History of the Plow. 



27 



necessarily be twenty-tlirec; then draw the diagonals, m d, Fig. 21, 

 on the upper face, and d o on that which is situ- 

 ated on the right; make the saw enter at the 

 point m, directing it towards d, and making it 

 descend along the line m Z, until it marks out 

 a straight line between d and I, Fig. 22. Then 

 make the saw enter at the point o, and preserving 

 the direction o d, make it descend along the line 

 o I, until it meets with the central diagonal, d I, 

 which had been formed by the first cut; the pyra- 

 mid, m n I d, Fig. 23, will drop out by itself 

 and leave the block in the form represented by 



Fig. 22. 



I^fff. 2f. 



It is here to be observed, that in the last ope- 

 ration, inst-ead of stopping the saw at the central 



diagonal, d I, if we had continued to notch the 

 !)l(>ck, keeping on the same plane, the wedge 

 / m n o d a, Fig. 21, would have been taken 

 away, and there would have remained another 

 wedge, I o d a b c, which, as was observed be- 

 fore, in speaking of the principle in regard to 

 the construction of the mould-board, would exhi- 

 bit the most perfect form, were the only object 

 to raise the sod; but as it must also be turned **- 

 over, the left half of the upper wedge has been 

 preserved, in order to continue on the same side 

 the bevel to be formed on the right half of the 

 lower wedge. 



Let us now proceed to lay down the means of 



producing this bevel; in order to oljtain which we 

 had the precaution to trace out lines around the 

 ij block before we removed the pyramid, Fig. 23. 

 Care must be taken not to confound these lines, 

 now that they are separated by the vacuity left 

 by the removal of that pyramid, Fig. 22. 



Bearing in mind that the line a d is thirteen 

 and a half inches in length, the line a h is twelve 

 inches, the line 6 c or / o is nine inches in length 

 and that the line d c slopes downward and inward 

 towards the left hand, as shown in Fig. 19. We make the saw 

 enter in the two points of tlio first dotted line situated nearest to 



J^/^ 



m 



