380 RECORD OF SCIENCK FOR 1S87 AND 1888. 



Because of the slight (;oudiictivitv of the air the i)i()ceiss of waviniiig can 

 progress only slowly from below upwards, so that, as is well known, 

 the temperature of the air steadily diminishes as we ascend. The 

 heated air expands. The pressure becomes less. If the heating takes 

 place uniformly over a large area there will be at first no reason for 

 horizontal currents. But vertical currents can certainly be brought 

 about by this means. If we imagine a circumscribed mass of air trans- 

 ported into a higher region without any increase or diminution of its 

 heat its temperature will sink because it has expanded itself propor- 

 tionately to the diminished pressure. If its temperature is then ecpial 

 to that prevailing in the upi)er stratum it will remain in equilibrium at 

 this altitude as well as below. The atmosphere in this case exists in a 

 state of indifferent equilibrium. If its temperature is lower the mass 

 of air will again sink down ; in the reverse case it will rise higher. The 

 air in these cases is then in stable or unstable equilibrium respectively. 

 In the latter case any vertical movement initiated by some accidental 

 disturbance will not again disappear, but rapidly assumes increasing 

 dimensions. The current will also continue uniform for a long time. 



This is the explanation given by Espy, 1831, William Thomson, 1801, 

 and Eeye, of Strasburg, 1868, of the ascending air currents in the whirl- 

 winds of the tropics. 



The winds of our (temperate) zone also presuppose such ascending 

 currents whose origin must have been quite similar. The ascending 

 current is in general restricted to a definite region that we can desig- 

 nate as the base. Since the ascending current consists of warmer air, 

 therefore above this base the pressure sinks.* A barometric depression 

 is inaugurated there. The pressure increases from this region outward 

 in all directions. The isobars therefore surround the region of ascend- 

 ing atmospheric currents in closed curves. At greater heiglits the u])- 

 per cooled air flows away to o)ie side, and in other regions gives occa- 

 sion to descending currents of air. At the earth's surface itself the air 

 flows towards the depression; its iniluence thus extends over an area 

 much greater than that of the base. If we neglect the curvature of the 

 earth's surface over this larger area we find there simple horizontal 

 movements. Mathematical computations should now reveal to us the 

 nature of such horizontal movements. To this end all the causes of 

 motion or the forces that come into consideration are first to be collected 

 together. 



The differences of pressure have already been several times spoken 

 of; these are measured by the gradient, and it gives for aiiy i)oint the 

 direction and amount of the greatest change in pressure. In horizontal 

 movements the effect of gravity can be omitted. 



*Tliu8 in tlie origiual ; — but tbe tliouglitful reader will perceive that in this "popu- 

 lar" presentation Oberbeck has not escaped tbe repetition of a ))opular error. Tbe 

 fall of pressure, due to tbe warming of tbe air, is (]uite inappreciable; tbe observed 

 barometic depression is due to cyclonic motions. 



