132 Prof. 0. Silvestri on the Eruption of Etna 



netic, and has a specific gravity of 2*3636 at 25° C. The lava-bed, 

 although its course was a very short one, nevertheless had a sur- 

 face-temperature of 70°, whilst at the depth of half a metre it had a 

 temperature of 90° ; and the temperature increases at the points 

 where the secondary phenomena are found. The outward mani- 

 festations of the eruption at the date of which I write may be said 

 to be over, since the mouth of the new crater and the thirty-five 

 eruptive openings of the chasm, though they are still open, only 

 emit a small quantity of vapour, now in an increasing, now in a 

 decreasing quantity, similar to that observed in the great central 

 crater. The activity observed in the central crater from the end 

 of May to the latter days of August, when the eruption broke 

 out, has now quite ceased. This sudden paroxysm of Etna, 

 which in such a short time (in less than a day) has left the mark 

 of a formidable eruption, has furnished us with a curious fact in 

 the special history of our volcano. It was known that from the 

 chief crater, in consequence of its elevation of 3314 metres above 

 the sea, there could only be eruptions of short duration; but it 

 was also known that if the volcanic force reached such a degree 

 of intensity as to produce a lateral outbreak, the eruptions that 

 followed would probably be of long duration, as they had been 

 formerly in this and in preceding centuries, similar to those 

 which usually happen in Etna at intervals of ten or twelve years. 

 How can we then explain the cessation of the recent phenomenon? 



If upon such a subject an opinion may be given which like- 

 wise goes to explain the occurrence of earthquakes permanently 

 which agitated the ground after the eruptive activity had ceased, 

 I would suggest that the violent shock felt on the N.N.E. side 

 of the mountain may have created an outlet extensive and deep 

 enough for the lava breaking forth from the axis of eruption at 

 Mongibello to find a more easy channel into subterranean caves 

 and channels than to be forced more slowly out to the surface. 



This idea would be confirmed by seeing all the preparation for 

 an eruption with the thirty-five mouths remaining inactive, 

 though open and smoking, and by hearing in their immediate 

 neighbourhood a deep noise as of a flowing subterranean mass. 

 It would also tend to support the theory which maintains that 

 the cause of earthquakes is the completion of some subterranean 

 eruption*, if we may so designate a movement of fluid and 

 gaseous matter in the interior of the earth. 



Such a conjecture would perfectly agree with the phenomenon 

 of the shocks which occurred when the outward flow of the erup- 

 tion continued nearly in full force; for it is generally the case 



* This theory is also maintained by Sig. Rudolph Falh, of Vienna, who 

 after my prophecy of July came to stay near Etna to observe the new 

 eruption. 



