therefore we infer that the total amount of heat received annually 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 surplus? 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, transported 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 northern continents the chimneys of the earth, but the difference of cloud in a continental and an oceanic sky must also greatly quicken 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 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 season in intertropical countries, as on the coast of Africa, there
