370 Reviews — Dr. Johnston- Lavis — Earthquakes of Ischia. 



from the observations on angles of emergence at various points, con- 

 clusions are deduced as to the form of the "focal cavity" and its 

 mean depth from the surface. It is admitted that the data obtained 

 by those observations are too few and imperfect to base absolutely 

 reliable conclusions upon ; but the careful plotting upon maps and 

 sections of those measurements which seem most worthy of con- 

 fidence, points to the conclusion that the centre of the disturbance 

 was the same in both cases, and must be sought on a line less than 

 one mile in length, running nearly north and south beneath Casane- 

 nella from near Lacco towards Fontana. The mean depth of the 

 focus was calculated for the earthquake of 1881 as 518-25 metres 

 from the sea-level, and that of 1883 as 528 metres. The author 

 suggests that the seat of disturbance is a vertical fissure, filled with 

 igneous matter, which is gradually extending itself upwards towards 

 the surface. 



In a chapter on the phenomena connected with the earthquakes, 

 the author points out that the Ischian earthquakes were accompanied 

 by muffled rumblings, which near the centre were simultaneous 

 with the shock, but which further away from the centre preceded 

 the shock. Dr. Johnson-Lavis' careful investigation of the reported 

 changes in thermal and other springs in the island before the shock 

 led him to the conclusion that these reports were for the most part 

 unworthy of credence ; but in the case of a well near Forio, what 

 seems to be satisfactory evidence is adduced in support of the 

 conclusion that shortly before and after the earthquake sudden 

 changes in the qualities of the water took place ; and Prof. Palmieri 

 suggests that the well might be in communication with a fumarole, 

 which from time to time exhibited increased activity. Landslips 

 affecting the soft tufaceous masses which cover so large a part of 

 the island are also shown to be a frequent accompaniment of earth- 

 quake-shocks, though they also sometimes occur quite independently 

 of seismic disturbances. 



In his two final chapters, and in an appendix, the author indulges 

 in some interesting speculations concerning the cause of the earth- 

 quakes of Ischia, and he ventures on some suggestions as to the best 

 methods by which their destructive effects may be guarded against 

 or even prevented. If, as he thinks, a fissure on the side of the 

 old volcano of Eporneo is being extended upwards, till at last the 

 disturbances will culminate in a volcanic outburst, producing another 

 parasitical cone on the old volcano, he suggests that a deep bore-hole 

 might serve as a safety-valve to prevent the destructive accumulation 

 of steam. More practical, perhaps, is the suggestion that careful 

 seismic observations with suitable instruments might result in ob- 

 taining useful warnings, as well as more exact knowledge of the 

 phenomena. And lastly he points out that not a little may be 

 done, during the rebuilding of the towns, in the selection of sites, 

 the arrangement of the positions and forms of houses, and in the 

 choosing of the materials and designs of their construction, to ward 

 off the terrible dangers which seem to beset them. 



In concluding this notice of a very interesting work, we must 



