214 



WEATHER AND CLIMATE 



FIGURE 85 



are not drawn for each degree, but only for each ten 



degrees. 



When the map has been constructed, copies are made in 



which the figures are left off and only the isotherms are 



preserved. In Figure 84 we have 

 a plan before the isotherms are 

 drawn, and in Figure 85 after 

 the isotherms are drawn. Figure 

 86 is a typical isothermal dia- 

 gram. If the map itself were 

 sketched, it would be an iso- 

 thermal map. 



Maps recording barometric 

 conditions are made in the same 



way as the isothermal maps, only their lines pass through 



places of equal barometric pressure instead of places of 



equal temperature. These lines are called isobars. 

 Weather maps are prepared by 



the United States Weather Bureau 



every day, on which are both the 



isotherms and isobars for that 



day. The data for these maps 



are telegraphed each morning 



from stations scattered all over 



the settled part of North America. 



FIGURE 86 



Weather Maps. Expensive 



weather bureaus are maintained not only by the United 

 States, but by all the other highly civilized countries of 

 the world. Records are kept also by sea captains and by 

 other observers throughout the world, and these are 

 gathered together by scientific men and from them are 



