HYGROMETRY. • 333 



condition of moisture of the atmosphere, by means of the dif- 

 ference of the deiv j^oint and of the temperature of the air. 

 Temperature, atmospheric pressure, and the direction of the 

 wind, are all intimately connected with the vivifying action 

 of atmospheric moisture. This influence is not, however, so 

 much a consequence of the quantity of moisture held in solu- 

 tion in different zones, as of the nature and frequency of the 

 precipitation which moistens the ground, whether in the form 

 of dew, mist, rain, or snow. According to the exposition made 

 by Dove of the law of rotation, and to the general views of 

 this distinguished physicist,* it would appear that, in our 

 northern zone, " the elastic force of the vapor is greatest with 

 a southwest, and least with a northeast wind. On the west- 

 ern side of the windrose this elasticity diminishes, while it in- 

 creases on the eastern side ; on the former side, for instance, 

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

 lighter current containing an abundance of aqueous vapor, 

 while on the eastern side it is the former current which is 

 repulsed by the latter. The southwest is the equatorial cur- 

 rent, while the northeast 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 Havana, 

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

 ation 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. t On the declivity of the Cordilleras, 



it 



See Dove, Meteorologische Vergleichnng von Nordamerika vnd Eu- 

 ropa, in Schumacher's Ja^r6McAy«r 1841, s. 311 ; and his Meteorologische 

 Unfersuchungen, s. 140. 



t 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; while at 

 Geneva the mean of thirty-two years' observation was 30*5 inches. In 

 Hindostan, near the coast, the quantity of rain is from 115 to 128 indies-; 

 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 

 diSerent periods of the year, see the admirable researches of Ga3[)arin, 

 Schouw, and Bravais, in the Bibliotheque Universelle, t. xxxviii., p. 54 



