DYNAMIC METEOROLOGY. 383 



Wo have as yet only spoken of the iuvariable systems of wind. In 

 fact, however, their duration is' relatively short. No sooner is a depres- 

 sion formed than it tills itself np. Furthermore, the central region of 

 depression generally does not remain long in the same place, but pro- 

 gresses often with great velocity, drawing the whole system of winds with 

 it. We must look to the density of the horizontal current flowing in 

 toward the ascending current of air as the cause of these changes. The 

 system of winds remains unchanged only when, as has hitherto been 

 silently assumed, the temperature and density of the horizontal and 

 vertical currents are alike. If the inflowing air is warmer, the depres- 

 sion increases in depth, in the opposite case it becomes shallower. 



Finally, if the inflowing air is not of the same temperature on all sides, 

 but is on the one side of higher and on the other side of lower tempera- 

 ture than the ascending air, then it will on the one side be strengthened 

 and its area increased, on the other side enfeebled and its area dimin- 

 ished. The consequence of this is that the current of air or the region 

 of depression moves along. The cyclone progresses. Since in the 

 cyclones of our zone the air entering on the east side comes from more 

 southern, therefore in general warmer, regions, while the air entering on 

 the west side comes from the north and is generally colder, therefore 

 the cyclone progresses from west to east or from southwest to northeast. 

 This is in fact the path of most cyclones in northern Europe. For a 

 moving cyclone the isobaric curves must have a different shape than for 

 a stationary. Therefore one can inversely, from the shape of the isobars, 

 infer the direction of motion. If the region of ascending air has a cir- 

 cular form the computation can be rigorously executed. Without going 

 into the details of this interesting ]>roblem in this place, I will only re- 

 mark that the isobars consist of closed curves similar to an ellipse. 

 There is one direction from the center outwards, in which the isobars 

 are most crowded together, while in the opposite direction they are 

 furthest apart. The movement of the cyclone is in a direction at right 

 angles to this line. With the solution of this problem we now stand 

 about at the limits of what analysis has thus far accomplished. Still 

 there is hope that it will make further progress so far as concerns the 

 relations between the pressure and the motion of the air at the earth's 

 surface. 



(21) (Jherhecl: — Oberbeck has added to his memoir of 1882 a further 

 investigation on the general movements of the atmosphere, which was 

 published in full in two communications to the Academy of Sciences at 

 Berlin in the year 1888. (See Sitzungsberichte, 1888, xiv, p. 383-395.) 

 On account of their importance I have, as in his former memoirs, given a 

 complete translation of these in the collection of translations i)reviously 

 noticed ; but to those who do not care to follow the mathematical inves- 

 tigation the following resume of his results is given mostly as expressed 

 in a popular [japer by Oberbeck himself and originally contributed by 

 bim to the Naturwisseuschaftliche Rundschau of June 9, 1888. 



