Clx NOTES. 



too much disposed to assume, as the mean atmospheric pressure on the high 

 plain and at the sea-shore, results inferred by them from observations made 

 during a few days only, and perhaps also at other seasons. " Dans la question 

 de savoir, si une mesure faite au moyen du barometre peut atteindre 1'exactitude 

 des ope'rations trigonome'triques, il ne s'agit que d'examiner, si dans un cas donne 

 les deux genres de mesures ont etc faites dans des circonstances e'galement favo- 

 rables, c'est-k-dire, en remplissant les conditions que la thdorie et une longue 

 expe'rience ont prescrites. Le gdometre redoute le jeu de refractions terrestres ; 

 le physicien doit craindre la distribution si ine'gale et peu simultane'e de la 

 temperature dans la coloune d'air, aux extremites de laquelle se trouvent places 

 les deux barometres. II est assez probable que pres de la surface de la terre le 

 de'croissement du calorique est plus lent qu'k de plus grandes e'le'vations ; et pour 

 connaitre avec pre'cision la densite' moyenne de toute la colonne d'air, il faudrait, 

 en s'e'levant dans un ballon, pouvoir examiner la tempe'rature de chaque tranche 

 ou couche d'air superpose'e." (Humboldt, Kecueil d'Observ. astron. vol. i. p. 138 

 and 371, in the Memoir on Eefraction and barometric Measurements.) While, 

 on the one hand, the barometric measurement of Messrs. Truqui and Craveri 

 gives only 17,158 feet for the height of Popocatepetl, and, on the other, Mr. 

 Glennie's makes it 17,884 feet, a newly published result, by a traveller who has 

 examined the environs of Mexico and the districts of Yucatan and Chiapa 

 (Gymnasial-Professor Carl Heller of Olmiitz), agrees with mine to within thirty- 

 two feet. (Compare my memoir entitled " Ueber die Hb'he des mexicanischen 

 Vulkans Popocatepetl," in Dr. Petermann's Mittheilungen aus Justus Perthes 

 geographischer Anstalt, 1856, S. 479481.) 



( 604 ) p. 432. In the Chimborazo-rock it is not possible to separate mecha- 

 nically the crystals of felspar from the ground-mass in which they are embedded, 

 as can be done in the Etna-rock; but the proportionally large amount of silicic 

 acid contained, and the lower specific gravity of the rock connected therewith, 

 enable the felspathic ingredient to be recognised as oligoclase. The amount of 

 silicic acid contained, and the specific weight, are, for the most part, in inverse 

 proportion : in oligoclase and labradorite respectively the first is 64 and 53 per 

 cent, while the latter is 2*66 and 2'71. Anorthite has, with only 44 per cent 

 of silicic acid, the great specific gravity of 2'76. In felspathic minerals, which 

 are also isomorphous, this inverse ratio, between the amount of silicic acid and 

 the specific gravity, does not hold good with different forms of crystals, as 

 Gustav Eose has remarked. Thus, for example, felspar and leucite have the 

 same constituents potash, alumina, and silicic acid; but felspar has 65, and 

 leucite only 56 per cent of^silicic acid ; and yet the former has a higher specific 

 gravity (2'56) than the latter (2'48). 



As, in the spring of 1854, 1 wished for a new analysis of the Chimborazo 



