460 Mr. W. Brown on the Influence of Atmospheric Currents 



consequent deficiency towards the equator. This, however, 

 will be better understood by the following illustration: — 



Let A B, C D, &c. represent atmospheric columns situated 

 anywhere, in a direction from south to north, having the equa- 



^^ 



E 



E 



^> -=^ 



M 



tor at the side of A B and the pole at that of G H, and de- 

 creasing in height from B to H, in inverse proportion to their 

 density, their upper limit being supposed as well defined as 

 their lower one. Their pressure upon the surface of the earth is 

 exactly equal ; and were the circulation of the air by means of 

 the current ascending at A B, flowing from A to G, and de- 

 scending at G H, kept up, it would remain so, because the air, 

 removed from the apex of the column A B by the upper cur- 

 rent, would be compensated by that flowing to its base. But 

 let the upper current, instead of being confined to the higher 

 strata until it reaches G, descend between A B and C D as 

 shown by the line a Zi ; if the air fell as a simply falling body, 

 it would immediately, on its arrival in the lower current, as- 

 sume its direction, and move as shown by the dotted line c b ; 

 thus this current would be undisturbed by its descent. Such, 

 however, we have seen to be far from being the case, for it has 

 acquired, on its arrival from the equator at the upper part of 

 the column A B, the momentum or velocity due to the height 

 of its fall from higher columns ; it therefore advances in its 

 original direction shown by e d, forcing back the air flowing 

 by the force of its gravity towards the column A B. The 

 effect of such a state of the currents upon the atmospheric 

 pressure is very evident. 



The upper current is flowing on in the upper strata without 

 interruption towards the lower columns G H, &c,, whilst the 

 lower one, which should restore the air carried from the top 

 of A B to its base, is wholly driven back. 



The air is thus advancing through the whole height of the 

 atmosphere towards the pole from the column A B, which can 

 receive no compensation but that afforded from more southern 

 regions, or from lateral columns, where the south wind does 



