Explanatory Note 



The series embodis^ 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 

 official use of Federal, State or cooperating agencies and in 

 processed form for economy and to avoid delay in publication. 



shington, D. C . 

 1951 



