298 Scientific Proceedings, Royal Dublin Society, 



The specimens of block pumice received bore, one and all, remains 

 of marine life, incrusting the walls of the larger sm-face cells. Mine- 

 ralogically they were not found to differ ; many, however, showed veins 

 or layers of black vesicular glass, giving a stratified appearance. The 

 specimens, some of which are now in the Museum of Science and Art, 

 Dublin, were picked up considerably to the south-west of the Straits of 

 Sunda. 



Dust from Batavia did not, on examination and comparison with 

 that gathered at half that distance from Krakatoa, indicate any such 

 sifting of its constituents in the course of its journey as M. Eenard 

 assumes in his Paper, read before the Academie Eoyale de Belgique 

 (November 3, 1883). The probable explanation is that the dust was 

 thrown so high, and in such quantities, that for many miles it was 

 descending almost vertically, and was not, therefore, subjected to such 

 a sifting process as it would have experienced had it been travelling 

 horizontally. 



Bearing on this point, the following computation respecting the 

 height to which the dust, &c., was projected may be of interest : — 



Captain W. Thomson heard the first explosion of Krakatoa at two 

 o'clock on August the 26th. He was then in Lat. 6° 56' S., Long, 

 106° 31' E. Immediately after, and before the second explosion, 

 which succeeded in ten minutes, a "black mass, rolling up, like 

 smoke, in clouds was observed to the westward." It attained an 

 altitude of about 12° ; and, as the Medea will be found to have been 

 just seventy-six miles, as the crow flies, from Krakatoa, a height of 

 about seventeen miles is obtained for the cloud. 



At five o'clock the Medea came to an anchor in Lat. 5° 58' S., Long. 

 106° 46' E., 89-3 miles direct from Krakatoa. The dust cloud had 

 meanwhile been going up suddenly with each explosion ; and now, as 

 Captain Thomson cast anchor, he read the angle of elevation of the cloud, 

 it was 40°. Obviously the column had advanced towards him ; but some 

 idea of its approach may be gathered from his subsequent statement 

 that " the wind during the whole of the eruption was W.S.W., force 

 between 3 and 4." Taking its velocity as twenty-one miles per hour, 

 its advance in the three hours since the first explosion would be sixty- 

 three miles. On this assumption the altitude of the cloud would be 

 twenty-one miles. Other observations from other points of view would 

 be of interest. 



Captain Thomson's narrative is one of much interest from the in- 

 telligence and care displayed in his observations, although made under 

 trying circumstances. 



