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ical rains; this air would flow north and south towards the poles. 

 Confining our view to that portion which would flow towards the 

 north pole, the larger part of it must descend to the earth within 

 thirty degrees of latitude, under the law as stated by Professor Henry ; 

 as in going north it continuously lias to pass over a portion of the 

 earth which is moving less rapidly than the portion it has left, it is 

 deflected and becomes a southwest wind. The greater part of this 

 upper current having descended to the earth within thirty degrees 

 and returned to join the trade wind, the remainder would flow 

 towards the pole, portions descending in its course at all points 

 where the rarefaction of the air near the earth's surface would per- 

 mit. These descending currents cause the local variable winds of 

 our temperate zone, but the aggregate of all of them is the prevailing 

 southwest return trade wind. The descending currents cannot give 

 rain, as they only fall to the earth when they become colder than the 

 air near the earth's surface. In falling they are constantly arriving 

 at places of warmer temperature than those they have left; they, 

 therefore, change to a condition of taking up moisture, rather than 

 of parting with it. Where the great body of the descending return 

 trade wind reaches the earth between latitudes twenty-eight degrees 

 and thirty-five degrees must, therefore, on this coast, be compara- 

 tively a rainless region. Other lessening portions of the upper cur- 

 rent would pass on until they met the prevailing northerly wind 

 from the polar regions, when their temperature would be lowered 

 and their moisture condensed and fall as rain. The conflict of this 

 descending current with the polar wind would create storms and 

 give rise to electrical phenomena. The prevailing northerly polar 

 wind reaches to about latitude sixty degrees, varied by the declina- 

 tion of the sun. 



This view of the causes of the tropical, temperate, and polar zones 

 of prevailing winds is in accordance with the theoretical deductions 

 of Professor Ferrell, concerning the course of atmospheric currents 

 moving on a sphere, and appears to be confirmed by the belts of low 

 barometer prevailing in the vicinities of the equator, and of latitude 

 sixty degrees. The polar wind, being colder, is heavier than the 

 return trade wind, and where they meet the tendency is for this polar 

 wind to become a surface wind, and prevent the upper current from 

 reaching the earth until it has been reduced to the same temperature. 

 The operation of these general laws can be more clearly seen on this 

 coast than on that of the Atlantic and Gulf States. There the north- 

 east trade winds are forced into the great cauldron of the Gulf of 

 Mexico. The Cordilleras of Central America and Mexico form a 

 wall against their progress; they rise, turn to the north as an upper 

 current, and return to the earth as southwest winds. 



The Rocky Mountains, one great chain of which extends from the 

 center of the continent northwesterly to the Arctic Ocean, assist in 

 the deflection. The great prairies extend in an unbroken line in the 

 same direction from the mouth of the Mississippi to the same frozen 

 ocean at the mouth of the Makenzie River, in about latitude sixty- 

 two degrees. Professor J. W. Foster, in his work on "The Physical 

 Geography of the Mississippi Valley," states that the sources of the 

 Mississippi River are but one thousand six hundred feet above the 

 ocean. Professor Coffin has shown from the records in the Smith- 

 sonian Institute, in his article on the "Winds of the Northern Hem- 

 isphere," that between latitudes sixty and sixty-six degrees there 



