History of the Plow. 



29 



jli 



a b m I, which in Fig. 22 hiid vertically and 

 to the left of the block, now lies down- 

 wards on its face, while the side of the 

 wedge a d m {a d being thirteen and a half 

 inches) now lies vertically and is on the 

 leftside of the block; now continue the 

 stroke along these traces until both ends 

 of the saAV approach within an inch, or any 

 other convenient distance of the finished 

 face of the mould- board. When each of 

 the traces have been thus sawn through, 

 remove the sawn parts with some conven- 

 ient tool, as before, and the mould-board 

 is finished. 



Any one who will have the patience to 

 fix in his mind carefully the line left by 



the saw when its posterior end rests in the line o h, and its ante- 

 rior end on the diagonal d /, through each of the twenty-three 

 traces made upon the block, will be enabled to conceive of the 

 exact twist of the surface of the mould-board; yet as some minds 

 have a difficulty in realizing such warped surfaces, we give 

 another method which may be more clear to such readers. 



^/^. 26. 



Suppose the saw cuts the lines m d and o d, Fig. 24, in the 

 points X and t, in the traces x z and t s, parallel to a b b o, and 

 the prolongations of wdiich on the triangles m d I and Ida are 

 the lines x iv and t lo; the saw must then penetrate the block, 

 remaining in the same plane in question, until its point has reached 

 the point s, and at the same time touch the point w of the central 

 diagonal d I. The fore end of the saw will come out at some 

 point, y, of the face a m d, so that three points, s lo y, will be in 

 the same straight line. But if this operation be repeated in dif- 

 ferent places of the lines, m d and o d, bringing the edge down 



