520 COSMOS. 



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

 compact configuration of jl continent having no littoral curv- 

 atures or bays ; the extension of land toward 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 theii 

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

 tension of these organs, and increases the surface that is cool- 

 ed 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, favoring the radi- 

 ation of heat.* 



The simultaneous action of these disturbing causes, wheth- 

 er productive of an increase or decrease of heat, determines, 

 as the total effect, the infl.ection of the isothermal lines, espe- 

 cially with relation to the expansion and configuration of solid 

 continental masses, as compared with the liquid oceanic. 

 These perturbations give rise to convex and concave summits 

 of the isothermal curves. There are, however, different or- 

 ders of disturbing causes, and each one must, therefore, be 

 considered separately, in order that their total effect may aft- 

 erward be investigated with reference to the motion (direc- 

 tion, local curvature) of the isothermal lines, and the actions 

 by which they are connected together, modified, destroyed, or 

 increased in intensity, as manifested in the contact and inter- 

 section of small oscillatory movements. Such is the method 

 by which, I hope, it may some day be possible to connect to- 

 gether, by empirical and numerically expressed laws, vast se- 

 ries of apparently isolated facts, and to exhibit the mutual de- 

 pendence which must necessarily exist among them. 



The trade winds — easterly winds blowing within the trop- 

 ics — give rise, in both temperate zones, to the west, or west- 



* Humboldt, Recherches sur les Causes des Inflexions des Li^nes Iso- 

 thermes, in Asie Centr., t. iii., p. 103-114, 118, 122, 188. 



