OF VOLCANOS. 385 



observations, and this not only where (as in the Peak of Teneriffe, 

 and in all the volcanos of the chain of the Andes) they are visibly 

 composed of trachyte, but also elsewhere. According to my last 

 determinations, the north-west edge of Vesuvius has, perhaps, not 

 altered at all since the time of Saussure, an interval of 49 years j 

 and the south-eastern side, on the side towards Bosche Tre Case, 

 which, in 1794, had become 400 French (426 English) feet lower, 

 has since then hardly altered 10 toises (60 French or 64 English 

 feet). 



If the public journals, in describing great eruptions, often state 

 the shape of Vesuvius to have undergone an entire change, and if 

 these assertions appear to be confirmed by picturesque views sketch- 

 ed at Naples, the cause of the error consists in the outlines of the 

 margin of the crater having been confounded with those of the cones 

 of eruption, accidentally formed in the middle of the crater, on its 

 floor or bottom, which has been upheaved by vapors. Such a cone 

 of eruption, consisting of loosely heaped-up rapilli and scoriae, had 

 in the course of the years 1816-1818 gradually risen so as to be 

 seen above the south-eastern margin of the crater; and the eruption 

 of the month of February 1822 augmented it so much, that it even 

 became from 100 to 110 (about 107 to 117 English) feet higher 

 than the north-western margin of the crater (the Rocea del Palo). 

 This remarkable cone, which it had become customary in Naples to 

 regard as the. true summit of the mountain, fell in, with a dreadful 

 noise, in the last eruption, on tne night of the 22d of October 

 (1822) : so that the floor of the crater, which had been constantly 

 accessible since 1811, is now 750 (almost 800 English) feet lower 

 than the northern, and 200 (213 English) feet lower than the 

 southern edge of the volcano. Variations in the form and relative 

 position of the cones of eruption the openings of which ought not 

 to be confounded, as they often are, with the crater of the- volcano 

 itself give to Vesuvius at different epochs a different appearance, 

 which would enable a person well acquainted with the history of the 

 volcano, on a mere inspection of Hackert's paintings in the palace 

 of Portici, to tell from the outlines of the summit, according as the 

 northern or the southern side of the mountain is represented as the 

 33 



