§ 367, 308. EASTING OF THE TRADE-WINDS, ETC. 173 



ing warmer or cooler, and therefore we infer that the total amount 

 of heat received annually by the whole earth is again annually ra- 

 diated from the whole earth. Nevertheless, the two hemispheres 

 may radiate very unequally. 



367. Direct observations concerning the amount of radiation 

 The northern radi- ^^^"^ different parts of the surface of our planet are 

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



conclusive ; but we have in the land and sea breezes a natural in- 

 dex to the actinometry of sea and land, which shows that the ra- 

 diating forces of the two are very different. JSTotwithstanding 

 the temperature of the land is raised so much above that of the 

 waters during the day, its powers of radiation are so much great- 

 er than those of water that its temperature falls during 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 dispenses most heat by radiation. 



2>^S. The question now may be well put: Since the two hemi- 

 Anothe- proof of the sphcrcs rcccive annually the same amount of heat 



crossingd at the culm J- , i • i i i • i 



belts. irom the sun, and since the northern hemisphere, 



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

 the surplus? Tlie 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 vapor that is taken up in the south- 

 ern hemisphere, transported by the winds across the calm belts, 

 and liberated, as the clouds drop down their fatness upon north- 

 ern fields. It is not only the difference of radiating power be- 

 tween land and water that makes the northern continents the 

 chimneys of the earthy but the difference of cloud in a continental 

 and an oceanic sky must also greatly quicken the radiating pow- 

 ers of the northern hemisphere. Radiation goes on from the up- 

 per surface of the clouds and from the atmosphere itself, but we 

 know that clouds in a great measure obstruct radiation from the 

 surface of the earth ; and as the surface 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 of it chiefly in three ways: some 

 is carried off by convection in the air ; some by evaporation ; and 

 some by radiation ; and such is the interference of clouds with 

 this last-named process, that we are told that during the rainy 



