6 PHYSICAL GEOGRAPHY OF THE SEA, AND ITS METEOKOLOGT. 



to Ally's formulae, tlie 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 2^ miles). 



17. Sj)ecijic gravity of sca-ivater. — 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 

 weio-ht between the two elements, the phenomena of which give 

 the physical geography of the sea its charms. 



19. Unequal distribution of ligJit, 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. TJie sun longer in northern declination. — In his annual round, 

 the sun tarries a week (7f 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 im- 

 pressed 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 gTowing 

 cooler than the southern: — 1. by the transfer of heat in the 



* Maui-y's Sailing Directions, vol. i. Sir Jobn Herschel quotes it at 1.0275 

 for 62^. 



