52 Report on Trials of Plows. 



stances of the medium on which it works. In the outset it is 

 assumed that the soil is homogeneous, and that it possesses such a 

 degree of tenacity and elasticity as to yield to the passing form 

 of the plow, and to resume, when laid in the due position, that 

 form which was first impressed upon the slice by the action of 

 iho share and coulter; the second consideration being the cuttino- 

 of a slice from the solid land. In a theoretical view, this must 

 be an operation through its whole depth and breadth; hence 

 the share is conceived to be a cutting edge, which shall have a 

 horizontal breadth equal to the breadth of the slice that is to be 

 raised, and that the face or land side of the coulter shall stand at 

 right angles to this. Another consideration is, that the slice now 

 supposed to be cut has to be raised on one side and turned over 

 through an angle of 135 deg., the turning over being performed 

 on the lower ri<>:lit hand edofe, as on a hino^e, throuo-h the first 90 

 deg., the remaining 45 deg. being performed on what was at first 

 the upper right hand edge (^m, Fig. 97). The slice, in going 

 through this evolution, has to undergo a twisting action and be 

 again returned to its original form of a right prism. To accom- 

 plish this last process, it is evident that a wed(/e, tivisted on its 

 upper surface, must be the agent; and to find the form and dimen- 

 sions of this wedge is solving the problem that gives the surface 

 of the mould-board required. 



We have seen. Fig. 97, that the slice, in passing through the 

 first ninety degrees, descrilios the quadrant d k with its lower 

 edge, and in doing so we can conceive a continued slice to form 

 the solid of revolution abed e. Fig. 123, Plate IV, which is 

 a quarter of a cylinder, as shown here in isometrical perspective; 

 the radius a b or a c, being equal to the 1)rcadth of the slice. We 

 have next to consider the ano-le of elevation of the twisted wedge; 

 and in doing so we must not only consider the least resistance, 

 but also the most convenient lenirth of the wed'»-e. In takino; a 

 low angle, which would present, of course, proportionally little 

 resistance, it would at the sam.e time yield a length of mould 

 board that would be highly inconvenient, seeing that the generat- 

 ing point, in any section of the slice, must ultimately reach the 

 same height, whether by a high or a low angle. From experience, 

 we find that from the point of the share to that point in the plow's 

 body where the slice arrives at the perpendicular position, which 

 I have liamed the zero, that thirty inches foi-m a convenient 

 length. The length c d ()f the solid is therefore made equal to 



