MECHANICAL PRINCIPLES OF THE PLOW 197 



ticity of the sod may not be overcome in shallow plowing. 

 In stubble plows, the moldboard is made steeper, and the inner 

 edge of the furrow slice must travel a much greater distance. 

 The inner edge being required to turn a sharp angle while the 

 outer edge remains stationary, the slice is broken by perpen- 

 dicular fissures which cross it at right angles to the landside. 



A third line of cleavage is secured by the effect of the curva- 

 ture of the moldboard. Since the soil layers do not slide over 

 each other with the freedom of the leaves of a book, the sur- 

 face layer, which is also more tenacious, often curves sharply 

 without crumbling. The layers nearer the moldboard must 

 therefore be extended sufficiently to form concentric arcs of 

 longer radius. The steeper and sharper the moldboard, the 

 more will the natural elasticity of the soil be overcome, and 

 deep fissures will extend parallel to the landside and perpen- 

 dicular to the bottom of the furrow. In practice, any one, or 

 all, of these three effects of the moldboards may be lost through 

 carelessness or avoided by design. 



The plow may be likened also to a plane. If the bit of the 

 plane is sharp and properly set, it cuts easily. If a thin 

 shaving is cut, the lifting action of the bit placed at a low angle 

 is not sufficient to overcome the elasticity of the wood, and 

 the shaving comes forth in a smooth, continuous band. If the 

 bit be set at a greater angle, even the thin shaving is sharply 

 broken at frequent intervals, and it requires greater force to 

 drive the plane. If a deep shaving is taken, then the bit must 

 be set at a practically impossible angle if the shaving is not to 

 be broken. Ordinarily, the difference in the distance traveled 

 by the upper and lower surfaces of the shaving, about the 

 angle formed by the board and the bit, is sufficient to over- 

 come the natural yielding of the wood, and since the fibers are 

 not free to glide over each other, the lowest layer, which trav- 

 els farthest must be broken at frequent intervals. This 

 accounts for the great amount of power required to move a 

 plane under improper conditions of adjustment. 



