312 PHYSICAL GEOGRAPHY. 



plains of Gobi to the interior of Russia, or whether the 

 direction of the current originates in the middle of the chain 

 of stations by a descent of the air from the upper regions. 

 "We shall thus learn in the strictest sense " where the wind 

 comes from/' If at the present moment we take a result 

 on this point exclusively from stations where observations 

 have now been made for above twenty years, we find (ac- 

 cording to Wilhelm Mahlmann's most recent and careful 

 calculation), that in the middle latitudes of the temperate 

 zone, and in both continents, the prevailing direction of the 

 wind is west south-west. 



We have gained a clearer insight into the distribution of 

 heat in the atmosphere, since the attempt has been made to 

 connect graphically by lines those points, where accurate 

 observations have indicated equality of mean annual, mean 

 summer, or mean winter temperature. The system of 

 Isothermal, Isotheral, and Isochimenal lines, which I fust 

 employed in 1817, if gradually perfected by the united 

 efforts of investigators, may perhaps prove one of the chiei 

 foundations of a Comparative Climatology. It is thus 

 that researches on terrestrial magnetism have become a 

 science, from the period when partial results were combined 

 in a graphic form in lines of equal Declination, equal Incli- 

 nation, and equal Intensity. 



The expression climate, taken in its most general sense, 

 signifies all those states and changes of the atmosphere which 

 sensibly affect our organs : temperature, humidity, variation 

 of barometric pressure, a calm state of the air or the effects 

 of different winds, the amount of electric tension, the purity 

 of the atmosphere or its admixture with more or less dele- 

 terious exhalations, and lastly the degree of habitual trans- 



