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ANNUAL REPORT SMITHSONIAN INSTITUTION, 19 31 



air mass, or in other words the ratio of the length of path of the ray in the 

 atmosphere to that obtaining with vertical sun. In its logarithmic form the 



formula is the equation of the straight line. We find it satisfactorily exact on 

 many days at our excellent high-altitude stations in California, Chile, and 

 South West Africa. 



