128 M. Humboldt on Volcanoes, [Aug. 



because, probably, as a low volcano, it has acted principally 

 through its summit, A few months ago, I succeeded not only 

 in repeating my former measurements on Vesuvius, but also in 

 ascertaining the elevation of all the edges of the crater. This 

 work, perhaps, deserves some consideration, for the periods at 

 which it was executed include those of the great eruptions from 1 805 

 to 1822, and it is, perhaps, the only admeasurement yet published 

 of any volcano which may be compared in all its parts. It proves 

 that the edges of the craters, not only where they evidently consist 

 of trachyte, as in the volcanoes of the Andes, but hkewise every- 

 where else, are much more constant phsBnomena than has 

 hitherto been believed. Simple angles of elevation ascertained 

 from the same points are more proper for these examinations than 

 barometrical and trigonometrical measurements. According to 

 my last determination, the north-western edge of the crater of 

 Vesuvius has not changed its form in the least since Saussure's 

 time, a period of 49 years. The south-eastern edge towards 

 Bosche tre Case, which became about 450 feet lower in 1794, 

 has sunk very little since that time. 



If in the description of great eruptions, in the pubhc papers, 

 the completely changed form of Vesuvius has frequently been 

 mentioned, if this opmion often seems to be corroborated by the 

 picturesque views of the mountain made at Naples, the cause of 

 this mistake may be found in the circumstance, that the outlines 

 of the edges of the crater have been confounded with those of 

 the cone of eruption which is accidentally formed in the middle 

 of the crater, upon a bottom that has been raised by vapours. 

 Such a cone of eruption, consisting of rapilli and slags loosely 

 heaped together, has become visible over the south-eastern edge 

 of the crater, since 1816 and 1818. The eruption of February, 

 1822, had so much increased it that it had become from 70 to 

 80 feet higher than the north-eastern edge of the crater, Rocca 

 del Palo. This remarkable cone, which, at Naples, they were 

 accustomed .to consider as the true summit of Vesuvius, fell in 

 with a tremendous noise, during the eruption of the 22d of 

 October, so that the bottom of the crater, which had been unin- 

 terruptedly accessible from the year 1811, now lies 850 English feet 

 beneath tne northern edge, and about 213 feet deeper than the 

 southern edge of the volcano. The variable form and relative 

 situation of the crater of eruption, the opening of which must 

 not be taken for the real crater of the volcano, as frequently has 

 been done, gives, at different times, a peculiar physiognomy to 

 Vesuvius ; and the historiographer of that volcano, from the mere 

 outline of the summit, and the relative height of the northern 

 or southern sid^ of the mountain, as it is drawn in Hackert's 

 Views in the palace of Portici, would guess the year in which 

 the artist made the sketch of his picture. 



In the night between the 23d and 24th of October, one day 

 after the fall of the cone of slags 400 feet in height, when small 



