ELEMENTS OF GENERAL SCIENCE 



with the pole which represents the north pole of the earth 

 pointing upward, and rotated while the water is poured upon it, 

 the paths which the drops of water mark on the surface of the 

 globe will not run directly from pole to pole, like meridians 

 (fig. 23). They will be curved to right or left. If the globe 

 is rotated in the same direction as the earth is rotating, the 

 paths of the water will curve to the right (westward) in the 

 northern hemisphere, but as soon as the drops cross the equa- 

 tor they begin to curve to the left (eastward). Precisely the 



same tiling occurs on the 

 surface of the earth. It is a 

 general law that anything 

 moving on the surface of 

 the earth will tend to fol- 

 low a curving path. In the 

 northern hemisphere this 

 path curves to the right, and 

 the direction of rotation of 

 cyclones and other storms 

 is opposite to the direction 

 which is followed by the 

 hands of a clock, or counter- 

 clockwise. In the southern 

 hemisphere deflection is to 

 the left and the rotation is 

 clockwise (fig. 24). 

 Having now learned this new law, we should no longer ex- 

 pect the air to blow at right angles to the isobars and there- 

 fore straight toward the center of the " lows." There are at 

 least two factors controlling the movement of the wind. One 

 of these is the result of differences of pressure, and it causes 

 the air to move across the isobars toward places of lower pres- 

 sure ; the other is the rotation of the earth, and this deflects the 

 moving air always a little to the right of the course which it 

 would otherwise follow. Since the effect of the earth's rotation 



LOW 



s 



FIG. 24. Deflection of air currents 



Diagram of air currents as affected by rota- 

 tion of the earth. Both northern (N) and 

 southern (S) hemispheres are shown 



