My South American fellow-traveler, Caldas, found that, at Santa Fe de Bogota, at an elevation of almost 8700 feet, it did not exceed 37 inches, being consequently little more than on some parts of the western shore of Europe. Boussingault occasionally observed at Quito that Saussure's hygrometer receded to 26 degrees with a temperature of from 53.6 degrees to 55.4 degrees. Gay-Lussac saw the same hygrometer standing at 25.3 degrees in his great aerostatic ascent in a stratum of air 7034 feet high, and with a temperature of 39.2 degrees. The greatest dryness that has yet been observed on the surface of the globe in the low lands is probably that which Gustav Rose, Ehrenberg, and myself found in Northern Asia, between the valleys of the Irtisch and the Oby. In the Steppe of Platowskaja, after southwest winds had blown for a long time from the interior of the Continent, with a temperature of 74.7 degrees, we found the dew point at 24 degrees. The air contained only 16/100ths of aqueous vapor.*
[footnote] *For the particulars of this observation, see my 'Asie Centrale', t. iii., p. 85-89 and 467; and regarding the amount of vapor in the atmosphere in the lowlands of tropical South America, consult my 'Relat. Hist.', t. i., p. 242-248; t. ii., p. 45, 164.
The accurate observers Kämtz, Bravais, and Martins have raised doubts during the last few years regarding the greater dryness of the mountain air, which appeared to be proved by the hygrometric measurements made by Saussure and myself in the higher regions of the Alps and the Cordilleras. The strata of air at Zurich and on the Faulhorn, which can not be considered as an elevated mountain when compared with non-European elevations, furnished the data employed in the comparisons made by these observers.*
[footnote] *Kämtz, 'Vorlesungen uber Meteorologie', s. 117.
In the tropical region of the Paramos (near the region where snow begins to fall, at an elevation of between 12,000 and 14,000 feet), some species of large flowering myrtle-leaved alpine shrubs are almost constantly bathed in moisture; but this fqact does not actually prove the existence of any great and absolute quantity of aqueous vapor at such an elevation, merely affording p 335 an evidence of the frequency of aqueous precipitation, in like manner as do the frequent mists with which the lovely plateau of Bogota is covered. Mists arise and disappear several times in the course of an hour in such elevations as these, and with a calm state of the atmosphere. These rapid alternations characterize the Paramos and the elevated plains of the chain of the Andes.
'The electricity of the atmosphere', whether considered in the lower or in the upper strata of the clouds, in its silent problematical diurnal course, or in the explosion of the lightning and thunder of the tempest, appears to stand in a manifold relation to all phenomena of the distribution of heat, of the pressure of the atmosphere and its disturbances, of hydrometeoric exhibitions, and probably, also, of the magnetism of the external crust of the earth. It exercises a powerful influence on the whole animal and vegetable world; not merely by meteorological processes, as precipitations of aqueous vapor, and of the acids and ammoniacal compounds to which it gives rise, but also directly as an electric force acting on the nerves, and promoting the circulation of the organic juices. This is not a place in which to renew the discussion that has been started regarding the actual source of atmospheric eletricity when the sky is clear, a phenomenon that has alternately been ascribed to the evaporation of impure fluids impregnated with earths and salts,* to the growth of plants,** or to some other chemical decompositions on the surface of the earth, to the unequal distribution of heat in the strata of the air,*** and, finally, according to Peltier's intelligent researches,**** to the agency of a constant charge of negative electricity in the terrestrial globe.
[footnote] *Regarding the conditions of electricity from evaporation at high temperatures, see Peltier, in the 'Annales de Chimie', t. lxxv., p. 330.
[footnote] **Pouillet, in the 'Annales de Chimie', t. xxxv., p. 405.
[footnote] ***De la Rive, in his admirable 'Essai Historique sur l'Electricite', p. 140.
[footnote] ****Peltier, in the 'Comptes Rendus de l'Acad. des Sciences', t. xii., p. 307; Becquerel, 'Traite de l'Electricite et du Magnetisme', t. iv., p. 107.