520 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 51 



For relatively small changes of pressure this gives us 



i «P - G>) = RT tLZl + ^ C^dt 



2 Po Po J M 



If an air mass is flowing toward a place of lower pressure and at 

 the same time the pressure at every point of its path is changing 

 with the time, then the increase of the living force of the moving 

 mass is no longer determined simply by its initial and final pressure. 

 If the pressure rises with the time then the increase in kinetic 

 energy is greater than it would be in a steady field, but if the 

 pressure falls, then the increase is less. 



Assume that the barometric pressure in the moving mass under 

 consideration falls io mm during ten hours, but that in the field 

 through which the moving mass describes its path it rises io mm , 

 then in this case the increase of the kinetic energy of the moving 

 mass is twice as great as in a steady field. But if the pressure 

 in the field surrounding the path had fallen io mm instead of rising 

 during these ten hours, then the moving mass would not have 

 needed to move at all and the increase of kinetic energy over that 

 of the stationary field would have been zero. 



PART II. ON THE MAINTENANCE OF A DIFFERENCE OF 

 PRESSURE BY THE ADDITION OF HEAT 



(7.) STEADY CIRCULATION IN A DRY ATMOSPHERE 



During movements of the air out of regions of higher pressure 

 into regions of lower pressure, work is being expended continuously 

 by the pressural forces drawing from a previously accumulated 

 supply. The potential energy of the system must exhaust itself 

 and the differences of pressure at any level must disappear, unless 

 there be compensation from some source. Movements against 

 the gradient could indeed reconvert kinetic energy into potential 

 energy, but then the process would develop some sort of wave 

 action and even then the loss by friction must be replaced. 



So far as the study of energy is concerned, one can imagine a 

 scheme for a steady circulation between regions of differing pres- 

 sures as explained in the following diagram and text. 



