BLASIUS'S THEORY OF STORMS. 2 g 7 







Atmospheric Currents. All storms owe their origin to the heat 

 of the sun, which produces differences of temperature in different por- 

 tions of the earth, and thereby causes all the movements and currents 

 which take place in the atmosphere around the globe. As the air at 

 the equator is more highly heated by the sun than that of any other 

 region, it expands, becomes lighter and rises, causing a partial vacuum 

 or deficiency there at the surface of the earth. The air north and 

 south of it at once moves forward from opposite directions to supply 

 this deficiency at the equator, and this in turn becomes heated and 

 ascends. Other air again moves forward from north and south to 

 replace it, and thus an upward current at the equator, and a north 

 and south polar current at the surface toward the equator, are estab- 

 lished. These north and south polar currents cause a deficiency of 

 air at the poles, and the heated air which has risen at the equator into 

 the upper region of the atmosphere divides and moves forward tow- 

 ard the opposite poles to supply the deficiency caused there. Thus, 

 upper currents in opposite directions from the equator to the poles 

 are also established in order to restore the equilibrium disturbed by 

 the surface polar currents flowing toward the equator. 



But by the time the air of the upper currents has reached the re- 

 gion of the tropics, it has become cooler and heavier, and descends to 

 the surface of the earth. Here it divides into two currents one 

 flowing back to the equator, forming the trade-winds ; and the other, 

 becoming warmer again at the surface, flows toward the poles, meet- 

 ing the polar current somewhere north of the tropic in the northern 

 hemisphere, and south of it in the southern. This meeting of the 

 equatorial or tropical and polar currents in the temperate zone, and the 

 various phenomena attending and resulting from it, are the most sig- 

 nificant and important facts ichich constitute the basis of Prof Bla- 

 sius's theory of storms, in distinction from the centripetal theory of 

 Espy, and the rotary theory of Colonel Clapper, as developed bv Pid- 

 dington, Thorn, Dove, and others, and better known in this country 

 as the cyclone theory of Redfield. 



The following diagram (Fig. l) will serve to indicate the move- 

 ments and courses of the general atmospheric currents of- the earth, 

 as above described, the arrows showing the directions in which thev 

 move. 



The two currents above referred to the polar and the equato- 

 rial or tropical are of different temperatures, and move horizontally 

 in opposite directions toward each other. When they meet they over- 

 lap 1 each other somewhat like two wedges with their sharp ends for- 

 ward. The warmer current, being lighter, glides obliquely over the 

 cooler current, and moves northward ; and the cooler current, being 

 heavier, moves beneath it on the surface of the earth southward, just 

 as two currents, warm and cold, flow over each other in opposite di- 

 rections through an open window or door of a heated room. 



