442 PHYSICAL GEOGRAPHY OF THE SEA, AXD ITS IVIETEOROLOGY. 



The air from all sides is continually seeking to restore tlie equi- 

 librium by rushing into those places of rarefaction and reduced 

 pressure ; consequently there ought to be between each pole and 

 the equator a place of high barometer from which the ak on one 

 side flows towards the equator, on the other towards the pole. Ob- 

 servation (p. 439) shows this high place to be between the parallels 

 of 25^^ and 40^ in the north, and of 20^ and 30° in the southern 

 hemisphere : thus the barometer as well as the winds, Plate XY., 

 are both indicative of a greater degree of rarefaction about the 

 south than about the north pole. Were there no friction, and were 

 the atmosphere ordained to move mthout resistance, the air from 

 these null belts would cajry with it to the polar calms the easterly 

 motion which it had acquired from the earth in its motion aromid 

 its axis at these null belts. Were this motion so impressed, the 

 "wind would arrive, rushing with an hourly velocity about the polar 

 calm places of 700 miles in the arctic, and 800 in the antarctic. 

 Such a velocity would impart a centrifugal force sufficient to keep 

 the air away from the poles and produce almost a vacuum there. 

 In this state of things, the same air would continue to revolve about 

 the poles were not some other agent, such as heat, brought m to 

 expand and drive it away. Being expanded and puffed out above 

 the general atmospherical level, but retaimng its velocity — ^for the 

 supposition is that it moves without friction — and returnuig through 

 the upper regions, it would flow back as it went, -sdz., as a westerly 

 wind, and arrive at its null belt in the dii'ection of the meridian. 

 But the wind lias friction, and is resisted in every movement ; the 

 atmosphere partakes of the spheroidal form, which has been im- 

 pressed upon the earth itseK by its axial rotation. That' form is to it 

 the form of stabihty. The water at the pole is about 13 miles nearer 

 to the centre of the earth than the water at the equator ; but there 

 is not on that account any tendency in the sea to flow back from 

 the equator towards the poles ; neither is there any tendency to 

 motion one way or the other in the atmospherical ocean by reason 

 of the oblateness of its sm-face. To produce the polar and equato- 

 rial movements of the air, there must be an agent both at the equa- 

 tor and the poles to prevent such stability by constantly disturbing 

 equihbrium there, and that agent is heat ; therefore, whatever be 

 the degree of depression due the polar barometer in consequence of 

 axial rotation, such depression could, of itself, produce neither trade 

 nor counter-trade wind ; it could no more produce currents hi the 

 air than m the sea, nor could axial rotation produce a high baro- 



