August 9, 1883] 



NATURE 



347 



by boob, boob -, boob — , boob , boob , boob 



, and so on 



In the short notice of the earthquake of March 4, 1881, 

 I pointed out that the centre of the mesoseismal area or 

 seismic vertical was at Casamenella, which occupies the 

 sime relative position to Eporaeo as do Montagnone, 

 Mount Rotaro, Cremate, and many other lateral cones. 

 Also that this earthquake belongs to that subterranean 

 class of movements that precede the bursting forth of an 

 eruption such as the Vesuvian shock of A.D. 63, and the 

 series that gave warning for some years before of the 

 appearance of Monte Nuovo. 



Prof. Samuel Haughton and myself are still engaged 

 on a memoir of the last earthquake, and so far as we 

 have gone we have found the following interesting 

 ficts : — 



1. That the area of injury is very small. 



2. That the angle of emergence rapidly diminishes as 

 we recede from the seismic vertical. 



3. That the focus is therefore very near the surface. 



4. That there is another seismic vertical at Fontana. 

 This is probably explained by conduction along a column 

 of trachyte which occupies the old vent of Epomeo, as 

 Fontana lies in the very centre of the old crater. 



Mallet pointed out that shallow foci must produce 

 violent effects in limited areas, that transmission to a 

 distance of the earth-wave diminishes rapidly — conditions 

 we find well illustrated in the present case. This led 

 Prof. Palmieri to believe that the shock of iSS 1 was the 

 result of the tumbling in of the clay caves near Casa- 

 micciola, and he again proposes a similar explanation, as 

 for this one he had not noticed any movement of the seis- 

 mographs of Naples of Vesuvius. On the former occasion I 

 pointed out in my letter at the time that such could not be 

 the case, not even as the result of the imagined spaces exca- 

 vated by the dissolving action of the mineral waters. The 

 real truth seems to be the inelastic nature of the tufas, 

 which vary much in density and dip in every imaginable 

 direction, so that the earth-wave has two powerful retard- 

 ing agencies at w'ork — the absorption by an inelastic 

 medium, and continual reflection and refraction from its 

 irregular structure. If really a falling-in had occurred as 

 the cause of the earthquake, we should expect some signs 

 of it, but such is not the case ; there is not a true fissure 

 in the locality, and no apparent changes of level. Nor 

 can we conceive that the houses of Forio, four miles 

 distant, would be shaken about the ears of their in- 

 habitants; besides in this one, unlike the preceding 

 shock, Naples felt the movement quite distinctly. 



In addition to the destruction of the houses by the 

 shock, fires have burst forth amongst the ruins, and two 

 large landslips have swept down from the flanks of 

 Epomeo, and converted gardens and vineyards into utter 

 ruin. 



In conclusion I would remark, as was done on a former 

 occasion, that we must expect other shocks more violent 

 in character, and that, as one follows the other, the 

 interval of tranquillity will be less, until the final eruption 

 bursts forth. What time such an occurrence may be ex- 

 pected it is only possible to judge by the force, character, 

 and frequency of future events. Only last week I advised 

 Dr. Dohrn of the peril of living at Casamicciola ; and 

 another friend, who would not heed my warning as to the 

 event and the dangerous position of certain rooms in his 

 house with regard to the seismic vertical, has lost his son 

 in the part of the building indicated. 



Would this not be a remarkably favourable occasion to 

 carry out a thorough investigation of the whole of the 

 phenomena accompanying this type of earthquake ? 

 If, for instance, some scientific society would choose 

 a committee, provide a number of suitable seismo- 

 graphs to be placed in different parts of the island, and 

 any other means that might be proposed, so as to study 

 the progress of the focus towards the surface, if such is 



really the case, the form of the focal cavity, and many 

 other points of interest, it might be the means of 

 preventing further catastrophes by sho ving the nearing 

 approach of volcanic matter to the surface. 



The horrors of the occasion I will not touch upon, as 

 it is the province of other newspapers, not to speak of the 

 hurry in which I send off this rough memorandum of my 

 visit to the island H. J. JOHNSTON-LAVIS 



Naples, July 30 



P.S. — Since writing the above,notices from Isernia in the 

 Apennines announce a severe earthquake in that locality, 

 besides others at Sorrento and in other parts of Italy The 

 three Ischian shocks were each accompanied by a period 

 of seismic activity in Italy and other parts of Europe. 

 Vesuvius is slightly more active. 



August 1. — Another slight sho:k occurred at Casa- 

 micciola about 4 p.m., and another a little before 12 pm. 



August 2. — At 12.30 another shock took place. 



There has not yet been time to collect data which 

 may throw light on the origin of the terrible cata- 

 strophe that visited Ischia on the 28th of last month. 

 As in the case of the previous earthquake on the 

 island, one of the most striking features of this last 

 calamity is its extremely local character. There does not 

 appear to have been any simultaneous perceptible tremol 

 at Naples, and Professor Palmieri's delicate seismom;ters 

 on Vesuvius registered no sympathetic movement on that 

 mountain. That the source of the shock at Ischia must 

 have lain comparatively near the surface may be con- 

 fidently inferred. Had it been more than a few hundred 

 feet deep, the waves of such a shock would assuredly 

 have been propagated to a considerable distance all 

 round. 



Various possible causes of earthquakes have been as- 

 signed, each of which may at different times and places 

 be effective in the production of the phenomena. The 

 sudden snap of large masses of rock under great strain 

 may be the origin of the frequent earthquakes of moun- 

 tain chains, such as those so constantly experienced along 

 the line of the Alps. On a smaller scale similar results 

 may arise on a line of dislocation, as is probably the case 

 at Comrie in Scotland. In volcanic regions the earth- 

 quakes that usually precede and accompany volcanic erup- 

 tions have been plausibly attributed to the explosions of 

 elastic vapours, and particularly of steam. Ischia lies 

 in a volcanic district, and is itself of volcanic origin. But 

 its earthquakes do not seem to be part of the active 

 volcanic phenomena of the district. So far as informa- 

 tion is yet available regarding the recent catastrophe, 

 there appear to have been no concomitant volcanic 

 manifestations, though there were active vents where 

 they might certainly have been expected to show them- 

 selves. The only facts yet known that might indicate a 

 connection between the Ischian earthquake and the vul- 

 canicity of the Neapolitan district are the reported out- 

 flow of lava from Vesuvius on the 31st, and the alleged 

 increase in volume and temperature of the Ischian hot 

 springs. As regards the descent of lava towards Torre 

 del Greco on Tuesday of last week, it did not take place 

 until three days after the calamity of the 28th ult., and 

 may have been entirely independent of it. Disturbance 

 of the thermal springs of the locality could hardly fail to 

 accompany so severe a shaking of the ground, from 

 whatever source the concussion might arise. 



So far as materials exist for forming a judgment on the 

 subject, the recent earthquake at Ischia appears to have 

 been caused by the sudden collapse of some subterranean 

 cavern, situated not far below the surface in the Casa- 

 micciola district. Such caverns no doubt frequently 

 exist underneath volcanic vents from which large masses 

 of material have been emitted. It is well known to 

 geologists that one of the final phases in the history of a 

 volcano is the subsidence of the cone. This downward 



