Sir Robert Ball

AN ASTRONOMER'S THOUGHTS ABOUT KRAKATOA

Sir Robert Stawell Ball

AN event like the great eruption of Krakatoa can only be studied properly when placed in suitable perspective. Accordingly years have been required before sufficient data could be collected to enable us to take an adequate view of the several incidents of the explosion. The eruption of Krakatoa in August, 1883, was not only a mighty and appalling incident in the neighbourhood of the Straits of Sunda. It was there no doubt that the fatal aspects of the disaster were exclusively developed.  It was along the shores of Sumatra and Java that the inundations took place in which 36,380 lives are said to have been lost. But the phenomena of Krakatoa, which give it a peculiar interest, are of an innocuous type, and have had a far wider range than those of a tragical character. The shock given to our globe was such that the influence of the explosion has extended in some degree to almost every part. To appreciate all that Krakatoa implies it is there-fore not sufficient merely to gather the information which can be procured at the seat of the volcano itself; we must extend our inquiries much farther afield. We have to learn what observers within many hundreds of miles tell us. Ships' logs have to be examined. The records of barometers and of magnetic instruments all over the globe, even to the very antipodes of Krakatoa, have to be brought together. The descriptions of extraordinary optical phenomena, such as wonderful ruddy glows at sunset and sunrise, or strange hues in which the sun and the moon were occasionally decked, have to be collected and scrutinized from numerous places scattered over both hemi-spheres. Need it be said that such a task as this must be a protracted one, but it has been accomplished, and now those interested in the matter have the opportunity of studying a unique chapter in the history of the earth.

It is to the Royal Society that we are indebted for the inception and the carrying out of this laborious undertaking. They appointed a Krakatoa Committee, under the chairmanship of Mr. G. J. Symons.  So multitudinous were the phenomena to be investigated that the committee was divided into sections. To examine the eruption itself and the volcanic phenomena generally, a geological section was necessary. To study the air-waves and the sounds, as well as the distribution of dust and pumice by wind and water, required the aid of meteorologists. On the border territory, between the sciences of meteorology and of astronomy, must be placed the investigation of the twilight effects and the strange coronas and weird colours of the sun and moon.   The great sea-waves must clearly be studied by hydrographers, and there were also some groups of facts connected with terrestrial magnetism and electricity. Immense numbers of letters and reports from all parts of the globe had to be brought to a focus, and  the extensive printed literature relating to Krakatoa had to be ransacked. At length, however, by the spring of 1887, the manuscript was completed, and, in the autumn of 1888, a superb quarto volume of nearly 500 pages, copiously illustrated both by artistic drawings and by charts and maps, was issued.

Midway between Sumatra and Java lies a group of small islands, which, prior to 1883, were beautified by the dense forests and glorious vegetation of the tropics. Of these islands Krakatoa was the chief, though even of it but little was known. Its appearance from the sea must, indeed, have been familiar to the crews of the many vessels that navigated the Straits of Sunda, but it was not regularly inhabited. Glowing with tropical verdure, such an island seemed an unlikely theatre for the display of an unparalleled effect of plutonic energy, but yet there were certain circumstances which may tend to lessen our surprise at the outbreak. In the first place, as Professor Judd has so clearly pointed out, not only is Krakatoa situated in a region famous, or perhaps infamous, for volcanoes and earthquakes, but it actually happens to lie at the intersection of two main lines, along which volcanic phenomena are, in some degree, perennial. In the second place, history records that there have been previous eruptions at Krakatoa.  The last of these appears to have occurred in May, 1680, but unfortunately only imperfect accounts of it have been preserved. It seems, however, to have annihilated the forests on the island, and to have ejected vast quantities of pumice, which cumbered the seas around. Krakatoa then remained active for a year and a half, after which the mighty fires subsided. The  irrepressible tropical vegetation again resumed possession. The desolated islet again became clothed with beauty, and for a couple of centuries reposed in peace.

A few significant warnings were given before the recent tremendous outbreak.  Admonitory earthquakes began to be felt in the vicinity some years before, and for a period of three months Krakatoa was gradually preparing for the majestic performance with which the world was astounded on August 26-27. The inhabitants of those regions were so accustomed to be threatened by volcanic phenomena that the early stages of the outbreak, which began on May 20, do not seem to have created any alarm; quite the reverse, indeed, for a pleasant excursion was organized from Batavia, and a trip made to Krakatoa in a steamer, to see what was going on. The party landed on the island, and found a large basin-shaped crater, more than half a mile across at the top, and almost 150 feet deep. In the centre of this was an aperture 150 feet in diameter, from which a column of steam issued with a terrific noise. Even at this early stage of the eruption the volcanic dust was projected aloft in quantities sufficient to be wafted to the adjoining shores of Sumatra and Java.

For the next fortnight or three weeks the intensity of the eruptive phenomena seemed at first to decline, but about the end of June other craters began to open on the island, and the volcanic energy from that date increased until the mighty climax. The actual nature of that awful event can only be imperfectly known. The Straits of Sunda were no longer a pleasant place for a steamboat excursion. They had become the theatre of an appalling catastrophe. For many hours the adjacent shores were wrapped in profound darkness, while the tremendous agitation of the volcano originated great sea waves which swept away entire towns and villages, and in a great measure destroyed their populations.

It was one o'clock in the afternoon of Sunday, August 26, 1883, when Krakatoa commenced a series of gigantic volcanic efforts. Detonations were heard which succeeded each other at intervals of about ten minutes. These were loud enough to penetrate as far as Batavia and Buitenzorg, distant 96 and 100 miles respectively from the volcano. A vast column of steam, smoke, and ashes ascended to a prodigious elevation. It was measured at 2 P.M. from a ship 76 miles away, and was then judged to be 17 miles high—that is, three times the height of the loftiest mountain in the world. As the Sunday afternoon wore on, the volcanic manifestations became ever fiercer.  At 3 P.M. the sounds were loudly heard in a town 150 miles away. At 5 P.M. every ear in the island of Java was engaged in listening to volcanic explosions, which were considered to be of quite unusual intensity even in that part of the world. These phenomena were, however, only introductory. Krakatoa was gathering strength. Between 5 and 6 P.M. the British ship Charles Sal, commanded by Captain Watson, was about ten miles south of the volcano. The ship had to shorten sail in the darkness, and a rain of pumice, in large pieces and quite warm, fell upon her decks. At 7 P.M. the mighty column of smoke is described as having the shape of a pine-tree, and as being brilliantly illuminated by electric flashes. The sulphurous air is laden with fine dust, while the lead dropped from a ship in its anxious navigation astounds the leadsman by coming up hot from the bottom of the sea.  From sunset on Sunday till midnight the tremendous detonations followed each other so quickly that a continuous roar may be said to have issued from the island. The full terrors of the eruption were now approaching.   The distance of 96 miles between Krakatoa and Batavia was not sufficient to permit the inhabitants of the town to enjoy their night's sleep. All night long the thunders of the volcano sounded like the discharges of artillery at their very doors, while the windows rattled with aerial vibrations.

On Monday morning, August 27, the eruption culminated in four terrific explosions, of which the third, shortly after 10 A.M. Krakatoa time, was by far the most violent.  The quantity of material ejected was now so great that darkness prevailed even as far as Batavia soon after 11 A.M., and there was a rain of dust until three in the afternoon. The explosions continued with more or less intensity all the afternoon of Monday and throughout Monday night. They finally ceased at about 2-30 A.M. on Tuesday, August 28. The entire series of grand phenomena thus occupied a little more than thirty-six hours.

We may imagine several different standards by which the significance of a volcanic outbreak is to be estimated. The most obvious standard of comparison is, of course, that of the quantity of materials which are extruded. Another would be the area covered by the clouds of volcanic dust and the duration of the darkness thus caused. Other standards would be sought in the incidental effects of the outbreak, such as the great waves which are thereby propagated in the sea, and the distances to which the

sounds are carried.   Other more subtle, but not less interesting, phenomena are the waves in the atmospheric ocean, which are neither seen nor heard, but of which the barometer gives no uncertain indications.  Among the remaining effects of a volcanic explosion are the curious sunset glows and the strange optical phenomena which are sometimes witnessed. We have thus a number of distinct points of view from which the significance of a volcano can be estimated.

We had all heard so much about Krakatoa that at nrst it is a little disappointing to read the assurances of Professor Judd that, so far as the first two of these standards are concerned, Krakatoa has been surpassed by other volcanoes. He enumerates three distinct out-breaks-viz., that of Papandayang, in Java, in 1772-of Skaptar Jokull (Yarmardalr), in Iceland, in 1783.' and of Tomboro, in Sumbawa, in 1815--in all of which the quantity of matter poured forth was considerably greater than that from Krakatoa. However, even in this respect the achievements of Krakatoa if second-rate are at least respectable.  The estimates made are necessarily founded on precarious data, but it seems to be certain that it all the materials poured forth from Krakatoa during the critical period could be collected together the mass they would form would be considerably over a cubic mile in volume. It is in the other standards of comparison that the importance of the explosion at Krakatoa is to be sought. The intensity of this outbreak in its last throes was such that mighty sounds were heard and mighty waves arose in the sea for which we can find no parallel. Every part of our globe's surface felt the pulse of the air-waves, and beautiful optical phenomena made the circuit of the globe even more than once or twice. In these last respects the eruption of Krakatoa is unique.

Professor Judd has satisfactorily accounted for the enormous manufacture of dust during the eruption. It appears to consist of comminuted pumice, and is produced by the attrition of the pumice masses, as in successive out-bursts they are hurled aloft, and then tumble back again into the crater.

It appears to me that the most remarkable incident connected with the eruption of Krakatoa was the production of the great air-wave by that particular explosion that occurred at ten o'clock on the morning of Monday, August 27. The great air-wave was truly of cosmical importance, affecting as it did every particle of the atmosphere on our globe. This phenomenon alone extends the study of Krakatoa beyond the province of vulcanology, and gives to the subject a particular interest in physical science.

A pebble tossed into a pond of unruffled water gives rise to a beautiful series of circular waves that gradually expand and ultimately become evanescent. A very large body falling into the ocean would originate waves that might diverge for miles from the centre of disturbance ere they became inappreciable. Waves can originate in air as well as in water. We are not at this moment speaking of those familiar air-waves by which sounds are conveyed. The waves we now mean are inaudible and apparently much longer undulations than those of sound.

Imagine a great globe, which for simplicity we may think of as smooth all over. Let us suppose that this globe has the stupendous dimensions, and expressed by a diameter of 8,000 miles, and imagine it to be enclosed in a uniform shell of air. Now, suppose that all is quiet, till at some point, which for the moment we may speak of as the pole, a mighty disturbance is originated. Let us regard this disturbance as produced by a sudden but local pushing up of the atmosphere by a force directed from the earth's surface outwards, and let us trace the effect thereby produced on the atmosphere. Such a sudden impulse will at once initiate a series of circular atmospheric waves, which will speed away from the centre of disturbance just like the waves caused by the pebble in the pond. If the original atmospheric impulse be large enough we shall find the circle growing larger and larger, its radius in-creasing from hundreds of miles to thousands of miles, until at last the wave reaches the equator. What is to happen when the diverging waves have attained the equator, and are now confronted by the opposite hemi-sphere ? This is one of those cases in which the mathematician can guide us where the experimentalist would be otherwise somewhat at fault. We know that as the wave entered the opposite hemisphere it would at once move through a similar series of changes to those through which it had already gone, but in the inverse order. The wave will thus, after leaving the equator, glide onwards into a parallel small circle, ever decreasing in diameter, and con-verging toward the anti-pole. Finally, just as the waves all radiated from the original pole, so will they all concentrate towards the opposite one. But what is now to happen ? Here, again, the mathematician will inform us. He can follow the oscillations after their confluence. He finds that from the anti-pole they will again commence to diverge.  Again they will expand, again they will reach the equator, and again will they gradually draw into concentration at the original pole. Nor will the process even here end. From the second confluence there will be a new divergence, and thus the oscillations will be sent quivering from one pole of the ^lobe to the other, until they gradually subside by friction.

This comprehensive series of phenomena wherein the atmosphere of the entire globe participates in an organized vibration has, so far as we know, only once been witnessed, and that was after the greatest outbreak at Krakatoa, at ten o'clock on the morning of August 27. But the ebb and the flow of these mighty undulations are not immediately appreciable to the senses. The great wave, for instance, passed and re-passed and passed again over London, and no inhabitant was conscious of the fact. But the automatic records of the barometer at Greenwich show that the vibration from Krakatoa to its antipodes, . and from the antipodes back to Krakatoa, was distinctly perceptible over London, not less then six or seven times. The instruments at the Kew, Observatory confirm those at Greenwich, and if further confirmation were required it can be had from the barograms at many other places in England. This is truly a memorable incident, and the scientific value of the labours of those who so diligently obtain automatic barometric records year after year would be amply demonstrated, if demonstration were required, by this single discovery of the great Krakatoa air-wave.

From all parts of Europe, from Berlin to Palermo, from St Petersburg to Valencia, we obtain the same indications.

Fortunately self-recording barometric instruments are now to be found all over the world. Almost all the instruments show distinctly the first great wave from Krakatoa to its antipodes in Central America, and the return wave from the antipodes to Krakatoa. They also all show the second great wave which sped from Krakatoa, as well as the second great wave which returned from the antipodes. Thus, the first four of the oscillations are depicted on up-wards of forty of the barograms. The fifth and sixth oscillations are also to be distinguished on several of the curves, and even the seventh is certainly established at some few places, of which Kew is one. Then the gradually increasing faintness of the indications renders them unrecognisable, from which we conclude that after seven pulsations our atmosphere had sensibly regained its former condition ere it was disturbed by Krakatoa.

Among the instruments which have yielded valuable information about the air-wave, we have, curiously enough, to mention the register of the recording gasometer-indicator at Batavia. This apparatus, designed and employed for a widely different purpose, shows that extraordinary fluctuations in the barometric pressure occurred at the time when the great wave passed over the town.

It is of particular interest, from a physical point of view, to study the numerical facts with reference to the speed at which this world-embracing wave was propagated. We shall for this purpose select the records taken at Greenwich. The phase of the wave found most convenient for measurement was the depression following the outbreak, and the moment at which this phase started from Krakatoa was 3 hrs. 32 mins. P.M. on August 17, Greenwich mean time. This is probably correct within two or three minutes. Diverging from its source this wave reached Greenwich after an interval of a little more than ten hours. The interesting point is, however, the determination of the period of a complete oscillation, that is to say, the interval between the passage of the wave over Greenwich and the next passage of the wave in the same direction also over Greenwich. It has been found convenient to designate the successive waves as i., ii., iii., iv., &c., the odd numbers being those from Krakatoa to its antipodes, and the even numbers being the return waves from the antipodes to Krakatoa.  At Greenwich, for example, we find the interval between i. and iii. to have been 36.47 hours, between iii. and v. 36.82 hours, and between v. and vii. 37.05 hours. For the return waves the intervals between ii. and iv. was 34.78 hours, and between iv. and vi. 35.25 hours. The similar values vary slightly when obtained at the several stations, but the average results indicate that for its first circuit of the earth the wave required 36 hrs. 24 mins., for the second 36 hrs. 30 mins., and for the third 36 hrs. 50 mins. The similar periods for the waves travelling in the reverse way were 34 hrs. 46 mins., and 35 hrs. 4 mins. respectively. The average of all is very nearly a day and a half.

Before leaving this part of the subject, I must refer to the approximate identity between the velocity of this aerial disturbance and the velocity of ordinary sound. This is well brought out by General Strachey. The speed of the wave varied from 674 to 726 miles per hour. The speed of sound propagation is 723 miles at zero Fahrenheit, and is 781 miles at 80° Fahrenheit. Considering that the waves had, of course, to cross the poles in their journeys, it would almost seem that within the limits of probable error the speed of the great wave and the speed of ordinary sound waves were identical. It would, I think, have been an improvement on the plates containing the barograms, if the scale had been given, so that it would have been possible to obtain some definite notion of the amplitudes of the oscillations at the different stations. The only pressure-diagram contained in the plates which does give any scale measures, is that of the gasholder at Batavia; from this it would appear that the barometric fluctuation produced by the great wave was about four-tenths of an inch of mercury at a distance of 100 miles from the source of disturbance.

While the chapter on the air-waves is the most novel scientific feature in the Report of the Krakatoa Committee, it will be admitted that the most amazing features of the same work are those contained in the section on " Sounds."  Here we find a collection of statements so marvelous that they would be well-nigh incredible were it not for the ample body of excellent testimony by which they are substantiated.  In the whole annals of noise there is nothing which can be compared to the records set forth in a table which occupies not less than eight pages of the volume. Let us select a few instances, almost at random.

Lloyd's agent at Batavia, 94 miles distant, says that on the morning of the 27th of August the reports and concussions were simply deafening. At Carimon, Java Island, reports were heard which led to the belief that some vessel offshore was making signals of distress, and  boats were accordingly put out to render succour, but no vessel was found, as the reports were from Krakatoa, at a distance of 355 miles. At Macassar, in Celebes, explosions were heard all over the province. Two steamers were sent out to discover the cause, for the authorities did not then know that what they heard came from Krakatoa, 969 miles away. But mere hundreds of miles will not suffice to exemplify the range of this stupendous siren. In St. Lucia Bay, in Borneo, a number of natives, who had been guilty of murder, thought they heard the sounds of vengeance in the approach of an attacking force. They fled from their village, little fancying that what alarmed them really came from Krakatoa, 1116 miles distant. All over the island of Timor alarming sounds were heard, and so urgent did the situation appear that the Government was aroused, and sent off a steamer to ascertain the cause. The sounds had, however, come 1351 miles, all the way from Krakatoa. In the Victoria Plains of West Australia the inhabitants were startled by the discharge of artillery— an unwonted noise in that peaceful district—but the artillery was at Krakatoa, 1700 miles distant. The inhabitants of Daly Waters, in South Australia, were rudely awakened at midnight on Sunday, August 26, by an explosion resembling the blasting of a rock, which lasted for a few minutes. The time and other circumstances show that here again was Krakatoa heard this time at the monstrous distance of 2023 miles. But there is undoubted testimony that to distances even greater than 2023 miles the waves of sound conveyed tidings of the mighty convulsion. Diego Garcia, in the Ohagos Islands, is 2267 miles from Krakatoa, but the thunders traversed even this distance, and created the belief that there must be some ship in distress, for which a diligent but necessarily ineffectual search was made. To pass at once to the most remarkable case of all, we have a report from Mr. James

Wallis, chief of police in Rodriguez, that “several times during the night of August 26-27, 1883, reports were heard coming from the eastward, like the distant roar of heavy guns.  These reports continued at intervals of between three and four hours." Were it not for the continuous chain of evidence from places at gradually increasing distances from Krakatoa, we might well hesitate to believe that the noises Mr. Wallis heard were really from the great volcano, but a glance at the map, which shows the several stations where the great sounds were heard, leaves no room for doubt. We have thus the astounding fact that almost across the whole wide extent of the Indian Ocean, that is, to a distance of nearly 3000 miles (2968), the sound of the throes of Krakatoa was propagated.

We appreciate this result more strikingly if we reflect on the velocity of sound. Seconds or minutes may elapse between the appearance of a flash of lightning and the arrival of the thunder. But the volcanic sounds could not have been heard at Rodriguez until four hours after they had commenced to travel from Krakatoa. Were Vesuvius now to break out as Krakatoa has done, every inhabitant of Great Britain would apparently be quite near enough to hear the awful detonation.

I shall content myself with the mention of three facts in illustration of the great sea waves which accompanied the eruption of Krakatoa. Of these, probably the most unusual is the magnitude of the area over which the undulations were perceived.   Thus, to mention but a single instance, and that not by any means an extreme one, we find that the tide gauge at Table Bay reveals waves which, notwithstanding that they have travelled 5100 miles from Krakatoa, have still a range of eighteen inches when they arrive at the southern coast of Africa. The second fact that I mention illustrates the magnitude of the seismic waves by the extraordinary inundations that they produced on the shores of the Straits of Sunda.

Captain Wharton shows that the waves, as they deluged the land, must have been fifty feet, or, in one well-authenticated case, seventy-two feet high. It was, of course, these vast floods which caused the fearful loss of life. The third illustrative fact concerns the fate of a man-of-war, the Berouw. This unhappy vessel was borne from its normal element and left high and dry in Sumatra, a mile and three-quarters inland, and thirty feet above the level of the sea.

Such incidents are not so unusual as the exquisite series of optical phenomena which has made most of the nations on the earth spectators in some degree of the wonders of Krakatoa. Resounding as were the crashes of the explosions, they still subsided thousands of miles to the east of Great Britain, and though the great aerial vibrations tingled to and fro through the air over every part of this globe, yet they were not perceptible to our unaided senses. But now we are to consider a splendid series of phenomena which scorned limitations of distance, and which obtruded their glories on our notice for weeks and even months together.

One of the most striking maps that the Report of the Royal Society contains is that which illustrates the progress of the main sky-phenomena from August 26 (evening) to September 9 1883. I doubt if the skies have ever presented to our vision, within atmospheric limits, a more singular series of phenomena than those which are most clearly depicted within the modest limits of this little map. (See Fig. 20.) Let me endeavour from the series of maps, of which this is one, as well as from the abundant body of matter so luminously set forth by the Hon. F. A. Rollo Russell and Mr. E. Douglas Archibald, to present a brief outline of this elaborately beautiful series of phenomena and their cause.

During the crisis on August 26-27, the volume of material blown into the air was sufficiently dense to • obscure the coasts of Sumatra to such a degree that at 10 A.M. the darkness there is stated to have been more intense than it is even in the blackest of nights. The fire-dust ascended to an elevation which, as we have already mentioned, is estimated to have been as much as seventeen miles. Borne aloft into these higher regions of our atmosphere, the clouds of dust at once became the sport of the winds and the currents which may be found there. If we had not previously known the prevailing tendency of the winds at these elevations and in these latitudes, the journey of the Krakatoa dust would have taught us. We shall confine our attention for the pre-sent to the chief phenomena, and we begin with the manifestation of these phenomena which were witnessed in the tropics.

It seems certain that, having attained their lofty elevation, the mighty clouds of dust were seized by easterly winds, and were swept along with a velocity which may not improbably be normal at a height of twenty miles above the earth's surface. It has been demonstrated by Dr. Vettin, at Berlin, that the upper cirrus clouds in winter at a height of only four or five miles have an aver-age velocity of 44-5 miles an hour. The Rev. W. Clement Ley has shown that the velocities of the upper cirrus clouds often amount to 120 miles an hour. These facts enable us without hesitation to attribute velocities to the great clouds of Krakatoa dust which shall be quite sufficient to account for the various phenomena.

It appears that this cloud of dust started immediately from Krakatoa for a series of voyages round the world. The highway which it at first pursued may, for our pre-sent purpose, be sufficiently defined by the Tropic of Cancer and the Tropic of Capricorn, though it hardly approached these margins at first. Westward the dust of Krakatoa takes its way. In three days it had crossed the Indian Ocean and was rapidly flying over the heart of Equatorial Africa; for another couple of days it was making a transatlantic journey; and then it might be found, for still a couple of days more, over the forests of Brazil ere it commenced the great Pacific voyage, which brought it back to the East Indies. The dust of Krakatoa had put a girdle round the earth in thirteen days ! The shape of the cloud appears to have been elongated, so that it took two or three days to complete the passage over any stated place.

When the dust-cloud had regained the Straits of Sunda the great eruption was over, but the winds were still the same as before, and again the comminuted pumice sped on its impetuous career. The density of the cloud had, however, lessened.  Doubtless much of the material was subsiding, and the remainder was becoming diffused over a wider area. Accordingly, we find that the track of the stream during this second revolution is some-what wider than it was on the first, though still mainly confined between the tropics. The speed with which the dust revolved was, however, unabated. Continents and oceans were again swept over with a velocity double that of an express train, and again the earth was surrounded within the fortnight.  The dust-cloud had now further widened its limits, but was still distinguishable, and with unlessened speed commenced for a third time to encircle the earth. The limits of the stream had spread themselves outside the tropics, though still falling short of Europe. There is no reason to think that there was any decline in the velocity of 76 miles per hour, but the gradual diffusion of the dust begins to obliterate the indications by which its movements could be perceived, so that during, and after, the third circuit the phenomena became so dif-fused that while their glory covered the earth, the distinction between the successive returns had vanished.  In November the area which contained the Krakatoa dust had sufficiently expanded from its original tropical limits to include Europe and the greater part of North America. During the winter months the suspended material gradually subsided or, at all events, became evanescent, and in the following spring the earth regained its normal state in so far as the Straits of Sunda were concerned.

It remains to give some brief account of the optical phenomena due to the presence of dust, unusual both in quantity and in character, in the upper atmosphere. The frontispiece of the volume shows some beautiful pictures of the twilight and after-glow effects as seen by Mr. W. Ascroft on the bank of the Thames a little west of London on the evening of November 26, 1883. Analogous phenomena to those here depicted were seen almost universally during November and December in the same year.

Who is there that does not remember the wondrous loveliness of the twilights and the after-glows during that remarkable winter! These appearances at sunrise and sunset are only the more generally recognised of a whole system of strange optical phenomena. One of the most striking indications of the presence of the dust-stream in its first voyage round the earth was given by the strange blue hue it imparted to the sun. The dust-stream was also visible in its rapid voyages as a lofty haze or extensive cloud of cirro-stratus. Then, too, strange haloes were often seen, there were occasional blue or green moons, and the sun was sometimes glorified by a corona that had its origin in our atmosphere. Everywhere in the world there were remarkable features in the sky that winter: from Tierra del Fuego to Lake Superior; from China to the Gulf of Guinea ; from Panama to Australia. Wherever on land there were inhabitants with sufficient intelligence to note the unusual, wherever on the sea there were mariners who kept a careful log, from all such observers we learn that in the autumn and winter months following the great eruption of Krakatoa, there were extraordinary manifestations witnessed in the heavens.

Just one point more in conclusion. We have recorded the great volcanic outbreak of Krakatoa, and we have recorded a wonderful series of optical phenomena. It remains to say a word as to the proof that the latter were indeed the consequence of the former. As the Committee have begun their book with pictures of sun-glows, and as they have occupied more than half of the work with descriptions of the purely optical effects, it seems as if they, at all events, entertained but little doubt that the dust of Krakatoa was responsible for the sunsets of

Chelsea. Still I notice that some members of the Committee seem to shrink from deliberately committing them-selves to this view. Indeed, the very title of their book exhibits a certain degree of caution on this point. They have called it “The Eruption of Krakatoa and subsequent

Phenomena.”  The word I have italicized would not improbably have been consequent had it not been for the existence of some such reserve as that I have indicated.

But the magnificent body of information which their labours have brought together will enable every one who will carefully study the volume to form his own opinion as to whether or not it was Krakatoa dust which painted our sunsets with those glorious hues. In attempting to decide this question we must first endeavour to conceive , the kind of evidence which would be necessary and sufficient to establish the fact that the optical phenomena were consequent upon, as well as subsequent to, the great eruption.

First of all it would be natural to ask whether the existence of volcanic dust in the air could have produced the optical effects that have been observed. This must be answered in the affirmative. Then it would be proper to inquire whether other volcanic outbreaks in other parts of the world, and on other occasions, had been known to have been followed by similar results. Here, again, we have page after page of carefully stated and striking historical facts which answer this question also in the affirmative. Next it would be right to see whether the sequence in which the phenomena were produced at different places in the autumn of 1883 tallied with the supposition that they all diverged from Krakatoa. The instances that could be produced in support of the affirmative number many hundreds, though it must be admitted that there are some few cases about which there are difficulties. Surely we have here what is practically a demonstration. It is certain that these optical phenomena existed. No cause can be assigned for them except the presence, at that particular time, of vast volumes of dust in the air. What brought that dust into the air except the explosion of Krakatoa ? Most people find themselves unable to share the scruples of those who think there can be a doubt on the matter. Would another eruption of Krakatoa, followed by a repetition of all the optical phenomena, convince them that in this case, at all events, post hoc was propter hoc ? Perhaps not, if they have already failed to be convinced by the fact that, when Krakatoa exploded two centuries ago, blood-red skies appear to have been seen shortly afterwards even as far away as Denmark.

When we reflect that an explosion on an insignificant islet in the Straits of Sunda has sufficed to set the whole atmospheric covering of our globe trembling, when we remember that the dust then poured forth in a few days of volcanic activity was adequate to adorn the sunsets of every, country in the earth, we are reminded once again of the old truth: “How small the world is after all!”