40 



ELEMENTS OF GENERAL SCIENCE 



if it remains in one place, its indicator goes up and down, 

 thus showing that the pressure is constantly changing. If 

 we should secure a record of the readings of barometers at 

 a great many places in the United States at the same time, 

 we should find that they would not be the same. The read- 

 ings would show that in some parts of the country the pres- 

 sure is high, while in others it is low. We might find, for 

 instance, that the barometers read 29.8 inches in Iowa but 



FIG. 25. Weather map for March 14, 1913 



only 29.5 inches in Indiana. An examination of the facts 

 will show that the pressure is never the same all over the 

 country and that it does not long remain stationary at any 

 one place. 



If the pressure of the air is high in Iowa and relatively 

 low in Indiana, we can see that the air next to the ground 

 m Iowa will tend to flow away toward places of less pressure. 

 It will move from Iowa toward Indiana across the interven- 

 ing state of Illinois, and Illinois will therefore have a wind 



