and Nations on the extent and situation of Forests. 241 



upon all geographical relations, and all the interests of nations, 

 are closely connected with the more or less favourable situa- 

 tion of forests, because it appears, from innumerable facts in 

 the physical history of countries, that the clearing of forests 

 produced considerable changes in the physical condition of 

 countries, which were and still remain connected with equally 

 momentous and influential changes in the economical, com- 

 mercial, and artificial, as well as in the moral, mental, and 

 political relations of the population : because forests, by 

 means of their powerful effect on the condition of flowing and 

 standing waters, on the atmospheric moisture, on the amount 

 and more or less frequent occurrence of rain, on the local tem- 

 perature, and, owing to this joint influence, on the productive 

 powers of the soil, and on the salubrity of the air, react upon the 

 physical condition of countries and the social relations of the 

 population. The dependence of the temperature on geo- 

 graphical situation, on the direction and rapidity of the winds, 

 on the presence and extent of flowing and standing waters, 

 on the greater or less elevation of the ground above the level 

 of the sea, and on the external shape as well as the internal 

 nature and condition of the soil, is attested by a great many 

 phenomena. Less known, however, is the influence of forests 

 upon temperature, because it has as yet, to a certain extent, 

 escaped examination, and because the method of comparison 

 has not been applied to demonstrate its reality. 



On considering, however, that the dark colour of forests not 

 only prevents reflection, but favours the absorption of caloric, 

 one may easily understand how they contribute to a decrease 

 of temperature, just as sandy plains cause its increase. This 

 latter case occurs, for instance, in the deserts of Africa, 

 which are densely covered with glassy sand ; whereas the im- 

 mense forests of Guyana, owing to their dark green colour, 

 considerably reduce the solar heat, and, under the same lati- 

 tude, produce in the annual mean temperature a diff*erence of 

 8° to 10° R. (18° to 22° F.), by which the thermometer stands 

 lower in the former than in the latter region. From all the 

 comparisons of the mean temperature of countries, situated 

 under the same degree of latitude, of which the one is clear, 

 the other full of forests, there always results a difference of 1 



