i62 Journal of Agricultural Research vo1.xii,no.4 



From a study of Tables II, IV, and VI it is evident that the share can 

 not be as accurately described by mathematical equations as can the 

 moldboard. However, the differences even upon the share are not very 

 great. It must be remembered that these surfaces have been developed 

 empirically; experience and an extensive knowledge of the conditions to 

 be met have been the chief guides. Yet this implement produced in the 

 school of experience has a surface approximately mathematically exact 

 in form. Further, the surfaces of cast bottoms, which, because of the 

 difficulty of manufacture, are not changed unless necessity demands, 

 consist in some cases approximately of a portion from each of two quadric 

 surfaces. It will be shown later in discussing the history of the plow that 

 the surfaces of the Holbrook bottoms were designed to be portions of 

 hyperboloids of one sheet. In the Utica (N. Y.) plow trials these 

 machines received many first awards and much commendation from the 

 judges for the excellence of their work. In addition to this, Mr. J. J. 

 Washburn, of Batavia, N. Y., who knew Mr. Holbrook and was 

 present at the Utica plow trials, stated that the Holbrook plows did as 

 good work as any that it has ever been his pleasure to witness. Thus, 

 there is considerable evidence, based upon field experience, which indi- 

 cates that a portion of a hyperboloid of one sheet is the proper form for 

 the surface of a plow bottom. So far as is known, this hypothesis awaits 

 definite proof. 



MOTION OF THE SOIL PARTICLES IN PLOWING 



For the purpose of studying the motion of the soil particles in plowing, 

 the work was limited to sod ground available in the vicinity of Ithaca, 

 N. Y. From observations on a sod plow at work in the field (PI. 7, C), 

 the following general facts regarding the furrow slice were noted : 



The lower outside ^ edge of the furrow slice did not appear to be either 

 stretched or compressed. 



The upper outside edge of the furrow slice appeared to be compressed. 



The inside of the furrow slice was stretched, the lower edge more than 

 the upper edge. 



As the furrow slice passed over the moldboard the cracks, which had 

 formed on the inside in traveling over the share and the front portion 

 of the moldboard, closed up as the soil passed over the rear of the plow 

 bottom, indicating a point of maximum stretching. 



The above considerations made it evident that a more detailed study 

 of the behavior of the furrow slice was desirable. For this purpose rows 

 of pins were set in the unplowed ground, the pins being driven in the 

 ground to the estimated depth of plowing, as shown in Plate 4, A. The 

 longitudinal rows are parallel to the line of motion of the plow, which is 

 also parallel to the Z-axis (fig. 8) and the transverse rows perpendicular 



»The portion of the furrow slice immediately adjacent to the furrow is called the "outside." 



