334 M. Humboldt on the difference of the 



after Dr Oudney'*s death, the temperature of the air was not be- 

 low 49° Fahr. (7i° R.). In South America, at a less distance 

 from the equator, near Bogota and Quito, I saw the water free 

 from ice, at the height of 8500 and 9000 feet, notwithstanding 

 the strong effect of the radiation of high plains in producing cold. 

 In the manuscripts of young Beaufort, who died lately in 

 Upper Senegal, a victim to scientific zeal, I find that under 

 the 16th degree of latitude, the thermometer marked in the 

 shade on the same day 113° Fahr. (36° R.) at noon, and 59° 

 Fahr. (12° R.) early in the morning. The temperature of 

 the air in the plains of America never sinks so low in the same 

 northern latitude. In laying before the Academy last year, 

 a detailed account of the excellent labours of Ehrenberg and 

 Hemperich, I have already mentioned the cold to which these 

 learned travellers were exposed, when in the Desert of Dongola 

 under the 19th degree of latitude. North winds penetrated into 

 this southern tropical country, and, in December, the thermo- 

 meter sunk to 38° Fahr. (2° 5^ R.) above the freezing point, con- 

 sequently 12° of R. lower than it had ever been observed, under 

 the same latitude, in the West Indies, according to the accounts 

 carefully collected by myself. It is astonishing to find Africa 

 in its deserts colder than America, with all its rich vegetation, 

 and this not on the margin of the tropics, but at the very centre 

 of them. The true causes of this singular cooling process 

 have not yet been sufficiently explained. Perhaps it is the ra- 

 diation of heat from the soil through the dry air towards a cloud- 

 less sky, or a sudden expansion produced by the pouring of 

 humid strata into this dry air, and the descent of the upper 

 parts of the atmosphere. 



It is generally known that more than two-thirds of our planet 

 are covered by a body of water, which, by its contact with the at- 

 mosphere, exercises the most powerful influence upon the climate 

 of the continents. The rays from the sun produce heat according 

 to different laws, as they fall either upon the water or upon the 

 solid surface of the earth. The mobility of the particles of 

 which we imagine fluid bodies to be composed, produces cur- 

 rents and an unequal distribution of temperature ; cooled and con- 

 densed by radiation, the particles of water sink to the bottom. 

 By ascending in balloons, climbing upon insulated peaks of 



