PREFACE 
illustrative material. All the drawings have been pre- 
pared by Mr. B. F. Williamson from sketches or photo- 
graphs furnished by the author. Special acknowledg- 
ment is made to The Pennsylvania State College for the 
use of numerous negatives, and to Professors J. W. 
Gregg and C. E. Myers of this institution for assistance 
in making photographs ; to Professor L. C. Corbett and 
Professor W. W. Tracy, of the United States Depart- 
ment of Agriculture, for Figures 82, 84, 90, 93, 94, 95, 
ioo, 102, 103, and 104; to H. B. Fullerton, of the Long 
Island Railroad Experimental Stations, for Figures 50, 
54, 73, 81 and 75 ; to Professor C. G. Woodbury, of the 
Indiana Experiment Station, for Figures 87 and 88; to 
Prof. H. H. Whetzel, of the Cornell Experiment Station, 
for Figures 62 and 72; to Dr. W. L. Howard, of the Mis- 
souri Experiment Station, for Figure 24; to Professor 
John W. Lloyd, of the Illinois Experiment Station, for 
Figure 89; to R. H. Garrahan, Kingston, Pa., for Fig- 
ure 21 ; to H. R. Pennypacker, Phoenixville, Pa., for 
Figure 78; to Lord & Burnham, for Figure 19; to the 
Skinner Irrigation Co., for Figures 16, 17 and 17A; to 
Robert J. Walton, Hummelstown, Pa., for Figures 55 
and 76; to J. G. Curtiss, Rochester, N. Y., for Figures 
69, 70 and 71 ; and to the Manaway Irrigation Co., New- 
held, N. J., for Figure 15. 
Ralph L. Watts. 
State College, Pa., 1922. 
