332 PHYSICAL GEOGRAPHY. 



saw Saussure's hygrometer recede to 26, with a temperature 

 of 12 to 13 C. (53.6 to 55.4 Pah.) Gay-Lussac, 

 in his great aerostatic ascent, in a stratum of air 6600 

 French feet (7034 English) high, with a temperature of 

 4 C. (39.2 Pah.) saw the same hygrometer at 25.3. The 

 greatest degree of dryness which has yet been observed on 

 the surface of the globe in the lowlands is probably that 

 which Gustav Rose, Ehrenberg, and myself, found in 

 Northern Asia, between the valleys of the Trtysch and the 

 Obi. In the steppe of Platowskaia, after south-west winds 

 had blown for a long time from the interior of the continent, 

 with a temperature of 23.7 C. (74.7 Fall.), we found the dew 

 point 4.3 C. (24 Pah.); the air, therefore, contained only 

 sixteen parts in a hundred of the quantity of vapour required 

 for saturation. ( 407 ) Accurate observers, Kamtz, Bravnis, and 

 Martins, have in the last few years raised doubts concerning 

 the greater dryness of mountain air, which appears to follow 

 from the hygrometric measurements made by De Saussure and 

 myself in the higher regions of the Alps and of the Cordilleras. 

 The more recent observations referred to furnish a com- 

 parison of the strata of air at Zurich and on the Paulhorn ; 

 but the latter scarcely deserves the name of an elevated 

 mountain station. ( 408 ) The moisture by which, in the 

 tropical region of the paramos (near the part where snow 

 begins to fall, between 12000 and 14000 English feet of eleva- 

 tion,) some kinds of large-flowered myrtle-leaved alpine shrubs 

 are almost perpetually bathed, does not necessarily imply 

 the presence of a large absolute quantity of aqueous vapour 

 at that height ; it only proves, as do the abundant mists on 

 the fine plateau of Bogota, the frequency of aqueous precipi- 

 tations. At such elevations, with a calm state of the atmo- 

 sphere, mists arise and disappear several times in the course 



