384 STRUCTURE, AND MODE OF ACTION 



determined, may be substituted for them; and for a comparison of 

 determinations made at different epochs, angles of altitude so mea- 

 sured may even be often preferable to the complication of circum- 

 stances which more complete operations may involve. 



Saussure had measured Mount Vesuvius, in 1773, when the two 

 margins of the crater, the north-western and the south-eastern, ap- 

 peared to him to be of equal height. He found their height above 

 the level of the sea 609 toises, 3,894 English feet, The eruption of 

 1794 occasioned a breaking down of the margin of the crater on the 

 southern side, and a consequent inequality between the height of the 

 two edges which the most unpracticed eye does not fail to distinguish 

 even at a considerable distance. In 1805, Leopold von Buch, Gray- 

 Lussac, and myself measured the height of Vesuvius three times, 

 and found the northern margin opposite to La Somma (the Rocca 

 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 elevation 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 Vesuvius 

 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 Vesuvius; probably be- 

 cause, being a low volcano, the action has been principally by the 

 summit. 



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

 former barometric measurements of the height of Vesuvius, but also, 

 during the course of three visits to the summit, to make a more 

 complete determination of all the edges of the crater. (*) These 

 determinations may not be without interest, since they include the 

 long period of great eruptions between 1805 and 1822, and consti- 

 tute perhaps the only known examination and measurement of a 

 volcano at different epochs, in winch the different parts of the ex- 

 amination are all truly comparable with each other. We learn 

 from it that the margins of craters are a phenomenon of far more 

 permanent character than had been previously inferred from passing 



