of Energy in the Atmosphere. 459 



the hot zone the equatorial ring of air, as described by Dove, 

 obviously similar to the solar prominences and flames which 

 must continually flow over towards the poles. 



This overflow is produced by the increasing pressure of 

 the air-masses driven out by the velocity gained in the ascent 

 over the pressure- equilibrium; the velocity which this pres- 

 sure communicates to the poleward-flowing highly rarefied 

 masses of air must therefore be equivalent to the maximum 

 velocity gained in the ascent. It is, however, only the 

 central strata, nearest to the equator, of the extensive region 

 of the equatorial up-current which can preserve the vertical 

 direction so as to destroy the vertical component of their 

 motion by gravitation. This follows at once from the con- 

 sideration that everywhere in the earth's atmosphere the 

 masses of air which flow towards the poles and towards the 

 equator must be equal for each latitude, unless local differ- 

 ences of pressure take place. The part of a mass of air rising 

 with accelerated velocity in the hot zone must therefore be 

 the sooner deflected polewards, the more distant it is from the 

 equator. If we trace the paths of these different air-currents, 

 we see that the strata of the masses of air streaming towards 

 the equator which lie nearest the earth's surface — which also 

 are the most heated by the sun's rays, in the neighbourhood of 

 the equator — stream upwards vertically, up to the greatest 

 height, and from thence are driven with the greatest velocity 

 towards the poles ; that the higher strata of the trade-winds do 

 not reach the greatest heights of the atmosphere, and are driven 

 so much the sooner polewards from the equator the greater 

 their distance from it, and the greater at the same time their 

 original height above the earth's surface. 



The course of the air-currents in the hot zone will then be 

 as follows : — The lower trade-wind, retarded at the earth's 

 surface by friction with the earth, increases in velocity as its 

 height above the earth's surface increases. Then at an unknown 

 height we have a space between the upper and lower trade- 

 winds filled with horizontal cyclones. Above that, the current 

 towards the pole prevails up to the greatest height of the 

 atmosphere, and moreover the velocity of this current increases 

 in rapid proportion with the height. 



It must here be observed that ascending and descending 

 masses of air retain their local velocity of rotation, and that with 

 increasing latitude the bed of the pole-directed current narrows 

 whilst that of the equatorial current enlarges. Hence there 

 results a constant increase of pressure in the pole-directed 

 current, and a decrease in the equatorial current. In conse- 

 quence of this combined action, a general return-flow, in- 

 creasing with the cosine of the latitude of the upper pole- 



