Page:Physical Geography of the Sea and its Meteorology.djvu/32

6 to Airy's formulæ, the average depth of the North Pacific between Japan and California is, by the path of the San Francisco wave, 2149 fathoms, by the San Diego, 2034 (say 2½ miles).

17. Specific gravity of sea-water.—At the temperature of 60°, the specific gravity of average sea-water is 1.0272, and the weight of a cubic foot is 64.003 lbs.

18. Of air.—With the barometer at 30 in. and the thermometer at 32°, the weight of a cubic foot of dry atmospheric air is 1.291 oz., and its specific gravity .00129. Such is the difference in weight between the two elements, the phenomena of which give the physical geography of the sea its charms.

19. Unequal distribution of light, land, and air.—There is in the northern hemisphere more land, less sea, more fresh water, more atmospheric air, and a longer annual duration of sunlight, than there is in the southern. And though the two hemispheres receive annually the same amount of heat directly from the sun, yet the northern, without growing cooler, dispenses the greater quantity by radiation.

20. The sun longer in northern declination.—In his annual round, the sun tarries a week (7¾ days) longer on the north than he does on the south side of the equator, and consequently the antarctic night and its winter are longer than the polar winter and night of the arctic regions. The southern hemisphere is said also to be cooler, but this is true only as to its torrid and temperate zones. In the summer of the southern hemisphere the sun is in perigee, and during the course of a diurnal revolution there the southern half of our planet receives more heat than the northern half during the same period of our summer. This difference, however, Sir John Herschel rightfully maintains is compensated by the longer duration of the northern summer. Therefore, admitting the total quantity of heat annually impressed upon the earth by the sun to be equally divided between the two hemispheres, it does not follow that their temperature should be the same, for their powers of radiation may be very different. The northern hemisphere having most land, radiates the more freely—the land and sea breezes tell us that the land dispenses heat more freely than the sea by radiation—but the northern hemisphere is prevented in two ways from growing cooler than the southern:—1. by the transfer of heat in the