DESCRIPTION OF a MOULD-BOARD. 315 



)perpenclicular, that it may fall over with its own weight. 

 And that this may be done fo as to give alio the lead re- 

 fiftence, it muft be made to rife gradually from the mo- 

 ment the fod is received. The mould-board then, in this 

 fecond office, operates as a tranfverfe, or rifmg wedge, the 

 point of which Hiding back horizontally on the ground, 

 the other end continues rifmg till it pafies the perpendi- 

 cular. Or, to vary the point of view, place on the 

 ground a wedge of the breadth of the ploughlhare, of its 

 length from the wing backwards, and as high at the heel 

 as it is wide. Draw a diagonal on its upper face from the 

 left angle at the point to the right upper angle of the heel. 

 Bevil the face from the diagonal to the right-bottom-edge 

 which lies on the ground. That half is then evidently in 

 the beft form for performing the two offices of raifmg and 

 turning the fod gradually, and with the leaft effort : and 

 if you will fuppofe the fame bevil continued acrofs the 

 left fide of the diagonal, that is, if you will fuppofe a 

 flraight line whofe length is at leaft equal to the breadth of 

 the wedge, applied on the face of the firft bevil and 

 moved backwards on it parallel with itfelf and with the 

 ends of the wedge, the lower end of the line moving 

 along the right-bottom-edge, a curved plane will be gene- 

 rated, whole chara£leriftic will be a combination ot the 

 principle of the wedge in crofs diredtions, and will give 

 what we feek, the mould-board of leajl rejiJleiKe. It offers 

 too this great advantage, that it may be made by the 

 coarfeft workman, by a procefs fo exadl that its form fhall 

 never be varied a fingle hair's breadth. One fatilt of all 

 other mould-boards is that, being copied by the eye, no 

 two will be alike. In truth it is eafier to form the mould- 

 board I fpeak of with precifion, when the method has 

 been once feen, than to defcribe that method either by 

 words or figures. I will attempt however to defcribe 

 it. Whatever may not be intelligible from the defcrip- 

 tioa may be fupplied from the model 1 fend you. 



T t 2 Let 



