326 COSMOS. 



within a portion of the tropical zone; the prevalence of 

 southerly or westerly winds on the western shore of a conti- 

 nent in the temperate northern zone; chains of mountains 

 acting as protecting walls against winds coming from colder 

 regions ; the infrequency of swamps, which, in the spring and 

 beginning of summer, long remain covered with ice, and the 

 absence of woods in a dry, sandy soil; finally, the constant 

 serenity of the sky in the summer months, and the vicinity of 

 an oceanic current, bringing water which is of a higher tem- 

 perature than that of the surrounding sea. 



Among the causes which tend to lower the mean annual 

 temperature I include the following: elevation above the 

 level of the sea, when not forming part of an extended plain; 

 the vicinity of an eastern coast in high and middle latitudes; 

 the compact configuration of a continent having no littoral 

 curvatures or bays; the extension of land towards the poles 

 into the region of perpetual ice, without the intervention of a 

 sea remaining open in the winter; a geographical position, in 

 which the equatorial and tropical regions are occupied by the 

 sea, and, consequently, the absence, under the same meridian, 

 of a continental tropical land having a strong capacity for the 

 absorption and radiation of heat; mountain-chains, whose 

 mural form and direction impede the access of warm winds ; 

 the vicinity of isolated peaks, occasioning the descent of cold 

 currents of air down their declivities ; extensive woods, which 

 hinder the insolation of the soil by the vital activity of their 

 foliage, which produces great evaporation, owing to the exten- 

 sion of these organs, and increases the surface that is cooled by 

 radiation, acting consequently in a three-fold manner, by shade, 

 evaporation, and radiation ; the frequency of swamps or marshes, 

 which in the north form a kind of subterranean glacier in the 

 plains, lasting till the middle of the summer; a cloudy summer 

 sky which weakens the action of the solar rays ; and finally, a 

 very clear winter sky favouring the radiation of heat.* 



The simultaneous action of these disturbing causes, whether 

 productive of an increase or decrease of heat, determines, 

 as the total effect, the inflection of the isothermal lines, 

 especially with relation to the expansion and configuration of 



* Humboldt, Recherclies sur les causes des Inflexions des Lignes 

 isothcrmes, in Asie centr., t. iii. p. 103-114, 118, 122, 188. 



