346 REACTION OF THE INTERIOR OF THE EARTH 



dangerous ascent of the volcano, was himself an eyewit- 

 ness, on the 1 1th of September, of the eruption of glowing 

 stones, ashes, and vapours from the summit, while, far be- 

 low, a considerable stream of lava poured itself forth from 

 a fissure on the western declivity. Here, also, the lava is 

 rich in obsidian. According to Erman (Beob. Bd. i. S. 

 400 403, and 419), the latitude of the volcano is 56 4', 

 and its height, in September 1829, was very exactly 

 1 5,763 feet. On the other hand, in August 1828, Admiral 

 Liitke, by angles of altitude taken from the sea at the 

 distance of forty nautical miles, made it 16,500 feet 

 (16,498). (Voyage, t. iii. p. 86 ; Landgrebe, Vulkane, 

 Bd. i. S. 375 386.) This measurement, and the com- 

 parison of the excellent outline drawings of Baron von 

 Kittliz, who accompanied Liitke's expedition in the Senia- 

 vin, with what Erman himself observed in September 

 1829, led the latter to believe that, in this short inter- 

 val of thirteen months, great changes had taken place 

 in the shape and height of the summit. He says 

 (Reise, Bd. iii. S. 359), " we can scarcely be much in 

 error if we assume the height of the summit in August 

 1828, to have been 250 French feet (266 English) 

 higher than in September 1829 ; making the height, at 

 the earlier epoch (in round numbers) 16,030 feet." 

 On Vesuvius, taking as my basis Saussure's barometric 

 measurement of the Rocca del Palo (the highest north- 

 ern edge of the crater) in 1773, I found, by my own 

 measurements, that in 1805 therefore, in an interval 

 of thirty-two years this edge had sunk thirty-eight 

 and one third feet; but that, from 1773 to 1822, or in 

 forty-nine years, it had gained (apparently ?) 102 feet. 

 (Ansichten der Natur, 1849, Bd. ii. S. 290.) In 1822, 



