HYGROMETRY. 831 



\ 



dew, mist, rain, or snow. According to Dove, f 404 ) in our 

 northern zone ' ' the elastic force of the vapour is greatest with 

 south-west, and least with north-east winds ; it diminishes 

 on the western side of the windrose, and rises on the eastern 

 side. On the western side of the windrose, the cold, dense, 

 and dry current represses the warm and light current con 

 taining an abundance of aqueous vapour, while, on the eastern 

 side, the contrary takes place. The south-west is the 

 equatorial current which has descended through the lower 

 current tc the surface of the earth ; the north-east is the 

 polar current prevailing undisturbed." 



The agreeable and fresh verdure which many trees preserve 

 in districts within the tropics, where for from five to seven 

 months no cloud is seen on the vault of heaven, and no per- 

 ceptible dew or rain falls, proves that their leaves are 

 capable of drawing water from the atmosphere by a 

 vital process of their own, wliich perhaps is not simply 

 that of producing cold by radiation. The absence of rain in 

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

 Brazil), is contrasted with the abundance of rain which 

 falls in other places within the tropics ; for example, at the 

 Havannah, where, by the average of six years of observation 

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

 is 102 Parisian inches, which is four or five times as much 

 as falls at Paris or at Geneva. ( 405 ) On the declivities of 

 the chain of the Andes, the quantity of rain as well as 

 the temperature decreases with increasing elevation. ( 406 ) 

 My South American travelling companion, Caldas, found 

 that the annual quantity of rain at Santa Fe de Bogota, at 

 an elevation of nearly 8700 English feet, did not exceed 

 37 Parisian inches, little more than on some of the western 

 coasts of Europe. At Quito, Boussingault sometimes 



