46-i THE PHYSICAL GEOGRAPHY OF THE SEA. 



calms, it has to cross a mountain range, we will suppose, the sum- 

 mits of which are pushed high up into the regions of perpetual 

 snow. As this air, with its moisture, rises, it expands, cools, and 

 liberates the latent heat of its vapor, which the air receives in the 

 sensible form. Now suppose the expansion due the height of the 

 mountain-top to be sufficient to lower the temperature of dry air 

 to —50° ; but, on account of the latent heat which is liberated from 

 the vapor of the moist air, the temperature of the air that has 

 ascended, instead of falling as it crosses the mountain to —50°, as 

 dry air would do, falls, in consequence of the condensation of its 

 vapor, no lower than — 30°. Thus, in the case supposed, heat 

 enough has been set free to raise the temperature of the newly-ar- 

 rived air 20°. Consequently, when this air, which, at the tempera- 

 ture of 40°, came from the sea loaded with vapor, passes the mount- 

 ain, it loses vapor, but receives heat : descending into the valleys 

 beyond, it is again compressed by the weight of the barometric 

 column, which, let us assume, is the same in the valley as at the 

 sea level on the other side of the mountain. The temperature of 

 this air now, instead of being 40°, will be 60°. A powerful mod- 

 ifier of climate is the latent heat of vapor in the air. 



869. There is not only reason to suppose that the topograph- 

 Aurora austraiis. ical fcaturcs and the climates of the antarctic re- 

 gions differ greatly from the topographical features and climates 

 of the arctic, but there is reason to suppose a difference in other 

 physical aspects also. The aurora points to these. "On the 

 morning of the second of September last," says Capt. B. P. Howes, 

 in his abstract log of the "Southern Cross," lat. 58° S., long. 70° 

 W., " at about half past one o'clock, the rare phenomenon of the 

 aurora austraiis manifested itself in a most magnificent man- 

 ner. Our ship was off Cape Horn in a violent gale, plunging fu- 

 riously into a heavy sea, flooding her decks, and sometimes bury- 

 ing her whole bows beneath the waves. The heavens were black 

 as death : not a star was to be seen when the brilliant spectacle 

 first appeared. I can not describe the awful grandeur of the 

 scene ; the heavens gradually changed from murky blackness till 

 they became like livid fire, reflecting a lurid, glowing brilliancy 

 over every thing. The ocean appeared like a sea of vermilion 

 lashed into fary by the storm ; the waves, dashing furiously over 

 our sides, ever and anon rushed to leeward in crimson torrents. 



