Explanatory Note 



The series embodies results of investigations , usually of restricted 

 scope, intended to aid or direct management or utilization practices and 

 as guides for administrative or legislative action. It is issued in 

 limited quantities for the offical use of Federal, State or cooperating 

 agencies and in processed form for economy and to avoid delay in publi- 

 cation. 



Washington, D.C. 

 May, 1951 



