31 S DESCRIPTION of a MOULD-BOARD. 



the hinder part will of courfe continue the fame ftraight 

 line, which will iffue fomevvhere on the top of the block. 

 Then enter the faw in like manner on the two proje(;:ting 

 points of the 2d fcribe, and faw in, along the fcribe, be- 

 fore and behind, till it reaches the fame bottom edge of 

 the right fide, and the central diagonal. Then the 3d, 

 4th, 5th, &c. fcribes fucceffively. After cutting in fe- 

 veral of the earlier fcribes, the hinder part of the faw 

 ^vill iffue at the left fide of the block, and all the fcribes 

 being cut, the fav/ wnll have left ftraight lines from the 

 bottom edge of the right fide of the block, acrols the cen- 

 tral diagonal. With an adze dub off all the fawed parts 

 to the bottoms of the faw- marks, juft leaving the traces 

 vifible, and the face of the mould-board is finiflied. 

 Thefe traces will fhew how the crofs wedge rifcs gradually 

 on the face of the direcfl: wedge, which is preferved in 

 trace of the central diagonal. A perfon may reprefent 

 to himfelf, fenfibly and eafily the manner in which the fod 

 is raifed on this mould-board, by defcribing on the ground 

 a parallelogram 2 feet long and 9 inches broad, as a. b. c. d. 

 Fig. 6. then reft one end of a ftick 27^ inches long on 

 the ground at b. and raife the other 1 2 inches high at e. 

 which is 4y inches from d^ and reprefents the overhang- 

 ing of that fide of the mould-board. Then prefent 

 another ftick 12 inches long from a. to b. and move 

 it backwards parallel with itfelf from a, h. to d. c. keeping 

 one end of it always on the line a. d. and letting the other 

 rife as it recedes along the diagonal ftick b. e. which re- 

 prefents our central diagonal. The motion of the crofs 

 iHck will be that of our rifing wedge, and will (hew how 

 every tranfverfe line of the fod is conduced from its firft 

 horizontal pofition, till it is raifed fo far beyond the per- 

 pendicular as to fall reverfed by its own weight. But to 

 return to our work. We have ftill to form the under fide 

 of the mould-boaid. Turn the block bottom up. Lnter 



the 



