VOLCANO POPOCATEPETL AND ITS VICINITY. 



77 



COMPUTATION OF HEIGHTS. 



POPOCATEPETL. Only a few of the barometrical observations in the city of 

 Mexico were made at exactly the same time as the observations at Tlamacas, and 

 at the trigonometrical station of the Ixtaccihuatl. For this reason all barometer 

 readings made on the same day have been reduced to the mean height of the 

 mercurial column at the freezing point for that day, and the differences of level 

 deduced from the means of the observations thus corrected. The reduction was 

 made by means of the following tables. 



CORRECTION TO BE APPLIED TO THE HOURLY READINGS TO REDUCE THEM TO THE MEAN HEIGHT 

 OF THE BAROMETER DURING THE DAY, FROM JANUARY 20 TO FEBRUARY 10. 



The table for the city of Mexico has been deduced from observations of days 

 on Avhich the mean height of the barometer was nearly the same as on the pre 

 ceding and following days. 



The other table has been interpolated from observations at Tlamacas, and at the 

 foot of the Ixtaccihuatl. 



Barometrical observations, which I made at very different heights in several 

 parts of the Mexican Republic, seem to indicate that the daily maximum and 

 minimum set in later in proportion as we ascend higher, at the rate of about six 

 minutes for every thousand feet change in elevation. I computed, therefore, from 

 the table of corrections for the city of Mexico, a table corresponding to a height of 

 12,500 feet, by supposing the time of maximum and minimum at this height half 

 an hour later than in Mexico, and reducing the corrections; then interpolated from 

 the table for the city of Mexico, in proportion to the mean daily range of the two 

 stations, which was assumed to be O.G15. In this way the following table was 

 obtained. 



