PHYSICAL GEOGRAPHY OF THE SEA, AND ITS IMETEOEOLOGY. 



18. Witli the barometer at 30 in. and the thermometer at 32°, 

 Of air. 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 j^hysical geography of the sea its charms. 



19. There is in the northern hemisphere more land, less sea, 

 Unequal distribution more frosli watcr, more atmospheric air, and a longer 

 ofiight. land, and air. g^j^j^^^^l duration of siinlight, than there is in the 

 southern. And though the two hemispheres receive amiually the 

 same amount of heat directly h'om the sun, yet the northern, without 

 growing cooler, dispenses the greater quantity by radiation. 



20. In his annual romid, the sun tarries a w^ek (7 J days) longer 

 The sun longer in on the nortli than he does on the south side of the 



northern declina- , -. ,i ii i i • • t i i 



tion. equator, and consequently tne antarctic night and 



its v/inter 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 smi is in perigee, and 

 during the course of a diimial revolution there the southern half of 

 our planet receives more heat than the northern half during the 

 same period of oiu' summer. This difference, however. Sir John 

 Herschel rightfully maintains is compensated by the longer dm-ation 

 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 fi'eely — 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 fi'om growing cooler 

 than the southern :^ — 1. by the transfer of heat in the latent form 

 with the vapours fi'om the southern seas ; — 2. by the transfer of 

 heat in the sensible form, by ciu'rents such as the Gulf Stream, et 

 al., from one climate to another in our hemisphere. Hence we 

 infer that the southern hemisphere is in certain zones cooler than 

 the northern, not by reason of its short summer or long winter, but 

 it is the cooler chiefly on account of the latent heat which is brought 

 thence by vapour, and set fi'ee here by condensation. 



21. Within the torrid zone the land is nearly equally divided 

 England about the north and south of the equator, the proportion be- 

 l,s°ith mJsUand.^'^^ iug as 5 to 4. In the temperate zones, however, the 



