44 



ELEMENTS OF GENERAL SCIENCE 



this path curves to the right, and the direction of the rota- 

 tion of cyclones and other storms is opposite to the direction 

 which is followed by the hands of a clock, or counterclock- 

 wise ; in the southern hemisphere deflection is to the left and 

 the rotation is clockwise (fig. 28). 



There are at least two factors controlling the movement 

 of the wind : the differences of pressure, which cause the au- 

 to move across the isobars toward places of lower pressure ; 



and the rotation of the 

 earth, which deflects the 

 moving air to the right of 

 the course which it would 

 otherwise follow. Since 

 the effect of the earth's 

 rotation is the same, re- 

 gardless of whether the air 

 is moving east, west, north, 

 south, or in some inter- 

 mediate direction, the air 

 about a "low" will circle 

 about it before reaching 

 the center. Each mass of 



Diagram of air currents as affected by rota- air will travel in a Spiral 

 tion of the earth. Both northern (N) and 

 southern (S) hemispheres are shown 



FIG. 28. Deflection of air currents 



direction, and the whole 

 body of air about the 

 "low" will rotate. The action of the air is like that of water 

 when it is running through a hole in the bottom of the 

 vessel containing it. We must therefore think of a "low" 

 as a great whirlpool of air sometimes as much as a thousand 

 miles in diameter and moving slowly eastward. Such a great 

 spiral whirl is called a cyclone. The cyclone must not be con- 

 fused with a very destructive storm that sometimes occurs and 

 is often called a cyclone, although the proper name is tornado. 

 The air about a "high" will be deflected to the right and thus 

 will circulate spirally outward. This is called an anticyclone. 



