462 PHYSICAL GEOGRAPHY OF THE 8EA, AND ITS METEOROLOGY. 



mount up* towards the north in consequence of the heavy winter 

 precipitation upon the Avestern slopes of these mountains. The 

 heat which is required to convert the water of the Columbia and 

 other rivers into vaj)our is set free on the mountain range, and 

 the upper Missouri is by this heat kept open for navigation long 

 after the lower and more southern portion of it is frozen up. 



868. The infiuencG of aqueous vapour upon winds and climates. — 

 The average evaporation of water from sea and land is estimated 

 to be from one third to one half as much daily as is contained in 

 the great chain of American lakes. The average precipitation 

 equals the evaporation. The heat that is absorbed and evolved 

 in the process of lifting up and letting down such a body of 

 water has a powerful influence iipon climates as well as upon 

 winds ; it is the chief source of that motive power which gives 

 to the winds their force, to the storm its violence. Six hundred 

 and twenty pounds of aqueous vapour occupy in the open air the 

 space which it takes one thousand pounds of dry air at the same 

 temperature to fill. Now to appreciate the wind -begetting power 

 of this vapour, and its heat, let us imagine the air over an area 

 of considerable extent to be saturated with vapour from the sea, 

 and that from some cause, as in a thunder-storm, this vapour is 

 suddenly, or even rapidly, condensed : — The aerial rarefaction 

 over such an area, and consequently the wind-begetting power 

 within it, would be immense, merely on account of the con- 

 densation of this vapour; but if we take into the account the 

 rarefying eifect of the heat that is set free during the process of 

 condensation and precipitation, we may cease to marvel either 

 at the force of the wind, or the violence of the rain which marks 

 the hurricane ; nor need we wonder at the low range of the 

 barometer or the mildness of temperature in all rainy latitudes. 



869. Blow the temperature of air may he raised by crossing moun- 

 tains, — In the preceding chapter the circumstances have been 

 considered which favour the idea that most of the unknown sur- 

 face of the antarctic circle is not only land, but that its coasts 

 are probably highlands ; that in its topographical features it 

 presents all the conditions that are required for the rapid con- 

 densation of the vapour with which the impinging air from the 

 sea is loaded, and that in the valleys bej-ond mild climates may 

 be expected. The aqueous vapour which the air carries along is 



Blodget's Climatology of the United States. 



