1890.] NATURAL SCIENX'ES OK PHILADELPHIA. 257 



high measurement given by Glennie is that of Truqui and ("ravcri, 

 who ascended the mountain in September, 1855, and found, at a point 

 estimated to be about 50 metres below the virtual summit, a baromet- 

 ric value (as computed by them) of 5230 metres (=17,159 feet.)' 

 Their barometric reading ^409 mm.), made a short distance beneath 

 the summit, would, if reduced to the summit, coincide almost exactly 

 with our own ; and I fail to see, in view of the common datum 

 accepted for Vera Cruz, how our culculated results should differ to 

 the extent that they do (380 feet). The difference should not exceed 

 some 50-75 feet, but since Truqui and Craveri give no formula of 

 their computation, it is impossible to determine whence the diver- 

 gence arises. 



The most extensive series of measurements made to determine the 

 height of Popocatepetl are those of August Sonntag (1857), published 

 in the Smithsonian Contributions to Knowledge (XI), and in- 

 corporated in an article entitled " Observations on Terrestrial 

 Magnetism in Mexico." The average of three series of measure- 

 ments (barometric and trigonometric, and varying from one another 

 in very narrow limits) gives 17,785 feet for the height of the mountain, 

 or almost exactly 100 feet less than that given by Glennie. We are 

 informed (p. 78 of paper) that the basal height upon which all other 

 elevations are calculated is that of the plaza of the City of Mexico, 

 which is assumed to be 7472.8 feet. The more recent leveling of the 

 Mexican Railway shows, however, that this figure is in excess bv 125 

 feet (Mexico 7347 feet\ an amount which has consequently to be 

 deducted from Sonntag's estimate. This would leave 17,660 feet, or 

 about 140 feet in excess of my own measurement. 



Most geographers still follow Humboldt's determination, made in 

 1804, which allows for the volcano little more than 17,700 feet. 

 This measurement was made from the Llano de Tetimba, lying on the 

 east or Puebla side of the mountain at an elevation, computed 

 barometrically, of 2405 metres. The trigonometrical determination 

 of the summit from this point gave 2993.7 metres, or an absolute 



1 Petermann's Mittheilungen, 1856, p. 361. 



2 Usually given as 17,720, 17,726, or 17,728 feet. The exact figure should 

 be 17,713 feet (.5399 metres, 277(1 toises), as given in a letter addressed by 

 Humboldt to Dr. Petermann, under date of December, 1856, and published in 

 Petermann's Mittheilungen for the same year (p. 479). But Humboldt himself, 

 or rather his translator, erroneously gives 17,729 and 17,728 feet, on pages 251 

 and 458 of the fifth vokime of his Cosmos (Bohn's Edition). 



18 



