THE STORY OF A VAST EXPLOSION.
The greatest physical convulsion of recent times occurred on the morning of the 27th of August last year, the scene of the catastrophe being a small island in the Sunda Straits, which separate Sumatra and Java. It is a region which there is much reason to regard as one of the intensest foci of volcanic activity on the earth’s surface. The main facts connected with this event, although slow in coming to hand, are now fairly within the records of science. Krakatoa, the volcanic island which a year or two ago was seven miles long by five broad, is about thirty miles from the Java coast. When surveyed in 1868-69, the island was found to be clothed from base to summit with a luxuriant growth of forest and tropical vegetation, but uninhabited. A few weeks prior to the eruption, the volcano, which had been dormant for two centuries, gave signs of an awakening. On the 20th of May several shocks, accompanied by loud explosions and hollow reverberations, startled the inhabitants of the towns of Batavia and Buitenzorg, about ninety miles distant.[1] These disturbances continued for the next three months with more or less activity. On the 11th and 18th of August the energy of the volcano increased, and there were symptoms of a crisis. On the 26th and the night following, several eruptions took place, until the climax was reached on the following morning. The submarine base of the mountain then seems, according to all available evidence, to have literally ‘caved in.’ This was apparently accompanied by an influx of the sea into the molten interior, the instantaneous development of superheated steam, and then an explosion which, for its colossal energy, is unparalleled in the annals of volcanic outbreaks.
The enormous power of this eruption can only be adequately understood by its effects; these we now briefly summarise. The explosion itself, according to Dr Verbeek, one of the Dutch Commission appointed to investigate the nature and results of this catastrophe, caused the north part of the island to be blown away, and to fall eight miles to the north, forming what is now named Steer’s Island. Moreover, the north-east portion of the island of Krakatoa was also hurled into the air, passed over Lang Island, and fell at a distance of seven miles, forming what is now known as Calmeyer Island. In proof of this, we have the fact elicited by the newly made marine survey of the Straits, that ‘the bottom surrounding these new islands has not risen.’ This would have been the case had they been upheaved in the usual way. Not only so, but the bottom round these new islands shows a slightly increased depth in the direction of the submarine pit, nearly one thousand feet deep, which now marks the place the peak of Krakatoa occupied prior to the convulsion. But out of the midst of this deep depression there rises ‘like a gigantic club’ a remarkable column of rock of an area not more than thirty-three square feet, which projects sixteen feet above the surface of the sea. The southern part is all that is now left of the island of Krakatoa, and this fragment on its north side is now bounded by a magnificent precipitous cliff more than two thousand five hundred feet high. It has been thought by some, however, that the first portion of the island was blown away on the evening of August 26th, and that on the following morning the larger mass, answering to Calmeyer Island, was shot out by an effort still more titanic.
The shock of the explosion was felt at a distance of four thousand miles, being equal to an area of one-sixth of the earth’s surface—that is, at Burmah, Ceylon and the Andaman Islands to the north-west, in some parts of India, at Saigon and Manila to the north, at Dorey in the Geelvink Bay (New Guinea) to the east, and throughout Northern Australia to the south-west. Lloyd’s agents at Batavia, in Java, stated that on the eve of this vast explosion, the detonations ‘grew louder, till in the early morning the reports and concussions were simply deafening, not to say alarming.’ So violent were the air-waves, due to this cause, that walls were rent by them at a distance of five hundred miles, and so great the volume of smoke and ashes, that Batavia, eighty miles off, was shrouded in complete darkness for two hours. Nearly four months after the eruption, masses of floating pumice, each several acres in extent, were seen in the Straits of Sunda.
Paradoxical as it appears, the sound was sometimes better heard in distant places than in those nearer the seat of disturbance. This singular effect has been thus explained—assuming, for example, the presence of a thick cloud of ashes between Krakatoa and Anjer, this would act on the sound-waves like a thick soft cushion; along and above such an ash-cloud the sound would be very easily propelled to more remote places, for instance, Batavia; whereas at Anjer, close behind the ash-cloud, no sounds, or only faint ones, would be heard. Other explanations seem to be less probable, though not impossible.
Dr Verbeek states that within a circle of nine and a-half miles’ radius (fifteen kilomètres) from the mountain, the layers of volcanic ash cover the ground to a depth of from sixty-five to one hundred and thirty feet, and at the back of the island the thickness of the ash-mountains is in some places even from one hundred and ninety-five to two hundred and sixty feet, and that the matter so projected extends over a known area of seven hundred and fifty thousand square kilomètres (285,170 square miles), or a space larger than the German Empire with the Netherlands and Belgium, including Denmark and Iceland, or nearly twenty-one times the size of the Netherlands. Moreover, he calculates that the quantity of solid substance ejected by the volcano was eighteen cubic kilomètres, or 4.14 cubic miles. To give some idea of the enormous volume this represents, we may take the following illustration: the largest of the Egyptian pyramids has upwards of eighty-two millions of cubic feet of masonry; it would therefore take about seven thousand three hundred and sixty of such structures to equal the bulk of matter thrown out by this eruption. Some of this matter was found to contain smooth round balls from five-eighths to two and a-quarter inches in diameter, and composed of fifty-five per cent. of carbonate of lime.
As may well be imagined, the final outburst by its awful energy gave rise to a succession of air-waves. These we now know went round the earth more than once, and recorded themselves on the registering barometers or barograms at the Mauritius, Berlin, Rome, St Petersburg, Valencia, Coimbra (Portugal), and other far-distant places. At some points, as many as seven such disturbances were noted; other instruments not so sensitive gave evidence of five, by which time the wave had pretty well spent itself.
Having collected the observations made at all the chief meteorological stations, General Strachey recently read a paper before the Royal Society which, in his opinion, conclusively shows that an immense air-wave started from Krakatoa at about thirty minutes past nine A.M. on August 27th. Spreading from this common centre, the wave went three and a-quarter times round the globe, and those parts of it which had travelled in opposite directions passed through one another ‘somewhere in the antipodes of Java.’ The velocity of the aërial undulations which travelled from east to west was calculated at six hundred and seventy-four miles per hour, those moving in the reverse direction at seven hundred and six miles per hour, or nearly the velocity of sound.
But another effect of the eruption was a series of ‘tidal waves,’ so called—although the term is objected to because not strictly scientific—which, like the air-wave, passed round the world. Whether this was synchronous with the final explosion, it is not possible to say. The highest of these seismic sea-waves, which was over one hundred feet high, swept the shores on either side of the Straits, and wrought terrible destruction to life and property. More than thirty-five thousand persons perished through it; the greater part of the district of North Bantam was destroyed, the towns of Anjer, Merak, Tjeringin, and others being overwhelmed.
The initial movement of this destructive agent was undoubtedly of the nature of a negative wave; but the best testimony to this is lost, since those who witnessed it were its victims. The sudden subsidence of so large an area of the sea-bottom in the Straits caused the sea to recede from the neighbouring shores. This negative wave was, however, seen by Captain Ferrat from his vessel, as she lay at anchor at Port Louis. He states that towards two P.M. he saw the water in the harbour roll back and suddenly fall four or five feet; and that, a quarter of an hour afterwards, the sea returned with great violence to its former level, causing his own and other vessels to roll terribly. The best witness of this remarkable phenomenon, however, is Captain Watson, of the British ship Charles Ball. His vessel was actually within the Straits, and he states that he and his helmsman ‘saw a wave rush right on to Button Island, apparently sweeping right over the south part, and rising half-way up to the north and east sides fifty or sixty feet, and then continuing on to the Java shore. This was evidently a wave of translation and not of progression, for it was not felt at the ship.’ This latter movement, beyond question, must have coincided with the great ‘tidal wave’ above mentioned, and which was felt at Aden, on the Ceylon coast, Port Blair, Nagapatam, Port Elizabeth, Kurrachee, Bombay, and half-way up to Calcutta on the Hooghly, the north-west coast of Australia, Honolulu, Kadiall in Alaska, San Celeto near San Francisco, and the east coast of New Zealand.