks by which whole cities are changed into a heap of ruins are by no means infrequent. Yet even in Peru, "the land of earthquakes," as Humboldt has termed it, no such catastrophe as that of August, 1868, had occurred within the memory of man. It was not one city which was laid in ruins, but a whole empire. Those who perished were counted by tens of thousands, while the property destroyed by the earthquake was valued at millions of pounds sterling.
Although so many months have passed since this terrible calamity took place, scientific men have been busily engaged, until quite recently, in endeavoring to [pg 212] ascertain the real significance of the various events which were observed during and after the occurrence of the earthquake. The geographers of Germany have taken a special interest in interpreting the evidence afforded by this great manifestation of Nature's powers. Two papers have been written recently on the great earthquake of August 13th, 1868—one by Professor von Hochsteter, the other by Herr von Tschudi, which present an interesting account of the various effects, by land and by sea, which resulted from the tremendous upheaving force to which the western flanks of the Peruvian Andes were subjected on that day. The effects on land, although surprising and terrible, only differ in degree from those which have been observed in other earthquakes. But the progress of the great sea-wave which was generated by the upheaval of the Peruvian shores and propagated over the whole of the Pacific Ocean differs altogether from any earthquake phenomena before observed. Other earthquakes have indeed been followed by oceanic disturbances; but these have been accompanied by terrestrial motions, so as to suggest the idea that they had been caused by the motion of the sea-bottom or of the neighboring land. In no instance has it ever before been known that a well-marked wave of enormous proportions should have been propagated over the largest ocean tract on our globe by an earth-shock whose direct action was limited to a relatively small region, and that region not situated in the centre, but on one side of the wide area traversed by the wave.
We propose to give a brief sketch of the history of this enormous sea-wave. In the first place, however, [pg 213] it may be well to remind the reader of a few of the more prominent features of the great shock to which this wave owed its origin.
It was at Arequipa, at the foot of the lofty volcanic mountain Misti, that the most terrible effects of the great earthquake were experienced. Within historic times Misti has poured forth no lava streams, but that the volcano is not extinct is clearly evidenced by the fact that in 1542 an enormous mass of dust and ashes was vomited forth from its crater. On August 13th. 1868, Misti showed no signs of being disturbed. So far as the volcanic neighbor was concerned, the forty-four thousand inhabitants of Arequipa had no reason to anticipate the catastrophe which presently befell them. At five minutes past five an earthquake shock was experienced, which, though severe, seems to have worked little mischief. Half a minute later, however, a terrible noise was heard beneath the earth; a second shock more violent than the first was felt, and then began a swaying motion, gradually increasing in intensity. In the-course of the first minute this motion had become so violent that the inhabitants ran in terror out of their houses into the streets and squares. In the next two minutes the swaying movement had so increased that the more lightly built houses were cast to the ground, and the flying people could scarcely keep their feet. "And now," says Von Tschudi, "there followed during two or three minutes a terrible scene. The swaying motion which had hitherto prevailed changed into fierce vertical upheaval. The subterranean roaring increased in the most terrifying manner; then were heard the heart-piercing shrieks of the [pg 214] wretched people, the bursting of walls, the crashing fall of houses and churches, while over all rolled thick clouds of a yellowishblack dust, which, had they been poured forth many minutes longer, would have suffocated thousands." Although the shocks had lasted but a few minutes, the whole town was destroyed. Not one building remained uninjured, and there were few which did not lie in shapeless heaps of ruins.
At Tacna and Arica the earth-shock was less severe, but strange and terrible phenomena followed it. At the former place a circumstance occurred the cause and nature of which yet remain a mystery. About three hours after the earthquake—in other words, at about eight o'clock in the evening—an intensely brilliant light made its appearance above the neighboring mountains. It lasted for fully half an hour, and has been ascribed to the eruption of some as yet unknown volcano.
At Arica the sea-wave produced even more destructive effects than had been caused by the earthquake. About twenty minutes after the first earth-shock the sea was seen to retire, as if about to leave the shores wholly dry; but presently its waters returned with tremendous force. A mighty wave, whose length seemed immeasurable, was seen advancing like a dark wall upon the unfortunate town, a large part of which was overwhelmed by it. Two ships, the Peruvian corvette America, and the United States "double-ender" Wateree, were carried nearly half a mile to the north of Arica beyond the railroad which runs to Tacna, and there left stranded high and dry. This enormous wave was considered by the English vice-consul at Arica to have been fully fifty feet in height.
[pg 215]At Chala three such waves swept in after the first shocks of earthquake. They overflowed nearly the whole of the town, the sea passing more than half a mile beyond its usual limits.
At Islay and Iquique similar phenomena were manifested. At the former town the lava flowed in no less than five times, and each time with greater force. Afterward the motion gradually diminished, but even an hour and a half after the commencement of this strange disturbance the waves still ran forty feet above the ordinary level. At Iquique the people beheld the inrushing wave while it was still a great way off. A dark blue mass of water some fifty feet in height was seen sweeping in upon the town with inconceivable rapidity. An island lying before the harbor was completely submerged by the great wave, which still came rushing on black with the mud and slime it had swept from the sea-bottom. Those who witnessed its progress from the upper balconies of their houses, and presently saw its black mass rushing close beneath their feet, looked on their safety as a miracle. Many buildings were indeed washed away, and in the low-lying parts of the town there was a terrible loss of life. After passing far inland, the wave slowly returned sea-ward, and, strangely enough, the sea, which elsewhere heaved and tossed for hours after the first great wave had swept over it, here came soon to rest.
At Callao a yet more singular instance was afforded of the effect which circumstances may have upon the motion of the sea after a great earthquake has disturbed it. In former earthquakes Callao has suffered terribly from the effects of the great sea-wave. In [pg 216] fact, on two occasions the whole town has been destroyed, and nearly all its inhabitants have been drowned, through the inrush of precisely such waves as flowed into the ports of Arica and Chala. But upon this occasion the centre of subterranean disturbance must have been so situated that either the wave was diverted from Callao, or, more probably, two waves reached Callao from different sources and at different times, so that the two undulations partly counteracted each other. Certain it is that, although the water retreated strangely from the coast near Callao, insomuch that a wide tract of the sea-bottom was uncovered, there was no inrushing wave comparable with those described above. The sea afterward rose and fell in an irregular manner, a circumstance confirming the supposition that the disturbance was caused by two distinct oscillations. Six hours after the occurrence of the earth-shock the double oscillations seemed for a while to have worked themselves into unison, for at this time three considerable waves rolled in upon the town. But clearly these waves must not be compared with those which in other instances had made their appearance within half an hour of the earth-throes. There is little reason to doubt that if the separate oscillations had re-enforced each other earlier, Callao would have been completely destroyed. As it was, a considerable amount of mischief was effected; but the motion of the sea presently became irregular again, and so continued until the morning of August 14th, when it began to ebb with some regularity. But during the 14th there were occasional renewals of the irregular motion, and several days [pg 217] elapsed before the regular ebb and flow of the sea were resumed.
Such were among the phenomena presented in the region where the earthquake itself was felt. It will be seen at once that within this region, or rather along that portion of the seacoast which falls within the central region of disturbance, the true character of the seawave generated by the earthquake could not be recognized. If a rock fall from a lofty cliff into a comparatively shallow sea, the water around the place where the rock has fallen is disturbed in an irregular manner. The sea seems at one place to leap up and down; elsewhere one wave seems to beat against another, and the sharpest eye can detect no law in the motion of the seething waters. But presently, outside the scene of disturbance, a circular wave is seen to form, and if the motion of this wave be watched it is seen to present the most striking contrast with the turmoil and confusion at its centre. It sweeps onward and outward in a regular undulation. Gradually it loses its circular figure (unless the sea-bottom happens to be unusually level), showing that although its motion is everywhere regular, it is not everywhere equally swift. A wave of this sort, though incomparably vaster, swept swiftly away on every side from the scene of the great earthquake near the Peruvian Andes. It has been calculated that the width of this wave varied from one million to five million feet, or, roughly, from two hundred to one thousand miles, while, when in mid-Pacific, the length of the wave, measured along its summit in a widely-curved path from one side to another of the great [pg 218] ocean, cannot have been less than eight thousand miles.
We cannot tell how deep-seated was the centre of subterranean action; but there can be no doubt it was very deep indeed, because otherwise the shock felt in towns separated from each other by hundreds of miles could not have been so nearly contemporaneous. Therefore the portion of the earth's crust upheaved must have been enormous, for the length of the region where the direct effects of the earthquake were perceived is estimated by Professor von Hochsteter at no [pg 219] less than two hundred and forty miles. The breadth of the region is unknown, because the slope of the Andes on one side and the ocean on the other concealed the motion of the earth's crust.
The great ocean-wave swept, as we have said, in all directions around the scene of the earth-throe. Over a large part of its course its passage was unnoted, because in the open sea the effects even of so vast an undulation could not be perceived. A ship would slowly rise as the crest of the great wave passed under her, and then as slowly sink again. This may seem strange, at first sight, when it is remembered that in reality the great sea-wave we are considering swept at the rate of three or four hundred sea-miles an hour over the larger part of the Pacific. But when the true character of ocean-waves is understood, when it is remembered that there is no transference of the water itself at this enormous rate, but simply a transmission of motion (precisely as when in a high wind waves sweep rapidly over a cornfield, while yet each cornstalk remains fixed in the ground), it will be seen that the effects of the great sea-wave could only be perceived near the shore. Even there, as we shall presently see, there was much to convey the impression that the land itself was rising and falling rather than that the deep was moved. But among the hundreds of ships which were sailing upon the Pacific when its length and breadth were traversed by the great sea-wave, there was not one in which any unusual motion was perceived.
In somewhat less than three hours after the occurrence of the earthquake the ocean-wave inundated the [pg 220] port of Coquimbo, on the Chilean seaboard, some eight hundred miles from Arica. An hour or so later it had reached Constitucion, four hundred and fifty miles farther south; and here for some three hours the sea rose and fell with strange violence. Farther south, along the shore of Chile, even to the island of Chiloe, the shore-wave travelled, though with continually diminishing force, owing, doubtless, to the resistance which the irregularities of the shore opposed to its progress.
The northerly shore-wave seems to have been more considerable; and a moment's study of a chart of the two Americas will show that this circumstance is highly significant. When we remember that the principal effects of the land-shock were experienced within that angle which the Peruvian Andes form with the long north-and-south line of the Chilean and Bolivian Andes, we see at once that, had the centre of the subterranean action been near the scene where the most destructive effects were perceived, no seawave, or but a small one, could have been sent toward the shores of North America. The projecting shores of northern Peru and Ecuador could not have failed to divert the seawave toward the west; and though a reflected wave might have reached California, it would only have been after a considerable interval of time, and with dimensions much less than those of the sea-wave which travelled southward. When we see that, on the contrary, a wave of even greater proportions travelled toward the shores of North America, we seem forced to the conclusion that the centre of the subterranean action must have been so far to the west that the sea[pg 221] wave generated by it had a free course to the shores of California.
Be this as it may, there can be no doubt that the wave which swept the shores of Southern California, rising upward of sixty feet above the ordinary sea-level, was absolutely the most imposing of all the indirect effects of the great earthquake. When we consider that even in San Pedro Bay, fully five thousand miles from the centre of disturbance, a wave twice the height of an ordinary house rolled in with unspeakable violence only a few hours after the occurrence of the earth-throe, we are most strikingly impressed with the tremendous energy of the earth's movement.
Turning to the open ocean, let us track the great wave on its course past the multitudinous islands which dot the surface of the Pacific.
The inhabitants of the Sandwich Islands, which lie about six thousand three hundred miles from Arica, might have imagined themselves safe from any effects which could be produced by an earthquake taking place so far away from them. But on the night between August 13th and 14th, the sea around this island group 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, indeed, 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 [pg 222] floods being noticed at Honolulu, on the island of Woahoo.
At Yokohama, in Japan, more than ten thousand five hundred miles from Arica, an enormous wave poured in on August 14th, but at what hour we have no satisfactory record. So far as distance is concerned, this wave affords most surprising evidence of the stupendous nature of the disturbance to which the waters of the Pacific Ocean had been subjected. The whole circumference of the earth is but twenty-five thousand miles, so that this wave had travelled over a distance considerably greater than two-fifths of the earth's circumference. A distance which the swiftest of our ships could not traverse in less than six or seven weeks had been swept over by this enormous undulation in the course of a few hours.
More complete details reach us from the Southern Pacific.Shortly before midnight the Marquesas Isles and the low-lying Tuamotu group were visited by the great wave, and some of these islands were completely submerged by it. The lonely Opara Isle, where the steamers which run between Panama and New Zealand have their coaling station, was visited at about half-past eleven in the evening by a billow which swept away a portion of the coal depot. Afterward great waves came rolling in at intervals of about twenty minutes, and several days elapsed before the sea resumed its ordinary ebb and flow.
It was not until about half-past two on the morning [pg 223] of August 14th that the Samoa Isles (sometimes called the Navigator Islands) were visited by the great wave. The watchmen startled the inhabitants from their sleep by the cry that the sea was about to overwhelm them; and already, when the terrified people rushed from their houses, the sea was found to have risen far above the highest water-mark. But it presently began to sink again, and then commenced a series of oscillations, which lasted for several days, and were of a very remarkable nature. Once in every quarter of an hour the sea rose and fell, but it was noticed that it rose twice as rapidly as it sank. This peculiarity is well worth remarking. The eminent physicist Mallet speaks thus (we follow Lyell's quotation) about the waves which traverse an open sea: "The great sea-wave, advancing at the rate of several miles in a minute, consists, in the deep ocean, of a long, low swell of enormous volume, having an equal slope before and behind, and that so gentle that it might pass under a ship without being noticed. But when it reaches the edge of soundings, its front slope becomes short and steep, while its rear slope is long and gentle." On the shores visited by such a wave, the sea would appear to rise more rapidly than it sank. We have seen that this happened on the shores of the Samoa group, and therefore the way in which the sea rose and fell on the days following the great earthquake gave significant evidence of the nature of the seabottom in the neighborhood of these islands. As the change of the great wave's figure could not have been quickly communicated, we may conclude with certainty that the Samoan Islands are the summits of lofty mountains, whose sloping sides extend far toward the east.
[pg 224]This conclusion affords interesting evidence of the necessity of observing even the seemingly trifling details of important phenomena.
The wave which visited the New Zealand Isles was altogether different in character, affording a noteworthy illustration of another remark of Mallet's. He says that where the sea-bottom slopes in such a way that there is water of some depth close inshore, the great wave may roll in and do little damage; and we have seen that so it happened in the case of the Samoan Islands. But he adds that, "where the shore is shelving there will be first a retreat of the water, and then the wave will break upon the beach and roll far in upon the land." This is precisely what happened when the great wave reached the eastern shores of New Zealand, which are known to shelve down to very shallow water, continuing far away to sea toward the east.
At about half-past three on the morning of August 14th the water began to retreat in a singular manner from the port of Littleton, on the eastern shores of the southernmost of the New Zealand Islands. At length the whole port was left entirely dry, and so remained for about twenty minutes. Then the water was seen returning like a wall of foam ten or twelve feet in height, which rushed with a tremendous noise upon the port and town. Toward five o'clock the water again retired, very slowly as before, not reaching its lowest ebb until six. An hour later a second huge wave inundated the port. Four times the sea retired and returned with great power at intervals of about two hours. Afterward the oscillation of the water was less[pg 225] considerable, but it had not wholly ceased until August 17th, and only on the 18th did the regular ebb and flow of the tide recommence.
Around the Samoa group the water rose and fell once in every fifteen minutes, while on the shores of New Zealand each oscillation lasted no less than two hours. Doubtless the different depths of water, the irregular conformation of the island groups, and other like circumstances, were principally concerned in producing these singular variations. Yet they do not seem fully sufficient to account for so wide a range of difference. Possibly a cause yet unnoticed may have had something to do with the peculiarity. In waves of such enormous extent it would be quite impossible to determine whether the course of the wave motion was directed full upon a line of shore or more or less obliquely. It is clear that in the former case the waves would seem to follow each other more swiftly than in the latter, even though there were no difference in their velocity.
Far on beyond the shores of New Zealand the great wave coursed, reaching at length the coast of Australia. At dawn of August 14th Moreton Bay was visited by five well-marked waves. At Newcastle, on the Hunter River, the sea rose and fell several times in a remarkable manner, the oscillatory motion commencing at half-past six in the morning. But the most significant evidence of the extent to which the sea-wave travelled in this direction was afforded at Port Fairy, Belfast, South Victoria. Here the oscillation of the water was distinctly perceived at midday on August 14th; and yet, to reach this point, the sea-wave must [pg 226] not only have travelled on a circuitous course nearly equal in length to half the circumference of the earth, but must have passed through Bass's Straits, between Australia and Van Diemen's Land, and so have lost a considerable portion of its force and dimensions. When w£ remember that had not the effects of the earth-shock on the water been limited by the shores of South America, a wave of disturbance equal in extent to that which travelled westward would have swept toward the east, we see that the force of the shock was sufficient to have disturbed the waters of an ocean covering the whole surface of the earth. For the sea-waves which reached Yokohama in one direction and Port Fairy in another had each traversed a distance nearly equal to half the earth's circumference; so that if the surface of the earth were all sea, waves setting out in opposite directions from the centre of disturbance would have met each other at the antipodes of their starting-point.
It is impossible to contemplate the effects which followed the great earthquake—the passage of a sea-wave of enormous volume over fully one third of the earth's surface, and the force with which, on the farthermost limits of its range, the wave rolled in upon shores more than ten thousand miles from its starting-place—without feeling that those geologists are right who deny that the subterranean forces of the earth are diminishing in intensity. It may be difficult, perhaps, to look on the effects which are ascribed to ancient earth-throes without imagining for a while that the power of modern earthquakes is altogether less. But when we consider fairly the share which time had in [pg 227] those ancient processes of change, when we see that while mountain ranges were being upheaved or valleys depressed to their present position, race after race, and type after type appeared on the earth, and lived out the long lives which belong to races and to types, we are recalled to the remembrance of the great work which the earth's subterranean forces are still engaged upon. Even now continents are being slowly depressed or upheaved; even now mountain ranges are being raised to a new level, tablelands are in process of formation, and great valleys are being gradually scooped out. It may need an occasional outburst, such as the earthquake of August, 1868, to remind us that great forces are at work beneath the earth's surface. But, in reality, the signs of change have long been noted. Old shore-lines shift their place, old soundings vary; the sea advances in one place and retires in another; on every side Nature's plastic hand is at work modelling and remodelling the earth, in order that it may always be a fit abode for those who are to dwell upon it.
(From Studies of Animated Nature.) By
W.S. DALLAS.
It is not merely on land that this phenomenon of phosphorescence is to be seen in living forms. Among marine animals, indeed, it is a phenomenon much more general, much more splendid, and, we may add, much more familiar to those who live on our coasts. There must be many in the British Isles who have never had the opportunity of seeing the light of the glow-worm, but there can be few of those who have frequented in summer any part of our coasts, who have never seen that beautiful greenish light which is then so often visible, especially on our southern shores, when the water is disturbed by the blade of an oar or the prow of a boat or ship. In some cases, even on our own shores, the phenomenon is much more brilliant, every rippling wave being crested with a line of the same peculiar light, and in warmer seas exhibitions of this kind are much more common. It is now [pg 229] known that this light is due to a minute living form, to which we will afterward return.
But before going on to speak in some detail of the organisms to which the phosphorescence of the sea is due, it will be as well to mention that the kind of phosphorescence just spoken of is only one mode in which the phenomenon is exhibited on the ocean. Though sometimes the light is shown in continuous lines whenever the surface is disturbed, at other times, and, according to M. de Quatrefages, more commonly, the light appears only in minute sparks, which, however numerous, never coalesce. "In the little channel known as the Sund de Chausez," he writes, "I have seen on a dark night each stroke of the oar kindle, as it were, myriads of stars, and the wake of the craft appeared in a manner besprinkled with diamonds." When such is the case the phosphorescence is due to various minute animals, especially crustaceans; that is, creatures which, microscopically small as they are, are yet constructed more or less on the type of the lobster or cray-fish.
At other times, again, the phosphorescence is still more partial. "Great domes of pale gold with long streamers," to use the eloquent words of Professor Martin Duncan, "move slowly along in endless succession; small silvery disks swim, now enlarging and now contracting, and here and there a green or bluish gleam marks the course of a tiny, but rapidly rising and sinking globe. Hour after hour the procession passes by, and the fishermen hauling in their nets from the midst drag out liquid light, and the soft sea jellies, crushed and torn piecemeal, shine in every clinging[pg 230] particle. The night grows dark, the wind rises and is cold, and the tide changes; so does the luminosity of the sea. The pale spectres below the surface sink deeper, and are lost to sight, but the increasing waves are tinged here and there with green and white, and often along a line, where the fresh water is mixing with the salt in an estuary, there is a brightness so intense that boats and shores are visible.... But if such sights are to be seen on the surface, what must not be the phosphorescence of the depths! Every sea-pen is glorious in its light, in fact, nearly every eight-armed Alcyonarian is thus resplendent, and the social Pyrosoma, bulky and a free swimmer, glows like a bar of hot metal with a white and green radiance."
Such accounts are enough to indicate how varied and how general a phenomenon is the phosphorescence of the sea. To take notice of one tithe of the points of interest summed up in the paragraph just quoted would occupy many pages, and we must therefore confine the attention to a few of the most interesting facts relating to marine phosphorescence.
We will return to that form of marine luminosity to which we first referred: what is known as the general or diffused phosphorescence of the sea. From this mode of describing it the reader must not infer that the surface of the ocean is ever to be seen all aglow in one sheet of continuous light. So far, at least, as was ever observed by M. de Quatrefages, who studied this phenomenon carefully and during long periods on the coasts of Brittany and elsewhere, no light was visible when the surface of the sea was perfectly still. On the other hand, when the sea exhibits in a high degree the [pg 231] phenomenon of diffused phosphorescence no disturbance can be too slight to cause the water to shine with that peculiar characteristic gleam. Drop but a grain of sand upon its surface, and you will see a point of light marking the spot where it falls, and from that point as a centre a number of increasing wavelets, each clearly defined by a line of light, will spread out in circles all around.
The cause of this diffused phosphorescence was long the subject of curiosity, and was long unknown, but more than a hundred years ago (in 1764) the light was stated by M. Kigaut to proceed from a minute and very lowly organism, now known as Noctiluca miliaris; and subsequent researches have confirmed this opinion. This Noctiluca is a spherical form of not more than one-fiftieth of an inch in size, with a slight depression or indentation at one point, marking the position of a mouth leading to a short digestive cavity, and having close beside it a filament, by means of which it probably moves about. The sphere is filled with protoplasm, in which there is a nucleus and one or more gaps, or "vacuoles." Such is nearly all the structure that can be discerned with the aid of the microscope in this simple organism.
Nevertheless, this lowly form is the chief cause of that diffused phosphorescence which is sometimes seen over a wide exten