EXPLANATORY ADDITIONS. 



(1) p. 363. " A more complete determination of the margins 



of the Crater of Mount Vesuvius."" 



My astronomical fellow-labourer, Oltmanns, who was un- 

 happily too early lost to science, has re-calculated the baro- 

 metric measurements I made on Mount Vesuvius (from the 

 22nd to the 25th of November, and on the 1st of December, 

 1822), and compared the results with those yielded by the 

 measurements given to me in manuscript by Lord Minto, 

 Visconti, Monticelli, Brioschi, and Poulett Scrope. 



A. Rocca del Palo, the highest northern margin of the Crater 



of Vesuvius, was estimated by 

 Saussure, in 1773, barometrically, probably according Feet 



to Deluc's formula . . ... . 3894 



Poli (1794), barometrically 3875 



Breislak (1794), barometrically, although, as in the 



case of Poli, it is uncertain what formula was used 3920 

 Gay-Lussac, Leopold von Buch, and Humboldt(1805) 



barometrically, according to the formula of Laplace 



as in all the following barometric results 

 Brioschi (1810), trigonometrically 

 Visconti (1816), trigonometrically 



3856 

 4079 

 3977 



.Lord Minto (1822), barometrically, and frequently re- 

 peated 3971 



Poulett Scrope (1822). This calculation is somewhat 

 uncertain, owing to the unknown relation of the 

 diameters of the tubes to those of the cistern . . 3862 



Monticelli and Covelli (1822) 3990 



Humboldt (1822) 4022 



The most probable final result is 2026 feet above the 

 hermitage, or 3996 feet above the level of the sea. 



B. The lowest south-eastern margin of the Crater, opposite 



Bosche Tre Case. 



After the eruption of 1794, this margin was 426 feet 

 lower than the Rocca del Palo, consequently, if the 

 latter be estimated at 3996 feet, it would be . . 3570 

 Gay-Lussac, Leopold von Buch, and Humboldt (1805), 



barometrically 3414 



Humboldt (1822), barometrically .... 3491 



