VOLCANO POPOCATEPETL AND ITS VICINITY. 81 



Vera Cruz during the time in March, when Mr. Craveri was observing. He gives as 

 the mean height of the barometer 759&quot;&quot;&quot;. 16 at the temperature 24.79. If we sup 

 pose this to be Centigrade, the mean height, reduced to the freezing point, would 

 be 756 mm .12, or reduced to the level of the sea, 757 mm .12 (the barometer being ll m .5 

 above the sea). But the mean height of the barometer in March is, in the northern 

 tropics, above the annual mean, which is at Vera Cruz (reduced to the level of the 

 sea and to the freezing point) 760 mm .5, according to the observation of Dr. Bcrendt, 

 which have been continued for nearly three years. If we suppose (as is the case in 

 Havana) the mean height of the barometer in March to be 0&quot;&quot;&quot;.7 above the mean of 

 the year, this height would become 7Gl mm .2, or 4 mm .08 higher than that M. Craveri 

 observed; this would make the city of Mexico 46.3 metres higher, and come 

 within 14 metres of Humboldt s determination and my own. 



GEOGRAPHICAL POSITIONS. 



At the Rancho Tlamacas I made several sets of circum-meridian observations of 

 the sun for determining the latitude, and also observations of equal altitudes in 

 the forenoon and afternoon for obtaining the local time; as I had only one pocket 

 chronometer, the rate of which was rather irregular in consequence of being trans 

 ported on horseback, the last observations arc of no value for the determination of 

 the difference of longitude between the Popocatepetl and the city of Mexico. I 

 have, therefore, given the longitudes of the different points, as deduced from the 

 triangulation, upon the western or highest peak of the Popocatepetl. The observa 

 tions were made with the instrument of Ertel, the circle being mounted vertically. 



The latitude from two sets of circum-meridian observations of the sun, was 

 found to be for 



RANCHO TLAMACAS, trig, station A. 19 3 0&quot;.() N. 



The co-ordinates of the principal points of the Popocatepetl, taking station A as 

 zero point and supposing X. positive when west, Y when north of A, were found 

 to be for 



Western peak. Eastern peak. Pico del Fraile. 



X 003 foot. - 2986 feet. + 945 feet. 



Y 13193 &quot; 12152 &quot; -10G21 &quot; 



From the first two sets of co-ordinates, combined with the difference of height of 

 the eastern and western peak (597.3 feet), was computed the 



Upper diameter of the crater .... 2668 Eng. feet. 

 Direction of this diameter . . . S. 66 24 W. (true). 



The diameter of the level part of the bottom of the crater, running from north 

 to south, was estimated by pacing to be 750 feet. 



The co-ordinates of the principal points determined at the trigonometrical station 

 on the Ixtaccihuatl, referred to station B as zero point, are 



IXT Arc i n ATI,. 1 orocATF.rETt,. 



Highest peak. South peak. Western peak. 



A 10043 feet. -19425 feet. 22123 feet. 



) + :~&amp;gt;:i-22 &quot; + 107 &quot; 49254 &quot; 



11 



