138 



FIRST YEAR SCIENCE 



Fig. 67. 



Maps recording barometric conditions are made in the 



same way as the isothermal maps, only their lines pass 



through places of equal baro- 

 metric 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. 

 67. 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 made charts of the weather 



conditions over the entire surface 



of the earth. Every year more 



and more data are being collected 



and these charts are becoming more and more reliable. 

 These charts are of great value, since they aid in the 



explanation of climatic conditions in different parts of the 



world. The results, of the data thus gathered together 



have been of untold service to commerce and each year 



have saved many lives and a vast amount of wealth^ On 



pages 136 and 137 are isothermal maps of the world for the 



months of January and July. 



