COMPONENTS OF THE AIR 385 



modify atmospheric conditions that unfailing predictions are 

 impossible, but the Weather Bureau predictions prove true in 

 about eight cases out of ten. 



The reports made out at Washington are telegraphed on 

 request to any locality and are frequently published in the 

 daily papers, together with the forecast of the local office. 

 A careful study of these reports enables one to forecast to 

 some extent the probable weather conditions of the day. 



The first impression of a weather map (Fig. 255) with its 

 various lines and signals is apt to be one of confusion, and the 

 temptation comes to abandon the task of finding an underly- 

 ing plan of the weather. If one will bear in mind a few sim- 

 ple rules, the complexity of the weather map will disappear 

 and a glance at it will give one information concerning gen- 

 eral weather conditions, just as a glance at the thermometer 

 in the morning will give some indication of the probable tem- 

 perature of the day. 



Components of the air. The best known constituent of 

 the air is oxygen, already familiar to us as an active agent in 

 the oxidation of most substances. Almost one fifth of the air 

 which envelops us is made up of the life-giving oxygen. This 

 supply of oxygen in the air is constantly being used up by 

 breathing animals and glowing fires, and unless there were 

 some constant source of additional supply, the quantity of 

 oxygen in the air would soon become insufficient to support 

 animal life. The unfailing constant source of atmospheric 

 oxygen is plant life. The leaves of plants absorb carbon 

 dioxide from the air and break it up into oxygen and carbon. 

 The plant makes use of the carbon but rejects the oxygen, 

 which passes back into the atmosphere through the pores of the 

 leaves. 



Although oxygen constitutes only one fifth of the atmos- 

 phere, it is one of the most abundant and widely scattered of 



