iGO COSMOS. 



Boussingault), Rucu-Piehincha, Antisana, Cotopaxi, Chim- 



Professor of Chemistry at the School of Mines in Mexico, the height of 

 the column of mercury at Mexico in six tubes, as follows : 



259.7 lines (old Paris foot) 



259.5 



259.9 



259.9 



260.0 



259.9 



" The two last tubes alone had, by means of heat, been deprived of air 

 by Bellardoni, the instrument maker at Mexico. As the exactness of 

 the experiment depends partly on the perfect cleanliness of the inside 

 of the empty tubes, which are so easily carried, it is a good plan to seal 

 them hermetically over a lamp." As the angles of altitude cannot, in 

 mountainous districts, be taken from the sea-shore, and the trigono- 

 metrical measurements are of a mixed nature and to a considerable 

 extent (frequently as much as i or -^y of the whole height) baro- 

 metrical, the determination of the height of the elevated plain in which 

 the base line may be measured is of great importance. As cor- 

 responding barometrical observations at sea are seldom obtained, or for 

 the most part only at too great a distance, travellers are too often in- 

 duced to take the results they have obtained from a few days' obser- 

 vations, conducted by them at different seasons of the year, as the 

 mean height of the pressure of the atmosphere on the elevated plain 

 and at the seashore. u In wishing to know whether a measurement 

 made by means of the barometer possesses the exactness of trigono- 

 metrical operations, it is only necessary to ascertain whether, in a given 

 case, the two kinds of measurement have been taken under equally 

 favourable circumstances, that is to say, by fulfilling those con- 

 ditions which both theory and long experience have prescribed. The 

 mathematical experimenter dreads the effect of terrestrial refrac- 

 tions, while the physical experimenter has reason to fear the 

 unequal and far from simultaneous distribution of the temperature 

 in the column of air at the extremities of which the two barometers 

 are placed. It is probable enough that near the surface of the earth 

 the decrease of caloric is slower than at greater elevations, and in 

 order to ascertain with precision the mean density of the whole column 

 of air, it would be necessary to ascend in a balloon so as to examine the 

 temperature of each successive stratum or layer of the superimposed 

 air" (Humboldt, Recueil d' Observ. Astron. vol. i, p. 138; see also 371, in 

 the appendix on refraction and barometrical measurements). While 

 the barometrical measurement of M. M. Truqui and Craveri gives only 

 17,159 feet to the summit of Popocatepetl, whereas Glennie gives 

 17,889 feet, I find that the lately published measurement of Professor 

 Carl Heller of Olmiitz, who has thoroughly investigated the district 

 surrounding Mexico, as well as the provinces of Yucatan and Chiapa, 



