ELEMENTS OF GENERAL SCIENCE 



several such maps and note the size of the areas included 

 within the low-pressure and high-pressure regions. 



31. Procession of high and low pressures. Observation of 

 maps on successive days will disclose the fact that a " low," 

 for instance, does not remain in the same place. It moves 

 eastward across the country at the rate of several hundred 

 miles per day, and often several of them may be on the map 

 at once. The centers of the " lows " commonly cross the 



FIG. 22. Average courses of " highs " and " lows " in the United States 

 After the United States Weather Bureau 



northern part of the United States and almost invariably pass 

 down the St. Lawrence River valley (fig. 22). The " highs " 

 occupy the spaces between the " lows " and also proceed east- 

 ward across the continent, but their course is usually inclined 

 toward the southeast, and their progress is not as regular as 

 that of the " lows." Both " highs " and " lows " are commonly 

 elongated in a north-and-south direction and are often com- 

 pared to a series of great waves in which the " highs " are 

 the crests and the " lows " are the troughs, 



