THE WEATHER 



43 



expect, the air turns a little to its right. That is, at a point 

 north of the "low" the air is moving not south bat south- 

 westerly; at a point east of the "low" it may be moving 

 northwest instead of west ; and so likewise for other places in 

 the vicinity of the "low." 

 This deviation of the 

 wind from the expected 

 direction is due to the 

 rotation of the earth on its 

 axis. The rotation does 

 not cause the wind to 

 blow, but it does affect 

 its direction. The effect 

 of rotation upon moving 

 bodies may be illustrated 

 by pouring water upon 

 a rotating globe. If a 

 smooth-surfaced globe is 

 placed with the pole which 

 represents the north pole 

 of the earth pointing up- 

 ward, and rotated while 

 the water is poured upon 

 it, the water marks on the 

 surface of the globe will 

 not run directly from pole 

 to pole, like meridians 

 (fig. 27), but 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 

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

 the same thing occurs on the surface of the earth. It is a gen- 

 eral law that anything moving on the surface of the earth will 

 tend to follow a curving path. In the northern hemisphere 



a rotating globe 



Water was poured upon the top of the globe 



while it was rotating from left to right. The 



direction of the water is reversed after cross- 



ing the equator 



