226 STRUCTURE AND MODE OF ACTION 



height above the level of the sea 609 toises, 3894 English 

 feet. The eruption of 1794 occasioned a breaking down of 

 the margin of the crater on the southern side, and a conse- 

 quent inequality between the height of the two edges which 

 the most unpractised eye does not fail to distinguish even at a 

 considerable distance. In 1805, Leopold von Buch, Gay- 

 Lussac, and myself, measured the height of Vesuvius three 

 times, and found the northern margin opposite to La Somma, 

 (the Eocca del Palo), exactly as given by Saussure, but the 

 southern margin 75 toises, or 450 French or 479 English 

 feet, lower than he had found it in 1773. The whole eleva- 

 tion of the volcano on the side of Torre del Greco (the side 

 towards which, for the last thirty years, the igneous action 

 has, as it were, been principally directed,) had at that time 

 diminished one-eighth. The height of the cone of ashes, as 

 compared with the whole height of the mountain, is in 

 Yesuvius as 1 to 3; in Pichincha, as 1 to 10 ; and in the 

 Peak of Teneriffe, as 1 to 22. In these three volcanic 

 mountains, the cone of ashes is therefore, relatively speaking, 

 highest in Yesuvius ; probably because, being a low volcano, 

 the action has been principally by the summit. 



A few months ago (in 1822) I was enabled not only to 

 repeat my former barometric measurements of the height of 

 Yesuvius, but also, during the course of three visits to the 

 summit, to make a more complete determination of all the 

 edges of the crater ( J ). These determinations may not be 

 without interest, since they include the long period of great 

 eruptions between 1 805 and 1822, and constitute perhaps 

 the only known examination and measurement of a volcano 



