380 RECORD OF SCIENCE FOR 1«87 AND 188^. 



Because of the .slight couductivity of the air the process of warming can 

 l)rogress 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 equal 

 to that prevailing in the upper 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 Jong time. 



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

 and Reye, 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 i^ressure 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 up- 

 per cooled air flows away to one 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 influence 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 And 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 any point the 

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

 movements the efl'ect of gravity can be omitted. 



*Tlms in tlie origiDal ;— but the tliougbtful reader will perceive that in this "popu- 

 lar" presentation Oberbeck has not escaped the repetition of a popular error. The 

 fall of pressure, due to the warming of the air, is quite inappreciable; the observed 

 barometic depression is due to cyclonic motions. 



