Feb. 14, 1884] 



NA TURE 



359 



earth-wave, at an interval ranging from half an hour 

 at Port Blair (in the Andamans), the nearest sta- 

 tion, to six hours at Dublat (in Sanger Island at the 

 mouth of the River Hooghly), the furthest station at 

 which such waves were certainly registered. At Ran- 

 goon, Moulmein, and various points in the Mergui Ar- 

 chipelago, the earth-wave was distinctly perceptible, 

 though its shock was here much less violent ; but no 

 trace of a sea-wave has been met with at any of the tidal 

 stations in this quarter ; the belt of islands and shoals 

 which extends from Cape Negrais to the Island of 

 Sumatra, practically dividing the Bay of Bengal into two 

 portions, must have formed a barrier to the sea-waves, 

 for though great and numerous at Port Blair, they died 

 awa\ in the deep sea beyond, and in no case reached the 

 eastern coast line. 



The position of the earthquake in the Bay of Beng.al 

 was necessarily not a matter of observation as at Kraka- 

 toa ; but it has been inferred by Major Rogers from the 

 following facts. The moment at which the shock of the 

 earth-wave was felt happens to have been recorded 

 with considerable accuracy at three places, two on the 

 west coast of the Bay, viz. the Madras Astronomical 

 Observatory and the tidal station at False Point ; the 

 third on the east coast, Kisseraing, a principal station of 

 the Great Trigonometrical Survey, where Major Rogers 

 was actually at the moment observing a distant station in 

 the field of the telescope of his theodolite. He reports that 

 " he saw the earthquake before feeling it," for he first 

 became sensible of its occurrence by noticing the object 

 which he was observing appear to rise and fall in the 

 tele-cope ; he immediately examined the spirit-levels of 

 his instrument, found they were violently agitated, and 

 made a note of the time. Subsequently he ascertained 

 that the shock he felt and those recorded at Madras and 

 False Point must have occurred almost simultaneously. 

 due allowance being made for the difterences of longitude, 

 Therefore, assuming the earth-wave to have travelled 

 from the centre of impulse with the same velocity in all 

 directions, the centre would be near that of the triangle 

 joining the three points of observation, but probably a 

 little to the south, towards the line joining Port Blair and 

 Negapatam, the stations at which the tidal disturbances 

 were the greatest. 



Having thus ascertained the probable position of the 

 centre of impulse, Major Rogers proceeded to ascertain 

 the probable time of the earthquake. Here again he was 

 favoured by his facts. It so happened that his assistant, 

 Mr. Rendell, had just completed an inspection of the 

 tidal station at False Point, and was at work on a line of 

 levels a few miles away, when he felt a violent shock of 

 earthquake; he noted the time; the clerk at the station 

 also felt the earthquake, and noted that the observatory 

 was much shaken ; afterwards it was found that at the 

 time recorded by Mr. Rendell the pencil of the tide-gauge 

 had been vibrating very sensibly on the diagram ; the 

 vibr.-ition must have been caused either by the shaking of 

 the ribservatory, or by a forced sea-wave such as is some- 

 times produced momentarily in shallow waters by a pass- 

 ing earth-wave. Thegreatsea-wave which was transmitted 

 from the centre of impulse arrived 3 hours 18 minutes 

 after .s-ards. Now there can be no question that the 

 vibration mark on the diagram correctly registers the 

 moment at which the earth-wa\e reached False Point ; 

 Major Rogers therefore conjectures, with much proba- 

 bility, that a similar very prominent vibration mark on 

 the I'ort Blair diagram registers the moment of the arrival 

 of the earth-wave at Port Blair ; thirteen minutes after 

 the time thus registered Major Rogers felt the earthquake 

 at Kisseraing, and as the distance between the two points 

 is 400 miles it may be infjrred that the earth-wave 

 travelled with a velocity of about 1800 miles an hour. 

 With this velocity, the distance of the assumed centre of 

 impulse from either of the three surrounding stations, 



and the lime of the occurrence of the earth-wave at either 

 station. Major Rogers calculates the time of the original 

 disturbance when both the earth-wave and the sea-wave 

 were initiated. Comparing this time with that of the 

 arrival of the sea-wave at his stations, he obtains the fol- 

 lowing velocities for the sea-wave : to Port Blair 360 miles 

 an hour, to Madras and Negapatam 240, to False Point 

 180, and to Dublat 120. The average depth of the sea is 

 known to diminish in every instance of diminished 

 velocity. 



The sea-wave here specifically referred to was the first 

 and generally the greatest of the supertidal waves ; its 

 amplitude from trough to crest was a maximum, 36 

 inches, at Negapatam, and 30 inches at Port Blair ; it 

 was always positive, the crest preceding the trough and 

 raising the sea-level. The latter point is to be specially 

 noticed because the first result of the great eruption at 

 Krakatoa was the reverse of this, namely, a negative 

 wave or general lowering of the sea-level at the stations 

 of observation, as will be shown more fully further on. 

 Secondary sea-waves followed the first, disturbing the 

 normal tides for some hours ; their greatest duration was 

 twenty-five hours at Port Blair, the nearest tidal station 

 to the centre of impulse. A single earth-wave of a few 

 seconds' duration is all that appears to have been per- 

 ceived at the tidal stations ; possibly, therefore, the whole 

 of the tidal disturbances were due to a single earthquake. 



Proceeding now to the eruptions at Krakatoa, we find 

 that while there is no uncertainty as regards their locality, 

 and there is evidence of one great eruption far exceeding 

 all the others in violence, there is as yet no certain in- 

 formation of their number nor of the times at which any 

 of them, even the greatest, occurred. No earth-waves 

 appear to have reached India ; but sea-waves of more or 

 less magnitude were transmitted lo all the tidal stations- 

 on both coasts of the peninsula, and not alone to those 

 on the east coast, as on the former occasion ; they were 

 also transmitted far beyond, to Aden, the Mauritius, and 

 the south-east coast of Africa, as e-hown in Major Baird's 

 report. Lately it has been announced that traces of the 

 sea-waves have been discovered at French tidal stations 

 on both coasts of the Atlantic. 



The principal facts set forth by Major Baird are the 

 following : — 



1. Distinct evidence of tidal disturbance was met with 

 at twelve of the seventeen Indian tidal stations, including 

 all which were fairly placed to receive the force of the im- 

 pulse from Krakatoa ; but, as in the previous instance, no 

 disturbance was perceived at the stations on the east_ 

 coast of the Bay of Bengal. 



2. The first result of the great eruption at Krakatoa 

 was a negative supertidal wave, or general fall of the sea- 

 level, at Major Baird's stations and also at the Mauritius. 



3. This negative wave was succeeded by a great 

 positive wave, at an interval ranging from seventy-five 

 minutes at Negapatam, the station nearest Krakatoa, to 

 twenty-four minutes at Aden, the most distant station. 



4. Supertidal waves of greater or less magnitude were 

 registered at the Indian stations some hours before the 

 negative wave of the great eruption, showing that there 

 must have been antecedent minor eruptions. They 

 appear at Aden about three hours before the negative 

 wave, and eighteen hours before at Negapatam, showing 

 that the explosions were at first comparatively feeble, 

 affecting only the nearer stations ; but afterwards they 

 increased in intensity and became sensible even at Aden, 

 a distance of over 4000 miles. 



5. Waves of amplitudes ranging from a maximum of 

 22 inches at Negapatam to a maximum of 9 inches at 

 Aden were registered at all the more favourably situated 

 stations. The first was the positive wave immediately 

 succeeding the primary negative wave, and it was gener- 

 ally of a greater amplitude than any i. ther wave, but in a 



