Explanatory Note 



The series embodies results of investigations, usually of re- 

 stricted 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. 

 July, 1951 



