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THE POPULAR SCIENCE MONTHLY 



The plane of meeting between these two currents is more or less 

 inclined northward in the northern hemisphere, for the reason just 

 stated; and the lower end of the plane, or the space of air between 

 these two currents where they meet on the surface of the earth, 1 con- 

 stitutes the centre line or area proper of the storm, and the region of 

 lowest barometer. The horizontal plane beneath this inclined plane a 



Fig. 1. Atmospheric Currents. 



is the geographical extent of the region affected by the storm and the 

 region of low barometer. The place where the currents meet is con- 

 stantly changing with the changing seasons, following the sun north- 

 ward in summer and southward in winter. These changes of locality 

 do not, however, take place in one continuous movement of the at- 

 mosphere ; but with successive oscillations, like the waves of a rising 

 tide, each succeeding wave advancing farther and receding less than 

 the one before it, until its most northern or southern limit is reached 

 as represented by the numbers 1 and 2 in the diagram when the 

 oscillations in the opposite direction again begin. Whenever the 

 lower end of the plane of meeting between the two opposing currents 

 at B oscillates or passes over any place on the surface of the earth, it 

 will cause storm or change of weather there a change of wind, of 

 temperature, and of atmospheric pressure. 



The inclination of the plane of meeting, or the slope of the tropi- 



1 As shown at B iu the diagram. 



- From B to D. 



