306 THE POPULAR SCIENCE MONTHLY. 



much more steep on the southward than on the northward side of this 

 area ; from which it follows that the rain-area is much less on the 

 southward than on the northward side of a progressive storm. 



All the atmospheric changes and phenomena above stated result 

 from the same general cause, but under different conditions and cir- 

 cumstances. This cause is the meeting of the polar and tropical cur- 

 rents in their movements northward and southward, to restore a dis- 

 turbed equilibrium in the atmosphere toward the equator or the poles. 



Applying this theory in brief explanation of the facts stated in 

 connection with Ballot's law, we find the area of loicest barometer at 

 the place where the two currents meet on the surface of the earth. It 

 is produced by the obliquely upward movement of the tropical cur- 

 rent over the polar current, and by its rising more or less vertically 

 in the vicinity of contact, after its horizontal progress northward has 

 been checked by encountering the polar current. This oblique and 

 upward movement of the tropical current diminishes the atmospheric 

 pressure there, as shown by the barometer, and produces that depress- 

 ing calm which is always felt by persons in any locality where this 

 meeting of currents takes place, or over which its area moves or oscil- 

 lates during the continuance of a storm. The elongated, elliptical 

 shape of this area is accounted for by the fact that it is the narrow 

 space between the two currents where they meet, and extends east- 

 ward and westward between them. It is rounded at the ends or mar- 

 gins of the currents, where the wind, in accordance with Ballot's law, 

 blows inward toward the centre line of contact, which is also the cen- 

 tre line of lowest barometer. And, as the two currents force each 

 other backward and forward during a storm, they necessarily carry 

 along the elliptical space between them, and thus its movements in the 

 direction of its shorter axis are accounted for. 



The rain-area, or that of low barometer, which surrounds the 

 elliptical region of lowest barometer where the currents meet on the 

 surface, as just explained, extends horizontally beneath the plane of 

 meeting, which is inclined northward. It is produced chiefly by the 

 oblique and upward movement of the tropical current over the polar. 



The gradients, or different degrees of pressure within the rain- 

 area, are caused by the same upward movement of the tropical cur- 

 rent over the polar, in connection with the constantly-varying heights 

 or depths of both polar and tropical air, which are vertically above 

 the space beneath the inclined plane from the region of lowest to that 

 of highest barometer northward; and the steeper or more abrupt 

 gradients southward are explained by the fact that when the tropical 

 current meets the polar current it is suddenly checked, and while a 

 portion of it moves obliquely over the polar current, as stated, another 

 portion of it rises, more or less vertically, for some distance around 

 the vicinity of contact, and the pressure is thus more suddenly dimin- 

 ished on the southward side of this area of low barometer than on 



