Explanatory Note 



The series embodies results of investigations, usually of res- 

 tricted 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 official use of Federal, 

 State or cooperating agencies and in processed form for economy and 

 to avoid delay in publication,, 



Washington, D. C. 

 March, 1951 



