1890.] NATURAL SCIENCES OF PHILADELI'HIA. 251 



BAROMETRIC OBSERVATIONS AMONG THE HIGH VOLCANOES OF 



MEXICO, WITH A CONSIDERATION OF THE CULMINATING 



POINT OF THE NORTH AMERICAN CONTINENT. 



BY PROFESSOR ANGELO HEILPRIN. 



Among the objects for which the Expedition, recently organized 

 under the auspices of the Academy of Natural Sciences of Phila- 

 delphia, was dispatched to Mexico was the determination of the 

 physical features of the giant volcanoes of the south, with special 

 reference to a study of the vertical distribution of animal and veg- 

 etable forms. While prosecuting our observations in this direction, 

 I took the op])ortunity, in company with one or more of ray associates, 

 of scaling the four loftiest summits of the land, namely: the peak 

 of Orizaba, Popocatepetl, Ixtaccihuatl, and the Nevado de Toluca. 

 This gave me the advantage of making personal comparisons between 

 the life that existed in difiereut regions of " cloud-land," at the same 

 time that it offered me the opportunity of more closely investigating 

 the geological features of some of the most gigantic volcanic mount- 

 ains known to us. Numerous measurements of altitude were made 

 during the ascents, and, in the higher regions, always with the same 

 instrument. This was a registered aneroid, tested and corrected at 

 Philadelphia — immediately before the starting, and shortly after the 

 return of the Expedition — at the sea-level of Vera Cruz, and in the 

 Central Meteorological Observatory of the City of Mexico, at an 

 elevation of 7403 feet. To the officers of the latter institution I am 

 indebted for the privilege of making comparisons with the standard 

 mercurial column. 



The results of our measurements show a striking accord in some 

 instances with those obtained from earlier measurements, while in 

 other cases they exhibit marked divergence. The fact that all the 

 summits were ascended within a period of three weeks, were measured 

 with the same instrument, and during a period of atmospheric 

 equability and stability which is offered to an unusual degree bv a 

 tropical dry season, renders the possibility of errors of any magnitude 

 almost nil ; at any i*ate, such errors as may have crept in will 

 probably not affect a general comparative result. The points of im- 

 portant difference are : 1, The highest summit of Mexico is not, as is 

 commonly supposed, Popocatepetl, but the peak of Orizaba (Citlal- 

 tepetl, the "Star Mountain"), which rises 700 feet higher (18,200 



