HYG HOME THY. 341 



the cold, dense, and dry current of air repels the warmer lightef 

 current containing an abundance of aqueous vapour, whils? 

 on the eastern side, it is the former current which is repulsec 

 by the latter. The south-west is the equatorial current, while 

 the north-east is the sole prevailing polar current." 



The agreeable and fresh verdure which is observed in many 

 trees in districts within the tropics, where, for five or seven 

 months of the year, not a cloud is seen on the vault of heaven, 

 and where no perceptible dew or rain falls, proves that the 

 leaves are capable of extracting water from the atmosphere by 

 a peculiar vital process of their own, which perhaps is not 

 alone that of producing cold by radiation. The absence of 

 rain in the arid plains of Cumana, Coro, and Ceara in North 

 Brazil, forms a striking contrast to the quantity of rain which 

 falls in some tropical regions, as for instance, in the Havannah, 

 where it would appear from the average of six years' observa- 

 tion by Ramon de la Sagra, the mean annual quantity of 

 rain is 109 inches, equal to four or five times that which 

 falls at Paris or at Geneva.* On the declivity of the Cordil- 

 leras the quantity of rain, as well as the temperature, dimi- 

 nishes with the increase in the elevation.f My South 

 American fellow-traveller, 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 



* The mean annual quantity of rain that fell in Paris between 

 1805 and 1822, was found by Arago to be 20 inches; in London, 

 between 1812 and 1827, it was determined by Howard at 25 inches; 

 whilst at Geneva the mean of thirty-two years' observation was 30 - 5 

 inches. In Hindustan, near the coast, the quantity of rain is from 115 

 to 128 inches; and in the island of Cuba, fully 142 inches fell in 

 the year 1821. With regard to the distribution of the quantity of rain 

 in Central Europe, at different periods of the year, see the admirable 

 researches of Gasparin, Schouw, and Bravais, in the BiUioiheque Uni- 

 versclle, t. xxxviii. pp. 54 and 264; Tableau du Climat de V Italic, 

 p. 76 ; and Martins' notes to hie excellent French translation of K'amtz'a 

 Vorlesungen ubtr Meteorologie, p. 142. 



+ According to Boussingault (Economic rurale, t. ii. p. 693), the 

 mean quantity of rain that fell at Marmato (latitude 5 27', altitude 

 4675 feet, and mean temperature 69.) in the years 1833 and 1834, 

 was 64 inches; whilst at Santa F6 de Bogota (latitude 4 36', alti- 

 tude 8685 feet, and mean temperature 58,) it only amounted to 39' 

 Inches. 



