428 COSMOS. 



Tolima (with the Paramo de Ruiz), Purace near Popayan, 



metrical observations taken nearly at the same time, it comes out fully 

 18,017 feet. On the other hand, a barometrical measurement by Sam- 

 uel Birbeck (10th of Nov., 1827), calculated according to the tables of 

 Oltmanns, gave only 17,854 feet ; and the measurement of Alex. Doig- 

 non (Gumprecht, Zeitschrift fur Attg. Erdkunde, bd. iv., 1855, s. 390), 

 coinciding almost too precisely with the trigonometrical measurement 

 of Tetimba, gives 5403 metres, equal to 1 7, 726 feet. The talented 

 Herr Von Gerolt, the present Prussian embassador in Washington, 

 accompanied by Baron Gros, likewise visited the summit of Popocate- 

 petl (28th of May, 1833), and found, by an exact barometrical .meas- 

 urement, the Roca del Fraile, below the crater, 16,896 feet above the 

 sea. Singularly contrasted with these chronologically-stated hypso- 

 metrical results appears a carefully conducted barometrical measure- 

 ment by M. Craven, published by Petermann in his valuable llitthci- 

 litngen iiber tcichtige neue Erforschungen der Gcographie, 1855 (heft x.), 

 s. 358-361. The traveler found, in September, 1855, the height of 

 the highest margin of the crater, the northwest, compared with what 

 he considered the mean height of the atmospheric pressure in Vera, 

 Cruz, only 5230 metres, or 17,159 feet, which is 555 feet (% of the 

 whole height under measurement) less than I found it by trigonomet- 

 rical measurement half a century previous. Craven, likewise, makes 

 the height of the city of Mexico above the sea 196 feet less than Burk- 

 art and I have found it to be at very different times; he reckons it at 

 only 2217 metres, or 7274 feet, instead of 2277 metres, or 7471 feet. 

 In Dr. Petermann's periodical above referred to, p. 479-481, I have 

 explained myself more particularly on the subject of these variations 

 plus or minus, as compared with the result of my trigonometrical 

 measurement, which unfortunately has never been repeated. The 453 

 determinations of height which I made from September, 1799, to Feb- 

 ruary, 1804, in Venezuela, on the woody shores of the Orinoco, the 

 Rio de la Magdalena, and the River Amazon ; in the Cordilleras of 

 New Granada, Quito, and Peru, and in the tropical region of Mexico, 

 all of which, recalculated by Professor Oltmanns, uniformly accord- 

 ing to the formula of Laplace and the coefficients of Ramond, have 

 been published in my Nivellement Barointtrique et Gcologique, 1810 (/?<?- 

 cueil d'Observ. Astron., t. i., p. 295-334), were performed without 

 exception with Ramsden's cistern barometers " a niveau constant," 

 and not with apparatus in which several fresh-filled Torricellian tubes 

 may be inserted one after another, nor by the instrument, projected by 

 myself, described in Laraetherie's Journal de Physique, t. iv., p. 468, 

 and occasionally used in Germany and France during the years 1796 

 and 1797. Gay-Lussac and I made use, to our mutual satisfaction, of 

 a portable Ramsden cistern barometer exactly similar in construction, 

 in the year 1805, during our journey through Italy and Switzerland. 

 The admirable observations of the Olmutz astronomer, Julius Schmidt, 

 on the margins of the crater of Vesuvius (Beschreibung der Eruption 

 im J/aj, 1855, s. 114-116) furnish, from their similarity, additional 

 motives of satisfaction. As I never have ascended the summit of 

 Popocatepetl, but measured it trigonometrically, there is no foundation 

 whatever for the extraordinary criticism (Craven, in Petermann's 

 Geoyr. Miitteilungen, heft x., s/359), " that the height of the mount- 

 ain as described by me is unsatisfactory, because, as I myself stated, 



