INTERCHANGE OF HEAT VON BEZOLD 379 



With these preliminaries we find the following as the number of 

 units of heat required. 



Kg. calories. 



Required to warm (l m ) 3 water 1° C 1000 



toynrarm (l m ) 3 earth 1° C 300-600* 



to evaporate l mm depth per (l m ) 2 water 600f 



to melt l mm depth per (l m ) 2 area — 1 cubic deci- 

 meter ice 76 



to warm by 1° C. the total atmosphere resting on 



(l m ) 2 of ground 2454$ 



to warm by 1° C. (l m ) 3 air at o° C. under pressure of 



760 mm 0.307 



* See p. 414. 



t Temperatures between o° C. and 30 C. principally occur in the case of 

 evaporation of water at the earth's surface. For these temperatures, accord- 

 ing to Regnault, the heat of evaporation lies between 606.5 an d 585.6, hence 

 600 can be adopted. 



X Under the assumption that the pressure at the earth's surface is 760 mm. 



Although this is a very elementary tabulation, yet it gives some 

 important indications. First, we see that the difference in the capaci- 

 ties of water and earth for heat, which is frequently adduced as a prin- 

 cipal basis for the explanation of the difference between the con- 

 tinental and oceanic climates, is greatly diminished when we compare 

 together not equal masses but, what is far more correct, equal vol- 

 umes, that is to say, when we consider the capacities per unit volume 

 instead of per unit weight. But above all it shows what an enor- 

 mous importance attaches to evaporation in the economy of nature, 

 and how it is that this, together with the mobility of water, assumes 

 the first place in the questions now at issue, a fact moreover that 

 Dove laid stress on in his memoir on monthly isotherms, 6 whereas 

 subsequently and notwithstanding this we often find the importance 

 of the difference of the thermal capacities greatly exaggerated. 



The powerful influence of evaporation is still more evident when, 

 by means of the figures above given, we come to clearly comprehend 

 that the evaporation of a depth of one millimeter of rainfall demands 

 as much heat as the melting of a layer of ice about eight times as 

 thick, and that this same amount of heat would suffice to warm up 

 the ground by i° C. to a depth of one or two meters, or to warm up 

 by one-fourth of a degree Centigrade the whole column of air stand- 

 ing on the same area of ground and reaching up to the extreme 

 limit of the atmosphere. 



Dove: Memoirs of the Berlin Acad., 1848, p. 219. 



