56o 



NATURE 



\_Feb. 14, 1 1 



few instances it was succeeded by greater waves. The 

 succeeding waves maintained considerable amplitudes — 

 not less than half the maxima values — for about twelve 

 hours, appearing at intervals of one or two hours apart at 

 all the more prominent stations. They were succeeded 

 by wavelets gradually diminishing in size, but continuing 

 for some time, being traceable on the diagrams for 

 August 29 and 30, the second and third days after the 

 great eruption. It is noticeable that they ceased first at 

 Port Blair and Negapatani, the two nearest stations, and 

 last at Aden, the farthest station. 



6. Loud reports, resembling the firing of distant guns, 

 were heard at Port Blair on August 26 and 27, and being 

 supposed to be signals from a vessel in distress a steamer 

 was sent out in search of the vessel ; similar reports were 

 heard on the 26th in Ceylon. 



These facts show that the great eruption at Krakatoa 

 was preceded by minor eruptions sufficiently powerful to 

 produce effects which were sensible at a distance of 

 upwards of 4000 miles ; also that it was probably followed 

 by minor eruption^, to the influence of which the long- 

 protracted continuance of tidal disturbance is due. 



The time at which the great eruption occurred is still 

 not known with any precision. Major Baird has endea- 

 voured to calculate it from the following data : he was 

 informed by Her Majesty's Consul in Java that the first 

 great (positive) wave reached Batavia at I2h. lom. local 

 time on the afternoon of August 27 ; as the distance from 

 Krakatoa by sea is 105 miles, and the average depth of 

 the sea about 186 feet, he infers from the table of the 

 velocity of free tide waves passing over seas of different 

 depths, in Sir George .'\iry's article on "Tides and Waves'' 

 in the Encyclopedia Mtiro/'oli/:i/iii, that the wa\'e must have 

 taken about two hours to reach Batavia, and therefore that 

 it must have started at 10.5 a.m. Krakatoa time, allowing 

 five minutes for the difference of longitude. Another 

 estimate has been recently furnished by General Strachey 

 in a paper — read before the Royal Society — on the "Baro- 

 metrical Disturbances which passed over Europe between 

 August 27 and 31 " ; General Strachey connects these 

 disturbances with the great eruption at Krakatoa, and 

 infers, from the recorded evidence of the times of transit 

 of the barometric waves over the European observatories, 

 that the initial barometric rise occurred at gh. 24m. 

 Krakatoa time on the morning of August 27. Now we 

 have seen that the first effect of the great eruption on the 

 ocean was the production of a negative wave which 

 preceded the great positive wave by an interval of seventy- 

 five minutes at Negapatani, and twenty-four minutes at 

 Aden ; if then we assume that the interval was somewhat 

 more than seventy-five minutes at Krakatoa itself — as is 

 to be inferred from the fact that wherever registered it 

 increases as the distance from the centre of impulse 

 diminishes — General Strachey's and Major Baird's deter- 

 minations will be seen to corroborate each other very 

 closely ; indeed, considering the absolute independence 

 of the two methods of deduction, the facts of observa- 

 tion being in one instance derived from the atmosphere, 

 in the other from the ocean, the coincidence between the 

 results is very striking. 



Major Baird has calculated the velocities with which 

 the great positive wave travelled from Krakatoa to the 

 more important of his own stations, and also to Port 

 Louis in the Mauritius, and Port.Elizabeth in South Africa.' 

 Startmg with the assumption that the wave left Krakatoa at 

 io'5 a.m , .'\ugust 27, local tiine, he finds that it attained its 

 maximum value, 467 statute miles per hour, in transit to 

 both Port Louis and Port Elizabeth. Considerable 

 interest attaches to this determination, in that it is identi- 

 cal with Sir George .'\iry's tabulated value of the velocity 

 of a free tide- wave passing over an ocean 15,000 feet 

 deep, which is supposed to be the average depth of the 



' For these ports he cm! toys the data t tiblished in Nature, vol. xxviii. 



ocean in this direction; moreover, the fact that the same 

 velocity is obtained for both the ports, and that the nearer 

 of the two is only 3400 miles from Krakatoa, while the 

 other port is 5450 miles distant, indicates that there is 

 probably no material error in Major Baird's adopted time 

 of starting. The velocity of the wave in all other direc- 

 tions is less, viz. to Galle 397 miles, to Negapatam 355 

 miles, and to Aden 371 miles. The velocities are neces- 

 sarily computed on the assumption of a uniform rate of 

 progress from the origin to the point reached ; but each 

 of the slower waves must have coincided with the wave 

 which impinged on Ports Louis and Elizabeth for a con- 

 siderable distance in the early portion of its course, and 

 it must then have travelled with the same high velocity ; 

 afterwards, on passing over shallower seas, the velocity 

 must have much diminished, and very possibly it may 

 have fallen to the smaller velocity values which Major 

 Rogers has calculated for the sea-waves in the Bay of 

 Bengal, on the occurrence of the earthquake of December 

 31, 1881. 



The Admiralty chart of the Eastern Archipelago shows 

 that Krakatoa is situated at the focus of what may be 

 regarded as a parabolic figure, formed by the contiguous 

 portions of the coasts of Java and Sumatra ; the axis of 

 the figure is directed towards the Indian Ocean. Thus 

 the waves generated by an eruption at Krakatoa would be 

 mostly propelled towards that ocean, both directly and 

 by reflection from the coasts ; but near the apex of the 

 parabola there is an opening, the Straits of Sunda, 

 through which a great wave passed, carrying widespread 

 destruction for some distance beyond along the con- 

 tiguous coasts. This wave may have impinged with great 

 force on the south-west corner of the Island of Borneo, 

 which is on the prolongation of a straight line drawn 

 from Krakatoa through the Straits. But it did not reach 

 Singapore, where a tide-gauge is established, and which 

 is within a third of the distance of the nearest Indian 

 station from Krakatoa ; the Master-Attendant at Singa- 

 pore reports that the gauge shows " no difference what- 

 ever in the tide." This is obviously due to the fact that 

 the wave which passed through the Straits of Sunda had 

 but a shallow sea to advance over towards Singapore, 

 and its course must have been greatly impeded by 

 numerous islands and shoals and the narrow straits and 

 passages between them. For similar reasons, and be- 

 cause the axis of the parabola in which Krakatoa is 

 situated is pointed towards the Indian Ocean, it is 

 probable that the eflects of the eruptions were not con- 

 veyed to anything like so great a distance along the 

 numerous groups of islands to the east and into the 

 Pacific Ocean. J. T. Walker 



THE INDIAN SURVEY^ 

 'T'HIS is the fifth report of the amalgamated Depart- 

 A ment of Surveys under the Government of India. 

 It is divided into two parts with an appendix. Part I. 

 gives a summary of the operations of the great trigono- 

 metrical, the topographical, and revenue survey parties ; 

 also of the geographical, geodetic, and tidal, and levelling 

 operations. Part II. describes the oper.itions at the 

 Head-Quarters Offices, viz. the Surveyor-General's Office, 

 the Revenue Survey Office, the Lithographic Ofiice, the 

 Photographic Office, and the Mathematical Instrument 

 Department, all in Calcutta ; and of the Trigonometrical 

 Survey Office in Dehra Dun. Index charts, coloured 

 maps, and sketches showing the present state of this very 

 important department accompany this report; to which 

 is prefixed, as frontispiece, a "Specimen o'i Heliogravure 

 by Major Waterhouse's Process," which invites the 



* '* General Report on the Operations of the Survey of India during the 

 year 1881-82. " Prepared under the superintendence of Lieut.-General 

 J. T. W.alker, C.B., R.E , F.R S-. &c., Surveyor-General of India. 

 iCalcutta. i88j ) 



