History of the Plow. 55 



iiio; divisions of leiiijth iii the transit of the slice throiiofh the curve 

 h d K, M'hich represents the periphery of the cylindrical surfaces 

 at the line of transit. The parts of the figure lying above that 

 line, represent those that must be superimposed above the quad- 

 rantal portion of the cylinder, to complete the upper regions of 

 the mould-board; tliese parts acting merely as a preventive against 

 the overfall of the soil into the waste of the plow, are of less 

 importance as to form, than those just described, but are quite 

 necessary in the practice of plowing. The parallelogram 1/ d 

 exhibits the relation in which the furrow slice stands to this form 

 of mould-board, when the slice has been raised to the perpendi- 

 cular, and y u in its ultimate position. 



Althousfh I hold this to be the true theoretical form, it is not 

 in this state fit to be employed as a practical mould-board ; but 

 the steps to render it so are very simple. The broad shovel- 

 mouth, a 6, Fig. 125, would meet with obstructions too numerous 

 to admit for a moment of its adoption in practice ; but we have 

 only to remove the right hand portion of the edge, a b, in the 

 direction b q, making the breadth, q m, six and one-half or seven 

 inches broad ; that portion also contained in T r 3 is to be cut 

 away, leaving m r about four inches broad ; b q r m will then 

 represent the share ; the mould-board being thus of the prolonged 

 form in the fore part. And, though this form has no peculiar 

 advantage over the truncated, in respect to working, it is better 

 adapted to admit of the body being constructed of malleable 

 iron, a practice which, though more expensive, is certainly the 

 most preferable, by reason of its greater durability, and being 

 less liable to fracture through the efiect of shocks when stones ©r 

 other obstructions are encountered. 



Besides the removal of tliese parts of the theoretical mould- 

 board, other slight modifications are admissible. When the parts 

 have been cut away as described, the edge, b q, of the share will 

 be found too thick for a cutting edge. If brought to a proper 

 thickness by removing the parts bdo%i\ making the edge to coin- 

 cide with the curved surface, the share so prepared would have 

 the character that belongs to the cresting plows. The lower edge 

 of the mould-board, from r to 2, would be also rather high, and 

 would present unnecessary resistance to the lower side of the 

 slice ; both parts, therefore, require to be reduced. The surface 

 of the feather, b q, is to be sloped down till it becomes straight 

 between tlie points b and q, q not being more tlian one-quarter of 



