178 THYSICAL GEOGRAPHY OF THE SEA, AND ITS METEOROLOGY. 



tlieroforo we infer that the total amount of heat received annu- 

 ally by the whole earth is again annually radiated from the whole 

 earth. Nevertheless, the two hemispheres may radiate very 

 unequally. 



367. The northern radiates most. — Direct observations concerning 

 the amount of radiation from different parts of the surface of our 

 planet are meagre, and the results as to quantity by no means 

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

 index to the actinometry of sea and land, which shows that the 

 radiating forces of the two are very different. Notwithstanding 

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

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

 greater 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 dispensed most heat by radiation. 



368. Another 'proof of the crossings at the calm belts. — The 

 question now may be well put : Since the two hemispheres 

 receive annually the same amount of heat from the sun, and 

 since the northern hemisphere, with its greater area of land, 

 radiates most, whence does it derive the sui-plus ? The theory of 

 the crossing at the calm belts indicates both the way and the 

 means, and suggests the answer ; for it points to the latent heat 

 of vapour that is taken up in the southern hemisphere, trans- 

 ported by the winds across the calm belts, and liberated, as the 

 clouds drop down their fatness upon northern fields. It is 

 not only the difference of radiating power between land and 

 water that makes the noi-thern continents the chimneys of the 

 earth, but the difference of cloud in a continental and an oceanic 

 sky must also greatly quicl^en the radiating powers of the 

 northern hemisphere. Radiation goes on from the upper 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 dis- 

 cussion 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 

 season in intertropical countries, as on the coast of Africa, there 



