TRUE VOLCANOES. 459 



Pasto and Cumbal (according to specimens collected by 



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 February, 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, re-calculated by Professor Oltmanns, 

 uniformly according to the formula of Laplace and the co-efficients 

 of Ramond, have been published in my Nivelkment JSarometrique et Geo- 

 logique, 1810 (Recueil d'Olserv. Astronom. t. i, pp. 295334) were^ per- 

 formed without exception with Ramsden's cistern-barometers "a niveau 

 constant," and not with apparatus in which several fresh-filled Torricel- 

 lian tubes may be inserted one after another, nor by the instrument, pro- 

 jected by myself, described in Lametherie's Journal de Physique, t. iv, 

 p. 468, and occasionally used in Germany and France during the years 

 1796 and 1797. Gay-Lussan and I made use, to our mutual satisfaction, 

 of a portable Ramsden cistern-barometer exactly similar in construc- 

 tion, in the year 1805, during our journey through Italy and Swit- 

 zerland. The admirable observations of the Olmutz astronomer, 

 Julius Schmidt, on the margins of the crater of Vesuvius (Beschreib un y 

 der Eruption im Mai, 1855, s. 114 116) furnish from their similarity 

 additional motives of satisfaction. As I never have ascended the sum- 

 mit of Popocatepetl, but measured it trigonometrically, there is no 

 foundation whatever for the extraordinary criticism (Craven, in Peter- 

 maun's Geogr. Mittheilungen, Heft x, s. 359), " that the height of the 

 mountain as described by me is unsatisfactory, because, as I my- 

 self stated, I had made use of fresh-filled Torricellian tubes." The 

 apparatus with several tubes ought never to be used in the open air, 

 more especially on the summit of a mountain. It is one of those 

 means which, from the conveniences furnished by large towns, may 

 be employed at long intervals, when the opeiator feels anxious as to 

 the state of his barometer. For my own part, I have had recourse to 

 it only on very rare occasions, but I would nevertheless still recom- 

 mend it to travellers, accompanied by a comparison with the boiling 

 point, as warmly as I did in my Observations Astronomiques (vol. i, 

 pp. 363 373): "As it is better not to observe at all than to make bad 

 observations, we ought to be less afraid of breaking the barometer than 

 of putting it out of order. M. Bonpland and I having four different 

 times traversed the Cordilleras of the Andes, the determinations which 

 chiefly interested us were repeated at different times, as we returned 

 to the places which seemed doubtful. We occasionally employed the 

 apparatus of Mutis, in which Torricelli's primary experiment is per- 

 formed, by applying- successively three or four strongly heated tubeK, 

 filled with mercury recently boiled in a stoneware crucible. When 

 there is no possibility of replacing the tubes, it is perhaps prudent 

 not to boil the mercury in the tubes themselves. In this way 

 I have found, in experiments made in conjunction with Lindner, 



