DESCRIPTION OF a MOULD-BOARD. 317 



the point m. being the left-fore-upper corner, and fiw 

 in, guiding the hinder part ot the law along the diagonal 

 m. k. (Fig. 2. 3.) and the fore part down the left edge of 

 the block at the fore-end m. I. (Fig. -2.) till it reaches /'. and 

 /. in a ftraight line. It will then have reached the true cen- 

 tral diagonal of the block k. I. Fig. 5. then enter the faw at 

 the point 0. being the right-fore-bottom corner, and lltw in, 

 guiding the hinder part of the faw along the diagonal 0. k. 

 (Fig 3.) and the fore part along the bottom edge of the 

 fore end 0. /. till it again reaches k. I. Fig. 5. the fame 

 central diagonal to which you had cut in the other direc- 

 tion. Confequently the pyramid /'. m. n. 0. /. Fig. 4. 

 drops out and leaves the block in the form Fig. 5. You 

 will now obferve that if in the laft operation, inftead of 

 flopping the faw at the central diagonal k. I. we had cut 

 through the block in the fame plane, we fhould have taken 

 off a wedge /. m. n. 0. k. b. Fig. 3. and left the block in 

 the form of a wedge alfo /. 0. k. b. a. p. k. which, when 

 fpeaking of the principle of the mould-board, I obferved 

 would be the mofl: perfect form if it had only to raife the 

 fod. But as it is to be turned over alfo, the left half of 

 the upper wedge is preferved, to furnifli on the left fide, 

 the continuation of the bevil which was propofed to be 

 made on the right half of the bottom wedge. We are now 

 to proceed to the bevil, for which purpofe the fcribes 

 round the block were formed before the pyramidal piece 

 was taken out ; and attention muft be ufed not to mifmatch 

 or miftake them, now that they are disjoined by the with- 

 drawing of that piece. F.nter the faw on the two points 

 of the ift fcribe where it has been disjoined, which is 

 exadly where it interfefted the two fuperficial diagonals, 

 and faw acrofs the hollow of the block, guiding the faw, 

 both before and behind, along the fame fcribe, till the 

 fore part of the faw reaches the bottom edge of the right 

 fide, and the middle of the faw reaches the central diagonal ; 



the 



