EASTIKG OF THE TEADE-WIKDS, ETC. 171 



makes up vatli heat for the greater intensity but shorter duration of 

 the southern summer. But though the amount of heat annually 

 impressed by the sun upon each hemisphere be identically the same, 

 it by no means follows that the amount radiated oS into space by 

 each hemisphere again is also identically the same. There is no 

 reason to beheve that the earth is growing warmer or cooler, and 

 therefore we infer that the total amount of heat received annually 

 by the u'lwle earth is again annually radiated from the whole 

 earth. Nevertheless, the two hemispheres may radiate very 

 unequally. 



367. Du'ect observations concerning the amount of radiation 

 The northern radi- ^^^m different pai^ts of the surfacc of our planet are 

 ates most. meagTo, and the results as to quantity by no means 

 conclusive ; but we have in the land and sea breezes a natm^al 

 index to the actinometry of sea and land, v\^hich shows that the 

 radiating forces of the two are very different. Notwithstanding the 

 temperatm^e of the land is raised so much above that of the waters 

 dm'ing the day, its powers of radiation are so much gTeater than 

 those of water that its temperature falls dm^ing the night below 

 that of the sea, and so low as to produce the land breeze. From 

 this fact it may be inferred that the hemisphere that has most land 

 dispensed most heat by radiation. 



368. The question now may be well put : Since the two hemi- 

 Another proof of the sphcrcs Toceive annuallv the same amount of heat 



crossings at the calm ^^ ,, t • ii n i '• i 



belts. ' from the sun, and smce the northern nemispnere, 



with its greater area of land, radiates most, whence does it derive 

 the sm-plus ? The theory of the crossing at the calm belts indi- 

 cates both the way and the means, and suggests the answer ; for it 

 points to the latent heat of vapom' that is taken up ia the southern 

 hemisphere, transported by the mnds across the calm belts, and 

 hberated, as the clouds drop dow^n their fatness uj^on northern fields. 

 It is not only the difference of radiating power between land and 

 water that makes the northern continents the chimneys of the earth, 

 but the difference of cloud ia a continental and an oceanic sky must 

 also greatly quicken the radiatiag powers of the northern hemi- 

 sphere. Kadiation goes on from the upper surface of the clouds 

 and fi'om the atmosphere itseK, but we know that clouds in a great 

 measure obstruct radiation from the surface of the earth ; and as 

 the smiace of the earth receives more of the direct heat of the sun 

 than the atmosphere, the point under discussion relates to the mode 

 in which the surface of the earth gets rid of that heat. It gets rid 



