THE AIR 



159 



bars have no definite direction, but they run north and 

 south more often than any other way. Sometimes they 

 form circles. They are indicated on the weather maps 

 by solid black lines. 



Isotherms are lines connecting places having equal 

 temperatures at the same time. Their general direction 



WEATHER BUREAU INSTRUMENTS AT THE ANNUAL EXPOSITION, 

 PITTSBURGH, PA. 



is east and west, but they shift to the north or south 

 according to the elevation of the country and the direc- 

 tion of the winds. The isotherms now put on the weather 

 maps indicate the temperatures which are multiples of 

 ten. They are indicated on the weather maps by dotted 

 lines. 



107. Weather Maps. In 128 different places in the 

 United States are Government Weather Bureau Stations, 

 at each of which accurate observations of the weather 

 are made. These observations consist of reading the 

 barometer and thermometer, ascertaining the humidity 

 of the air and the amount of rainfall or precipitation 



