342 Mary Somerville. 



been unusually great and continuous. There were 

 frequent thunder-storms ; and, on one occasion, the 

 quantity of rain that fell was so great, as to cause 

 a land-slip in Pizzifalcone, by which several houses 

 were overwhelmed ; and, on another occasion, the 

 torrent of rain was so violent, that the Eiviera di 

 Chiaja was covered, to the depth of half a metre, 

 with mud, and stones brought down by the water 

 from the heights above. This enormous quantity of 

 water pouring on the slopes of Vesuvius, and per- 

 colating through the crust of the earth into the 

 fiery caverns, where volcanic forces are generated, 

 being resolved into steam, and possibly aided by the 

 expansion of volcanic gases, may have been a partial 

 agent in propelling the formidable stream of lava 

 which has caused such destruction. We observed, 

 that when lava abounded, the projection of rocks 

 and lapilli either ceased altogether, or became of 

 small amount. The whole eruption ended in a 

 shower of impalpable ashes, which hid the moun- 

 tain for many days, and which were carried to a 

 great distance by the wind. Sometimes the ashes 

 were pure white, giving the mountain the appear- 

 ance of being covered with snow. Vapour con- 

 tinued to rise from Vesuvius in beautiful silvery 

 clouds, which ceased and left the edge of the crater 

 white with sublimations. I owe to Vesuvius the 



