252 PROCKEDINGS OF THE ACADEMY OF [1890. 



feet) ; 2, Ixtaccilmatl, tlie foniiliar " White Woman " of the plain 

 of Auahuac, is but a few hundred feet (about 550) lower than 

 Popocatepetl, 



In the following suniniary of our results I have taken up the 

 history of each volcano individually. The peak of Orizaba was 

 ascended on the 6th and 7th of April, Popocatepetl on the 16th and 

 17th of the same month, the Nevado de Toluca on the 21st, and Ix- 

 taccihuatl on the 26th and 27th. 



The Peak of Orizaba. 



The ascent of this mountain was made from the side of San 

 Andres Chalchicomula, which lies not far from the west foot of the 

 volcano, at an elevation of some 8,200 feet above the level of the sea. 

 The attacking party consisted of myself, three of my scientific as- 

 sociates, Messrs. Roberts Le Boutillier, J. E. Ives, and Witraer 

 Stone, and eleven guides and carriers. The two last-named gentle- 

 men desisted from the attempt when not further than 300 feet below 

 the summit, while the strength of the first gave out already 

 at an altitude of about 14,000 feet. My measurements were made at a 

 point on the rim of the crater which I estimated to be approximatelu 

 40 yards below the actual apex of the mountain ; the result obtained 

 is 18,205 feet. I append herewith the data for the determination : 



Reading of the barometer at the summit, with the correction for 

 error (.26) kindly given to me by the authorities of the Mexican 

 Central Observatory, 15,56 inches (395.4 mm.). 



Barometer at the same hour (5 p. m.) at the IMexican Ob- 

 servatory, 23.02 inches (584.87 mm.). 



Temperature of the air on the summit, 35° F. 



Temperature of the air, City of Mexico, 78°5 F. 



During the ascent of the second day, and continued through the 

 greater part of the following day, the barometer indicated a drojj of 

 .1 inch ; this I determined by a re-measurement of ray positions on 

 the down slope, and through a subsequent examination of the 

 barometric reading made at the sea-level of Vera Cruz.i 



The calculation is based upon the tables prepared by Delcros and 

 Guyot, published in the Smithsonian Miscellaneous Collections, I. 



The elements of the calculation are as follows : 



1 For the use of the daily and hourly barometric readings made at this point 

 I am indebted to the courtesy of Captain Powell, constructing engineer of docks of 

 the Mexican Railway. 



