414 SCIENTIFIC WRITINGS. 



current takes place in the upper regions of the at- 

 mosphere towards the poles, which however only in 

 a small part can reach polar latitudes. The reason 

 of this is that, owing to the narrowing of the upper 

 and simultaneous expanding of the lower stratum - 

 in consequence of the decrease of the latitudinal circles 

 on approaching the poles - - a partial passage of the 

 upper poleward moving current must continually take 

 place into the lower current towards the equator. It 

 is the inertia of the upper poleward travelling current, 

 which carries back the air in the lower one to the 

 equator. By this circulating current, continued for 

 untold thousands of years, the air of the higher lati- 

 tudes is intimately mixed with that of the lower ones, 

 and the whole aerial ocean must therefore rotate with 

 the mean easterly velocity of the earth's surface. The 

 westerly course of the trade -winds is thereby ex- 

 plained, as well as the mean easterly direction of the 

 aerial currents in the intermediate and polar latitudes. 

 The maxima and minima are essentially concomitant 

 phenomena of the alternation of temperature and of 

 the velocity of motion of the upper equatorial air- 

 current, and always depend on disturbances of the 

 indifferent equilibrium of the overlying air - strata. 

 When an aerial current, which has a higher or lower 

 temperature than corresponds to its altitude in the 

 adiabatic curve of temperature, breaks into the highest 

 regions of the aerial ocean, the indifferent equilibrium 

 of the whole aerial column is thereby disturbed, and 

 neutralization must take place by ascending or descend- 



