VOLCANO POPOCATEPETL AND ITS VICINITY. 



69 



OBSERVATIONS OF HORIZONTAL AND VERTICAL ANGLES. 



All observations of altitudes and horizontal angles, except the measurement of 

 the elevation of the highest peak above the edge of the crater, were made with the 

 instrument of Ertel, before described. One division of the level of this instrument 

 corresponds to three seconds. 



For the determination of the elevation of the highest peak of " Popocatepetl" 

 above the crater, a small theodolite, belonging to the set of Smithsonian magnetic 

 instruments, was used. It had a vertical circle reading to single minutes. 



OBSERVATIONS OF VERTICAL ANGLES. 

 Rancho Tlamacas, January 23, 1857. — Ertel's altitude and azimuth instrument. 



