IV PREFACE 



Reddick, Cornell University; H. C. Thompson, U. S. Depart- 

 ment of Agriculture; F. H. Ballou, Ohio Agricultural Experi- 

 ment Station, and W. H. Wicks, Arkansas Agricultural 

 College, for the figures for which they are credited in the 

 text. 



Acknowledgment is also due the R. M. Kellogg Co., of 

 Three Rivers, Michigan, the Bateman Manufacturing Co. 

 and the International Harvester Company for furnishing 

 photographs and illustrations, for which due credit is given 

 under each figure. 



The author wishes to express his appreciation to Profes- 

 sors R. F. Howard and J. R. Cooper, of the University of 

 Nebraska, for reading and criticising the manuscript and for 

 the many valuable suggestions which they offered. He also 

 wishes to thank all others who contributed in any way to the 

 work and to whom it is hoped due credit has been given in 

 every case. 



G. W. H. 



Lincoln, Nebraska, 1919. 



