PAPER BY PROF. OBERBECK. 175 



temperature 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 diminished. 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 north temperate 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 one that is stationary; therefore 

 one can inversely from the shape of the isobars infer the direction of 

 motion. If the region of ascending air has a circular form the compu- 

 tation can be rigorously executed. Without going into the details of 

 this interesting problem in this place I will only remark that the isobars 

 consist in closed curves similar to an ellipse. There is one direction 

 from the center outward 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 wbat 

 analysis has thus far accomplished. Still there is hope that it will 

 make further progress so far as concerns the relation between the 

 pressure and the motion of the air at the earth's surface. 



