HYTSOMETRICAL DETEEMINATIONS, 



BAROMETRICAL AND TRIGONOMETRICAL MEASUREMENTS OF HEIGHTS. 



Determinations of the heights, derived from one or even from several barome- 

 trical observations, are liable to a considerable error, if they are continued only for a 

 few hours at the upper station, and if the lower station is very distant ; but for a 

 sojourn of several days at a height of nearly 18,000 English feet above the sea — 

 though within the tropics — preparations on an extensive scale become so neces- 

 sary, that I could not think of adopting this plan, although it would have been 

 very interesting in many respects. 



I concluded, therefore, to make a series of barometrical observations extendinof 

 over several days, at the highest convenient point; to determine the elevations of 

 the different peaks above this point trigonometrically; and to observe barometers 

 at the highest points to Avhich I might ascend, so that the two measurements would 

 check each other. 



For the lower station I took the city of Mexico as the nearest place of known 

 altitude, at which I could obtain corresponding observations. The most convenient 

 places I found for trigonometrical stations were; on the base of the cone of the 

 Popocatepetl; the "Rancho Tlamacas," which is 5000 feet below the highest peak, 

 and a little more than 5000 feet above the city of Mexico; and on the base of the 

 Ixtaccihuatl, a small plain, 11,600 English feet above the sea. At the two latter 

 stations a French syphon-barometer was employed, and before and afterwards com- 

 pared with two barometers, which were observed simultaneously in Mexico. 



This barometer could not be used at much greater heights than those of the 

 trigonometrical stations, and several other French and English syphon-barometers 

 which were at that time at my disposal were also rendered useless at these great 

 elevations: the mercury in the long arm retreating into the brass tube, or below 

 the limit of the scale; and in the short arm rising above the divisions. Besides this, 

 it was very difficult to transport long glass tubes, filled with mercury, over such 

 rough ground as the upper part of these mountains presents. I took, therefore, 

 two barometer tubes of large diameter, and cut them off, one to the length of 500 

 and the other to 440 millimetres. The open end was then heated under the blow- 

 pipe, and the aperture reduced to so small a size that it could be easily closed 

 air-tight by the point of the finger; to each of the tubes a scale of dry cedar- 

 wood was attached, on which minute divisions miglit be read without difficulty, 



