Weather Study 



873 



is that two great whirlpools develop in the atmosphere ; one whirling about 

 the north and the other whirling about the south pole. The outer margins 

 of these whirlpools coincide with the tropical belts of high pressure. 



As an example of a whirlpool we may take a basin having a vent at the 

 center of the bottom. If the basin is filled with water, the plug withdrawn 

 and the water given a slight rotary motion, its velocity will increase as it 

 approaches the center and the rapid whirling will develop sufficient cen- 

 trifugal force to open an empty core. Those who have visited the great 

 whirlpool at Niagara, undoubtedly noticed that the whirling waters are 

 held away from the center and piled up around the margins by the centrif- 

 ugal force developed. Let us suppose that air starting from the equator, 

 moves without friction or other resistances toward the pole. Its velocity 

 must increase as its radius shortens, because the law of the conservation of 



CIRCUMPOLAR WHIRL 



areas requires that the radius must always sweep over equal areas in a given 

 unit of time. (See law of conservation of areas.) At the equator, the air 

 has an easterly motion equal to the eastward motion of the earth, which is 

 1,000 miles per hour. At latitude 60 the radius will have decreased one- 

 half and the velocity, therefore, doubled; but at latitude 60 the eastward 

 motion of the earth is only 500 miles per hour, so the air would be moving 

 1500 miles per hour faster than the earth. At a distance of 40 miles from 

 the pole the wind would attain an easterly velocity of 100,000 miles per 

 hour, and moving on so short a radius would develop sufficient centrifugal 

 force to hold all the air away from the pole and thus form a vacuum. That 

 the supposed case of no friction is far from the truth is evidenced by the 

 fact that the pressure at the north pole is but little less than at the equator; 

 but the centrifugal force developed by the gyration winds, in thus with- 

 drawing the air from the poles and piling it up at the tropics, may be fairly 

 taken as sufficient cause for the low pressure found at the poles and the 

 belts of high pressure at the tropics. 



The questions that remain to be considered are : ( i ) the low pressure at 

 the south pole as compared with the pressure at the north pole and (2) the 



