-1859] WRITINGS OF JOSEPH HENRY. 279 



On account of the heat evolved, the air in the ascending 

 belt receives an additional momentum which carries it con- 

 siderably beyond the point of statical equilibrium, and con- 

 sequently it descends with a greater velocity, which is further 

 accelerated by the cooling to which it is subjected at this 

 high altitude by radiating its heat into celestial space. In 

 its descent it brings down with it (at about the average lati- 

 tude of 30°) the air north of this latitude, giving rise to a 

 reverse current, and thus producing two separate systems, 

 A and B. (Fig. 5.) The air at the foot of the descending belt 

 at the latitude of 30° will press with greater weight than 

 that of the average of the atmosphere, hence in this belt at 

 the surface of the earth the barometer will stand higher, and 

 while the belt of rains is called the middle belt of low bar- 

 ometer, the belt of 30° is frequently known as the belt of 

 high barometer. At the foot of this belt the air will be 

 pressed out toward the north and south ; southward to supply 

 trade winds and the air which ascends at the belt of calms, 

 and northward to form the current from the southwest, (as 

 shown in Fig. 6,) which latter is the prevailing wind of the 

 north temperate zone. 



We have thus seen that there would be a tendency to 

 separate into the two systems A and B. (Fig. 5.) There 

 would also be a tendency in the remaining air to separate at 

 the point g, giving rise to the polar system C. Were the air 

 within the circle of 60° north latitude entirely isolated from 

 the other part of the atmosphere, a circulation would take 

 place in this such as is indicated by C, the difference of tem- 

 perature between the surface of the earth at the circumfer- 

 ence of this circle and the regions in the vicinity of the cold 

 pole would be suflBicient to produce such a circulation. The 

 column of air in the polar region, on account of its low tem- 

 perature, would be denser and consequently heavier than 

 the surrounding air; it would therefore sink down and 

 spread out in every direction from the centre of the column ; 

 the air would flow in above to supply the level, while the cur- 

 rent below would become heated as it passed southward and 

 rise as shown at the point g. In its ascent it would tend to 



