ATMOSPHERIC PRESSURE 



267 



is swift ; the gentle wind is gentle for much the same reason that 

 the slow river is sluggish; while the absence of wind may be com- 

 pared to the pond in which there is no perceptible current because 

 there is no slope of the surface. 



Fig. 225. A series of isobaric lines showing increasing pressure toward the 



center. 



Returning now to Fig. 222 several points are readily seen: 

 (1) The isobars have a general east-west course, though many of 

 them are not straight; (2) on the average, they show greater pres- 

 sure in low latitudes than in high latitudes; (3) they are highest 



\s\\ 



Fig. 226. Section through the area represented in Fig. 225, showing the posi- 

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

 area shown in Fig. 225 increases, the isobaric surface bends upward. 



(that is, they show highest pressure) in the latitudes just outside 

 the tropics; (4) they are more regular in the southern hemisphere 

 than in the northern; and (5) they are, on the whole, more regular 

 on the sea than on the land. 



Isobars and parallels. It will be remembered that isotherms 

 have a general east-west course. Is it the latitude, or the tempera- 



