of the department library, in the collection of material for this work. Miss C. L. Feldkamp, of the library of the Office of Experiment Stations, put the bibliography in proper form. D. J. Crosby, late of the faculty of the New York College of Agriculture at Cornell University, but also associated for many years with the author in the educational work of the Office of Experiment Stations and States Relations Service, carefully read the manuscript and made many helpful suggestions for its improvement.