THE ANTArXTIC EEGIOI^S AXD THEIR CLIICATOLOGY. 445 



same position "svitli regard to the wind. Labrador is almost imin- 

 habitable on accomit of the seventy of its cHmate, but in the Falk- 

 land Islands and their neighbouring shores the cattle find pasturage 

 throughout the winter. The the rmometr iced difierence of chmate at 

 these two places, north and south, may be taken as a sort of index 

 to the relative difference between the arctic and antarctic chmates 

 of our planet. 



867. Along the eastern base of the Eocky Mountains the iso- 

 TTiermai difference thorms moimt up * towards the north in consequence 

 antarctic climates, of the heavj wiutcr precipitation upon the western 

 slopes of these mountains. The heat which is reqiured to convert 

 the water of the Columbia and other rivers into vapour 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 fi'ozen up. 



868. The average evaporation of water from sea and land is 

 The influeuce of estimated to be from one third to one half as much 



aqueous vapour upon -, .-, . l • 2 • l^ j. i • p a 



winds and climates, daily as IS contamed m the great cham oi 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 upon 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 

 ak the space w^hich it takes one thousand pounds of dry air at the 

 same temperatui'e 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 condensation of 

 this vapour; but if we take iuto the account the rarefy iug effect 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 ^dolence of the raiu which marks the hm-ricane ; nor 

 need we wonder at the low range of the barometer or the mildness 

 of temperature iu all rainy latitudes. 



869. In the preceding chapter the circumstances have been 

 considered which favour the idea that most of the unknown sm'- 



* Blodget's Climatology of the United States. 



