_Secretary of Agriculture_ Arthur M. Hyde. _Assistant Secretary_ R. W. Dunlap. _Director of Scientific Work_ A. F. Woods. _Director of Regulatory Work_ Walter G. Campbell. _Director of Extension Work_ C. W. Warburton. _Director of Personnel and Business W. W. Stockberger. Administration_ _Director of Information_ M. S. Eisenhower. _Solicitor_ R. W. Williams. _Weather Bureau_ Charles F. Marvin, _Chief_. _Bureau of Animal Industry_ John R. Mohler, _Chief_. _Bureau of Dairy Industry_ O. E. Reed, _Chief_. _Bureau of Plant Industry_ William A. Taylor, _Chief_. _Forest Service_ R. Y. Stuart, _Chief_. _Bureau of Chemistry and Soils_ H. G. Knight, _Chief_. _Bureau of Entomology_ C. L. Marlatt, _Chief_. _Bureau of Biological Survey_ Paul G. Redington, _Chief_. _Bureau of Public Roads_ Thomas H. MacDonald, _Chief_. _Bureau of Agricultural Economics_ Nils A. Olsen, _Chief_. _Bureau of Home Economics_ Louise Stanley, _Chief_. _Plant Quarantine and Control C. L. Marlatt, _Chief_. Administration_ _Grain Futures Administration_ J. W. T. Duvel, _Chief_. _Food, Drug, and Insecticide Walter G. Campbell, _Director of Administration_ Regulatory Work, in Charge_. _Office of Experiment Stations_ E. W. Allen, _Chief_. _Office of Cooperative Extension Work_ C. B. Smith, _Chief_. _Library_ Claribel R. Barnett, _Librarian_. This bulletin is a contribution from _Bureau of Public Roads_ Thomas H. MacDonald, _Chief_. _Division of Agricultural Engineering_ S. H. McCrory, _in Charge_. U. S. GOVERNMENT PRINTING OFFICE: 1929 * * * * * Transcriber Notes All illustrations were moved so as to not split paragraphs.