THE MOULD-BOAED. 125 



shorter twist towards its rear. This form is distinctly 

 shown in the figure of Holbrook's Stubble Plow, on 

 a future page ; and it contributes largely to that crumbling 

 movement of the sod, so important for effecting pulveriza- 

 tion. 



The mould-board of a plow is capable of an almost infi- 

 nite variety of forms, and the multitude of inventors have 

 each ado|)ted a different one. Some have made their 

 selections by repeated random trials ; while others, among 

 whom Thomas Jefferson was the first, devised a series of 

 straight lines, mathematically arranged, by which uniform- 

 ity was given to the shape. The limits of this work pre- 

 clude a full explanation. Many modifications in com- 

 bining lines have been adopted, the most successful of 

 which is that of Ex-Governor Holbrook, of Vermont, 

 whose plows made according to these rules have perform- 

 ed admirably. It is less essential that farmers generally 

 should understand these mathematical principles, provided 

 they find a plow that will do good work ; because, as al- 

 ready shown, the form of the mould-board has compara- 

 tively little to do with the required draught of the team. 

 It will be readily understood, however, 

 that more force will be needed for draw- 

 ing a short or blunt j)low, like Fig. 131, 

 than one in the form of a longer wedge, 

 as in fig. 132, the latter, like a sharp 

 boat in water, moving more easily. Care 

 must be taken, however, that this slender wedge be not 

 too long, else the friction of the sod on the extended sur- 

 fiice may overbalance the advantage. 

 The cutting part of the plow may be 

 improperly formed like the square end 

 of a chisel, and the sod may slide back- 

 ward on a rise, with a very slight turn, 

 until elevated to a considerable height before inversion ; this 

 must require more force of the team, and make the plow hard 



