42 ELEMENTS OF GENERAL SCIENCE 



The map is made in the following manner : Each morning 

 at eight o'clock (Washington time), at every weather-bureau 

 station in the United States, the barometer is read and the 

 reading telegraphed to Washington. There the readings are 

 placed^ upon a map, each at the location of the city from 

 which it was telegraphed. The lines are then drawn through 

 places having the same pressures. These lines thus make it 

 possible to see very easily where the high-pressure and low- 

 pressure centers are located. Usually a map will show several 

 well-defined areas of this kind. 



41. Progression of high and low pressures. Observation of 

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

 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 northern part of the United States and almost 

 invariably pass through the St. Lawrence River valley. The 

 " highs " occupy the spaces between the " lows " and also pro- 

 ceed eastward across the continent, but their course is usually 

 inclined toward the southeast, and their progress is not so 

 regular as that of the 'Mows." Both "highs" and "lows" 

 are commonly elongated in a north-and-south direction and 

 are often compared to a series of great waves in which the 

 " highs " are the crests and the " lows " the troughs. 



42. Winds about a "low" cyclones. Since a low area 

 usually has a more or less circular form and is surrounded 

 by higher pressures, it follows that the air will tend to move 

 into the low area from all sides. If there were nothing to 

 direct the course of the wind except its tendency to blow 

 toward the lowest pressure, it would blow from all directions 

 straight across the isobars toward the center of low pressure. 

 An examination of the weather maps, in which the wind direc- 

 tion is indicated by arrows, shows that while the wind does 

 blow into the low areas, it does not blow straight toward 

 the center of the "low." Instead of moving as we should 



