of Energy in the Atmosphere. 461 



The same holds good of the meridional velocity, which in 

 the higher strata is very little diminished by friction, but in 

 the lower strata very much diminished. 



If, now, with increase of latitude the upper current-bed has 

 become so contracted that a block takes place, the resulting 

 local increase of pressure produces at the same time a disturb- 

 ance in the curve of neutral equilibrium of the atmosphere. 

 The excess of air flowing in must therefore at first compress 

 the lower strata of air in such a way that the curve of equili- 

 brium shall be reestablished down to the earth's surface. 

 There results therefore a descending current of air, and an 

 increase of pressure on the earth's surface depending upon 

 the ratio of the increase of pressure in the higher regions of 

 the air to its normal pressure— that is, a local maximum of 

 atmospheric pressure. From this region of higher pressure 

 currents will issue in radial directions along the earth's sur- 

 face, which prevent the complete reestablishment of the neutral 

 equilibrium corresponding to the increase of pressure in the 

 higher rarefied strata of air. 



Such a maximum of pressure may therefore persist for a 

 long time ; and, since it continuously supplies the excess of 

 the inflowing equatorial air to the lower return-current, may 

 even hinder for a long time the formation of a regular deflec- 

 tion of the upper current into the lower one. This must, 

 however, finally take place, and with the cessation of the 

 block in the upper strata of air ceases also the cause of the 

 maximum. 



The formation of the return-branch of the equatorial cur- 

 rent is to be conceived as occurring in consequence of the 

 continually increasing deflection towards the east of the cur- 

 rent hemmed in, in consequence of the block in its progress 

 towards the pole, so that it carries with it by internal friction 

 the lower strata of air, relatively quiescent or moving in the 

 opposite direction. It will therefore approach the earth's 

 surface on a wide curve of gentle fall, until it finally unites 

 with the polar current and. enters upon its return journey to 

 the equator. But by thus "carrying with it'''' the lower 

 strata of air, it will produce a rarefaction of the bounding 

 strata of air under it, and consequently one of the already- 

 described opposite disturbances of the neutral equilibrium. 

 There must therefore result an up-current of the lower strata 

 of air to restore the neutral equilibrium, and a local minimum 

 of atmospheric pressure on the surface of the earth results. 

 The magnitude of the decrease of atmospheric pressure here 

 observed is, just as with the maximum, not equal to the 

 actual decrease of pressure caused by the carrying-power of 



