with Height in the Atmosphere. 211 



th6 annual range diminishes as we rise, and, at a certain height, 

 would probably sensibly vanish. The third is, that the influ- 

 ence of seasons begins to be felt at the plains, and is later 

 communicated to the mountains. The two former of these 

 facts obtain with reference to the diurnal as well as annual 

 variation of temperature ; the last appears to be in that case 

 reversed.* 



The shift of the annual curve, or retardation of epochs, and 

 likewise the decreased range, is common to the strata of the 

 air above the surface of the earthy and to those of the soil be- 

 neath it. Both ultimately, no doubt, exhibit a limit, first 

 where the diurnal variations disappear, then the annual. The 

 cause, however, is very different in the tWo oases, the one being 

 chiefly the result of the radiation and the other of the conduc- 

 tion of heat. 



It is only curious that the diurnal curve seems to follow so 

 different a law, at least in summer ; — perhaps the reason is, 

 that the direct solar radiation is more energetic in that case, 

 and the vehicular conveyance of heat by the air (or convection) 

 less. Thus, with respect to the process of annual heating, the 

 earth's surface (considered as an extensive plain) is the point 

 where the sun's rays freely transmitted by the atmosphere first 

 become productive of any considerable warmth. That warmth 

 is propagated slowly and progressively by conduction to the 

 inferior strata of earth, and by convection to the superior 

 strata of air ; in either case, as I have said, a later and v^ feebler 

 impress of the annual curve is found. The diurnal tempera- 

 ture is probably much more modified by the direct effects of 

 radiation. The detached mountain tops, more exposed and 

 less massive, receive and part with the solar heat more rapidly 

 than the low country, presenting a complete analogy, the for- 

 mer with an insular, the later with a continental climate. The 

 summits change temperature rapidly, the extremes are less ; 

 but the changes of the heat of the plain follow later, and are 

 more marked. This is not conjecture ; many facts might 

 be quoted to support it, but the following is sufficient, that 



^ Saussurc, Voyages dans les Alpes, torn. iv. § 2050; &c. See also 

 Kaemtz, Lehrbucli, band ii. s. 133, 



