28 Report on Trials of Plows. 



m and o, Fig. 22, and on the diagonals m d and o d, continning 

 the stroke on that first line till it reach on the one hand the cen- 

 tral diagonal d I, and on the other the lower right edge, o /t, of 

 the block. Fig. 22; the posterior end of the saw will come out at 

 some point situated on the upper trace in a straight line with the 

 corresponding points of the edge and the central diagonal. Con- 

 tinue to do the same thing on all the points formed by the inter- 

 section of the exterior dias^onals and lines traced out around the 

 block, taking always the central diagonal and the edge o h, as the 

 term and the traces as directors; the result will be, that when 

 you have formed several cuts with the saw, the end of that instru- 

 ment, which came out before at the upper face of the block, will 

 come out at the face situated on the left of the latter; and all 

 these different cuts of the saw will have marked out as many 

 straight lines, which, extending from the lower edge, o h, of the 

 block, will proceed to cut the central diagonal. Now, by the 

 help of any proper tool, remove the sawn parts, taking care to 

 leave visible the traces of the saw, and this face of the mould- 

 board will be finished. 



An attentive consideration of the processes which have been 

 gone through with the block, Fig. 22, will show that they are 

 precisely equivalent to those described in elucidating Fig. 18; the 

 diagonal, d I, in the first, corresponds with the diagonal b d in 

 the second, and the line o h in the one corresponds with the line 

 c e in the other. 



It must not be forgotten that the point d overhangs the point 

 c four and a half inches towards the right; hence there will be a 

 point in the line o h, at /, which will be exactly vertical on the 

 diagonal d I, it is obvious that in sawing the traces bewteen/ and 

 0, the point of the saw will be directed upward and forward 

 towards the line am. At / it will be directed vertically upward, 

 and after it passes the point /, towards c, it will be directed 

 lownwards and backwards towards the line b I; the saw marks 

 'A'hich are left on the face of the mould-board are the representa- 

 tives of the successive positions of the twelve-inch stick as it was 

 progressively moved backward in a line parallel to itself from c 

 towards e, in Fig. 18. 



It now remains to construct the opposite side of the mould- 

 board so as to be parallel at all points with the face. Invert the 

 block, and make the saw enter at the points where the line b I, 

 Fiij. 24, meets with the traces, and remembering that the side. 



