504 W. von Siemens on the General 



lower air, which will continue until the indifferent equilibrium 

 disturbed by the lower pressure of the upper strata of the 

 atmosphere is again restored. When the pressure of the air 

 of the upper strata increases beyond that due to their elevation, 

 through cooling and condensation, in consequence of the dimi- 

 nution of the width of the current with increasing latitude 

 the opposite effect will be produced, the limiting strata will 

 sink down, condensing the lower strata of air, and thereby 

 increasing their pressure. In both cases the disturbed indif- 

 ferent equilibrium must eventually be again restored by the 

 strata of air lying below the source of disturbance delivering 

 by means of up-currents, or taking up by means of down- 

 currents, as much air as is required to restore the condition of 

 indifferent equilibrium in the whole height of the atmosphere. 

 In order to effect this the pressure of the air of the lower 

 strata must increase or diminish, in the first instance, until it 

 has adjusted itself to the scale of pressure of the indifferent 

 equilibrium of the disturbing upper air-strata. In other 

 words, the pressure on the earth's surface must vary propor- 

 tionately with the change of pressure above, by which means 

 the sudden changes observed in the pressure of the air on the 

 earth's surface are perfectly explained. This alteration in the 

 condition of the lower strata of air will continue even after 

 this adjustment has been effected so long as the cause in the 

 upper strata of the atmosphere continues ; and during that 

 time minimum pressures of air with ascending currents, and 

 maximum pressures with descending currents, of air must 

 occur, and set the atmosphere for a considerable distance in 

 whirling motion. It is only when the flow of air in the higher 

 strata of the atmosphere has again become normal, that a mean 

 position of the barometer and relative rest of the atmosphere- 

 on the earth's surface will again prevail. 



The theory of the general system of winds may therefore 

 be summed up in the following statements : — 



1. All motions of the air depend upon disturbances of the 

 indifferent equilibrium of the atmosphere, and tend to bring 

 about its restoration. 



2. These disturbances are caused by the superheating of the 

 strata of air lying nearest to the earth's surface through solar 

 heat, by unsymmetrical cooling of the upper layers of the air 

 through radiation, and by the piling up of masses of air in 

 motion through obstructions occurring to the current. 



o. The disturbances are balanced by means of ascending, 

 currents, which possess an acceleration of such a kind that the- 

 increase of velocity of the air is proportional to the diminution 

 of its pressure. 



