The Atmosphere 



The polar regions at this time receive practically 

 no heat from the Sun as it is on the horizon during 

 the whole twenty-four hours, and the air therefore 

 becomes chilled, contracts, becomes heavier, and there- 

 fore tends to descend towards the surface. Thus a 

 circulation is set up, upwards at the equator, outwards 

 in the higher regions towards the poles, downwards 

 in the polar regions, and back along the surface towards 

 the equator. 



The current of air along the surface, from poles 

 towards equator, tends to carry the mass of the atmo- 

 sphere in that direction, while the pole-ward currents 

 in the upper air tend to shift it in the opposite sense. 



These tendencies balance each other in the regions 

 of the tropics of Cancer and Capricorn (latitude 23^- 

 North and 23^ South respectively). Owing to this, 

 the pressure of the lower part of the atmosphere is 

 highest in the regions of the tropics, and the effect is 

 to complicate the circulation, and to produce surface 

 winds from the tropics towards the poles (Fig. i). 



An examination of the diagram will show that at 

 the equator the air currents are moving upwards and 

 at the tropics downwards, consequently to persons 

 situated on these lines there would appear to be no 

 wind. These are the regions known to navigators as 

 the calms of Cancer, the Doldrums, and the calms of 

 Capricorn. 



On the supposition that the Earth did not rotate 

 upon its axis, and that it was entirely covered by the 

 waters of the ocean, this would be the state of things 

 at the equinox, while between the tropic of Cancer and 

 the equator there would be constant north winds, and 

 c 33 



