April 15, 1886] 



NATURE 



561 



of sunspots likewise occurred, of which those of 1837 and 

 1870 were almost twice as great as the maximum of 1884, 

 whereas those years were not marked by any special vol- 

 canic activity. Mr. Verbeek cannot, therefore, see any 

 connection between the activity of the sun and that of 

 the earth in 1883, nor does he believe the position of the 

 earth with respect to the moon and sun had any relation 

 to the great eruption of August. The following is IVIr. 

 Verbeek's summary of what the eruption has specially 

 taught us : — 



(1) The extraordinary loudness of the sounds deserves, 

 in the first place, our attention. The substances were 

 shot out of the crater with great velocity up to a very 

 considerable height, and this was accompanied with de- 

 tonations which far surpassed in power all the sounds 

 with which we are acquainted. Never were sounds heard 

 over such a large area of the earth's surface during any 

 previous catastrophe. Some time ago, in the French 

 Academy (Cf/;///6' J- rendus du 9 Mars, 1S85), the possibility 

 even of the transmission of these sounds through the 

 earth's centre, straight to the antipodes of Krakatab, was 

 suggested. According to a communication from M. F. A. 

 Forel, reports were heard on August 26 on the Island of 

 Caiman-Brae in the Caribbean Sea (south of Cuba, 80° 

 W. long, from Greenwich, and 20° N. lat.). The exact hour 

 is not given ; but from the account it appears clearly 

 that the reports were heard in the daytime. (The sky was 

 clear. People ran to the shore to see whether a ship was 

 approaching, &c.) For several reasons it does not seem 

 to me very probable that these sounds proceeded from 

 Krakatao. In the first place, at the time of the great 

 eruption in the Straits of Sunda, there appear to have 

 been eruptions near the antipodes, though the details are 

 wanting. In the second place, it is probable, for other 

 reasons, that an earthquake or eruption occurred in or 

 near the Caribbean Sea. Thirdly, the time does not 

 agree ; for if, according to M. Forel, an hour be allowed 

 for the transmission of the sound through the earth's 

 centre, which is probably too little, the great detonations 

 of Krakatati could not have been heard at the antipodes 

 in the daytime, but only late in the evening of August 26. 



(2) These stupendous detonations caused such violent 

 disturbances in the atmosphere that many objects at long 

 distances from the volcano underwent a corresponding 

 vibration of such intensity as to suggest the idea of an 

 earthquake. 



(3) The formation also of an atmospheric disturbance 

 which propagated itself in very long air-waves round the 

 earth's surface is a phenomenon which had not yet been 

 observed in connection with eruptions, though other at- 

 mospheric disturbances had been previously understood 

 to have regular wave-motions. 



(4) The phenomena of the green and blue sun and 

 moon, and of the beautiful red glow, had been already 

 observed after eruptions, but not with such intensity as 

 after August 1883. 



(5) Though the truncated cones of many volcanoes 

 had been previously recognised as the remains of conical 

 mountains which had had their summits flattened by a 

 subsidence of the central part, we have here for the first 

 time witnessed a subsidence which agrees in dimensions 

 with the Tengger in East Java, known to be one of the 

 largest subsided crater areas in the world. The explana- i 

 tion, which was hitherto considered a doubtful one by 

 some on account of the enormous dimensions ascribed to 

 the subsided part, has been entirely confirmed by the 

 catastrophe at Krakata'b in 1883. 



(6) The almost vertical section of the peak Rakata,' 

 accidentally formed by the eruption, has given us a very 

 valuable insight into the internal structure of a volcano. 

 Of course this formation is by no means the same in all 

 volcanic conical mountains ; thus for instance in many 



_^ The old name, of which Kr.ikat.ru is a corruption, and which is still 

 given to the peak itself. 



volcanoes the existence of a hollow space or a compact 

 kernel may be considered probable, though the Rakata 

 section does not exhibit this. 



(7) By the subsidence into the sea of part of the peak, 

 waves arose which fai surpassed in elevation the biggest 

 ever formed in a storm. To this additional catastrophe, 

 which caused the inundation of the coasts in the Straits 

 of Sunda, the large number of victims of the Krakatab 

 eruption must be attributed. 



(S) The propagation of this wave-motion is very remark- 

 able. Not only in the whole of the Indian Ocean, but even 

 in the Atlantic and the Pacific Oceans, disturbances were 

 observed which were caused by the Krakatab wave. 

 Part of the disturbances, however, observed in the state 

 of the water on the coasts of America and Europe, which 

 were originally also attributed to Krakatab, must have 

 had another cause. 



(9) It is known that, from the velocity with which the 

 wave-motion is propelled, the medium depth of the sea 

 may be calculated along its path. Upon the route from 

 Krakatab to South Georgia it is found that the rate of 

 velocity is equal to the extraordinary depth of 6340 

 metres. Over that track there lies to all appearance a 

 deep basin, the probable existence of which will, I hope, 

 be shortly confirmed by deep-sea soundings. 



(10) Finally, a very remarkable result of the analysis 

 of the Krakata'b ashes deserves mention. These ashes, 

 namely, are the first rocks in which a very great number 

 of plagioclase species of felspar have been found together. 

 While in many rocks of volcanic origin various triclinic 

 felspar species (usually two) had been previously supposed 

 or shown to exist, it was proved for the first time by the 

 analysis of the Krakatab ashes that one rock may con- 

 tain a!/ the plagioclases, from the most basic to the most 

 acid. There appears, besides, a little sanidine. All 

 these felspars are, in this instance, products of the first 

 crystallisation, as the second crystallisation was prevented 

 by the sudden cooling and consolidation of the melted 

 rock magma. Tschermak's theory of felspar finds a 

 fresh support in this simultaneous presence of plagio- 

 clases whose specific gravity diminishes gradually from 

 that of pure anorthite to that of pure albite. 



That which has been easily proved in this case by the 

 isolated condition of the crystals, we may take as highly 

 probable for many compact andesites, and even for a 

 large number of other eruptive rocks, namely, that 

 among the porphyritic felspars there are a variety of 

 species, which differ in specific gravity, and therefore 

 also in chemical composition. 



If on two corresponding rectangular axes, with the 

 specific weights as abscissa, the analogous quantities of 

 plagioclase which a rock contains are put as ordinates, 

 a number of points are the result, of which the junction 

 gives a curve which may be called the " felspar curve " of 

 the rock. This curve must necessarily take a very dif- 

 ferent course for the different acid and basic stones. 

 It is a new and interesting task for petrographers to 

 determine the " felspar curve " of the principal eruptive 

 rocks. 



THE PLEIADES 

 T T is a singular circumstance that the oldest-known mode 

 A of determining the seasons and directing the recurrent 

 operations of human industry, should also have been the 

 most widely diffused. Nor was this an obvious one. It 

 regarded directly neither of the two great luminaries, 

 which, as they move, might almost be said visibly to pay 

 out a golden cable of time ; but turned, instead, to a 

 comparatively inconspicuous, though beautiful and emi- 

 nently interesting, group of stars. The periodical shift- 

 ings in the sky of the sun and moon force themselves 

 upon the dullest apprehension ; one must, however, be 

 already something of an astronomer to take any close 



