266 



PHYSIOGRAPHY 



the pressure greatest at the center, and Fig. 226 shows the slope of 

 isobaric surfaces in the same region. 



If a surface of water had the form shown by the uppermost line 

 in Fig. 224, the water would flow in from all sides until the surface 



Fig. 223. A series of isobaric lines showing diminishing pressure toward the 



center. 



became level. If the water surface had the form shown in Fig. 226 

 the water would flow away from the top in all directions. The 

 air, which is more fluid than water, acts in a similar way, and 

 moves down the slope of any isobaric surface which has slope. Such 

 movements of air are winds. When the isobaric slope (or isobaric 



Fig. 224. Section through the area represented in Fig. 223, showing the posi- 

 tion of isobaric surfaces. As the pressure toward the center of the area 

 shown in Fig. 223 diminishes, the isobaric surface bends downward. It 

 will be seen that isobaric lines are the lines where isobaric surfaces cut sea- 

 level. 



gradient) is high, the wind is strong; when the isobaric gradient is 

 low, the wind is gentle; and when there is no isobaric gradient, 

 that is, when the isobaric surface is level, there is no wind. The 

 strong wind is strong for much the same reason that a swift river 



