156 THE WEATHER, AND WEATHER PROPHETS. 



the equatorial region of the earth habitually hotter by 

 about 80 Fahr. on an average of all seasons and hours, 

 than the Polar, 



(18.) Hot air under equal barometric pressure is 

 lighter than cold. The equatorial portion of the atmo- 

 sphere, then, in comparison with the polar, is dilated 

 upwards ; the only direction in which the lateral pressure 

 it experiences will permit it to dilate. Hence, the ex- 

 ternal form of the atmosphere, and of each of its upper 

 strata, instead of conforming in exact parallelism to the 

 spherical* form of the globe on which it reposes, as the 

 laws of equilibrium would require ; are unduly elevated, 

 and bulged out, equatorially, into elliptic forms, a state 

 of things inconsistent with repose. The prominent 

 portion rests, in fact, either way, on a slope, and being 

 unsupported laterally, flows down on either side that is, 

 from the equator towards the poles. In so doing, how- 

 ever, it deserts its place, and ceases to contribute by its 

 weight to the total pressure on the equatorial region ; 

 while at the same time it goes to add to the weight in- 

 cumbent on the polar. Thus the hydrostatic equilibrium 

 of pressure is subverted, and air is pressed inwards to- 

 wards the equator from the poles below, to make good 

 the efflux aloft. A circulation is established in each 

 hemisphere by inferior currents of air running in on both 

 sides towards the equator and superior ones setting out- 

 wards, all around the globe, from the equator towards 

 the poles. Both these, were the earth at rest, would 



* We neglect the spheroidal form of the earth, which in meteor- 

 ology is never worth considering. 



