TRUE VOLCANOES. 343 



riod of thirteen months, great changes had taken place in 

 the form and height of the summit. " I am of opinion," says 

 Erman {Reise, vol. iii., p. 359), "that we can scarcely be wrong 

 in assuming the height of the summit in August, 1828, to have 

 been 266 feet more than in September, 1829, during my stay 

 in the neighborhood of Kliutschi, and that therefore its height 

 at the former of these periods must have been 16,029 feet." 

 In the case of Vesuvius, I found, by my own calculations 

 (founded on Saussure's barometrical measurement in 1773), 

 of the Rocca del Palo, the highest northern margin of the 

 crater, that up to the year 1805 — that is to say, in the course 

 of thirty-two years — this northern margin of the crater had 

 sunk 35tJt feet ; while from 1773 to 1822, or forty-nine years, 

 it had risen (apparently) 102 feet (Vieics of Nature, 1850, p. 

 376-378). In the year 1822 Monticelli and Covelli calcu- 

 lated the Rocca del Palo at 3990 feet, and I at 4022 feet; I 

 then gave 3996 as the most probable result for that period. 

 In the spring of 1855, thirty-three years later, the delicate 

 barometrical measurements of the Olmutz astronomer, Julius 

 Schmidt, again brought out 3990 feet {Neue Bestimm. Am. 

 Vesuv., 1856, s. i., 16 and 33). It would be curious to know 

 how much should here be attributed to imperfection of meas- 

 urement and barometrical formula. Investigations of this 

 kind might to be multiplied on a larger scale and with greater 

 certainty if, instead of often-repeated completed trigonomet- 

 rical operations or, in the case of accessible summits, the 

 more practicable though less satisfactory barometrical meas- 

 urement, operators would confine themselves to determining, 

 even to fractions of seconds, at comparative periods of twen- 

 ty-five or fifty years, the simple angle of altitude of the mar- 

 gin of the summit, from the same point of observation, and 

 one which could with certainty be found again. On account 

 of the. influence of terrestrial refraction, I would recommend 

 that, in each of the normal epochs, the mean result of three 

 days' observations at different hours should be taken. In 

 order to obtain not only the general result of the increase or 

 diminution of the angle, but also the absolute amount of the 

 change in feet, the distance would required to be determined 

 previously only once for all. What a rich source of knowl- 

 edge, relative to the twenty volcanic Colossi of the Cordille- 

 ras of Quito, would not the angles of altitude, determined for 

 more than a century by the labors of Bouguer and La Con- 

 damine, have provided had those travelers accurately desig- 

 nated as fixed and permanent points the stations whence they 



