ON ITS EXTERIOR. EARTHQUAKES. 181 



existence of the Island Sabrina or Julia), there may be 

 remarked at points where yet the mariner would not 

 feel any shock, an unusual heaving and rolling of the 

 waves. The inhabitants of the Peruvian coast often 

 directed my attention to this phenomenon. In the 

 harbour of Callao, and at the Island of San Lorenzo 

 which is opposite to it, in perfectly calm, windless 

 nights, in this exceedingly tranquil part of the Pacific, 

 I have myself seen suddenly wave rise upon wave to a 

 height of eleven to fourteen feet, continuing for a few 

 hours. Nor can we, in these latitudes, as we might do 

 elsewhere, explain the phenomenon by the supposition 

 of a violent storm having occurred far out at sea. 



To begin with agitations of the earth limited to the 

 smallest space, and obviously owing their origin to the 

 activity of a volcano, I will mention first what I ob- 

 served when seated with my chronometer in my hand 

 at night in the crater of Vesuvius, at the foot of a 

 small cone of eruption; it was after the great earth- 

 quake at Naples, on the 26th of July 1805, and after 

 the eruption of lava which followed seventeen days 

 later. I regularly felt the ground of the crater shake 

 every twenty or twenty-five seconds, immediately before 

 each eruption of glowing scoriae or cinders. Of these, 

 which were thrown to a height of fifty or sixty feet, part 

 fell back into the opening from which they had issued, 

 and part covered the sides of the cone. The regularity 

 of the phaenomenon rendered the observation free from 

 danger. The repeated small shocks which I felt were 

 not sensible beyond the crater, i. e. at the Atrio del 

 Cavallo, or at the hermitage del Salvatore. The pe- 

 riodicity of the shaking showed that it' depended on 



