316 PHYSICAL GEOGEAPHY. 



the winter) ; a geographical position in which the tropical 

 portions of the same meridians are occupied by sea, implying 

 the absence under those meridians of extensive tropical land 

 powerfully heated by the sun's rays, and giving out great 

 heat by radiation; chains of mountains which, by their 

 direction and precipitous form, impede the access of warm 

 winds; the neighbourhood of isolated peaks, causing the 

 descent of currents of cold air on their declivities ; extensive 

 forests, preventing the heating of the ground by the direct ef- 

 fect of the sun's rays, and, by means of the vital organic action 

 of their leafy appendages, causing great evaporation, while, 

 by the extension of those organs they increase the quantity 

 of surface cooled by radiation, thus operating in a threefold 

 manner, by shade, evaporation, and radiation; extensive 

 marshes, which, in the north, form a kind of subterranean 

 glacier in the plains, lasting until the middle of summer ; 

 a cloudy summer sky, or frequent mists, which impede the 

 action of the sun's rays ; and lastly, a very serene and clear 

 winter sky, favouring the escape of heat by radiation. ( 39 ) 



The inflections of the isothermal lines are determined by 

 the joint action, or total effect of the simultaneous operation, 

 of all the disturbing causes, whether belonging to those 

 which raise or to those which depress the temperature, but 

 especially by the relative extent and configuration of the 

 continental and oceanic masses. Local perturbations occa- 

 sion the convex and concave summits of the isothermal 

 curves. Since there are different orders of disturbing causes, 

 each should first be viewed singly ; and afterwards, in order 

 to learn their total effect on the form of the isothermal line 

 in the part of the earth under consideration, we must consider 

 their joint influence; and the operation of each, in modi- 



