1805.] Description of a Mould Board,&6: 9

'These traces will Show how the wedge which is at the right angle, rises gradually upon the face of the direct or inferior wedge, of which the declivity is preserved in the central dia- gonal. It is easy to represent and render Sensible, the manner M which the elod of earth, is raised upon the earth board we are describing, by tracing upon the earth a parallelogram, two feet long, by nine inches wide, abcd, fig. 6, then placing at the point b, the end oft a tick twenty-Seven inches and a half, and raising the other end to twelve inches above the point e(the line de equal to ſour inches and a halt, repre- Sents the quantity which the beight oftheeartb board passesthe perpendicular.) This done, take another stick of twelve inch- es, and placing it upon a b, move it backwords and parallel to itzelf, from ab to cd, taking care to hold one of its ends, always upon the line a d, whilst the other is woved the length of be Stick be, which represents here the central diagonal. The motion of the Stick ot twelve inches, will be that of our riSing wedge, and will Show how each transversal line of the elod of earth, is conducted from its first horizontal position, üll it is elevated to a height 50 far above the perpendicular, that it falls reversed by it3 own proper weight.

But to return to our operation, it noW remains to execute the bottom part of the earth board. Reverse the block, and make the saw enter by the points where the line a(, tig. 9, meets the traces, and continue your cut the length of these traces, till 1ihe two ends of the Saw approach within an inch, (or any other proper thickness) of the oppogsite face of tbe mould board. When these traces are finiShed, take away as before, the pieces 5awed, and the mould board will be com- pleated.Y

Theſe two curves being traced, we will ſuppoſe the right lines condutted to the places where the(aw is ſtopped, every time that it touches the dia- gonal k 1, and of which the one as we have before ſaid, palles by the points SZ y; and we conceive a ſurface tanjent at all th« ſe places, and 6f which the limits are at one part the curves k yn, n 1 and on the other the edge o'h, this Lurface which will be diſcovered by the ſe&tions made with a proper initru= ment, will form one of the faces ot the earth board. This 1Ss repretented at big 19, and the face in queſtion, is th;t which was ſhown before and indi- cated by inlor. It ſovuld"be remarked, that the angle fituated towards bs Fig 9, upon the part kcediehot the block, has been alſo cut off by a ſec tion made from d tor, in conformity to what will be ſaid further on.

+ We will add to rhis detail, an explanation ſimilar to that which we h“ve given relative to the front of the earth board. The thick' els ot this being determined by that of the part k«dieb, fig. 9, or what amounts to he ſame thing by the length of the lines c k, d1, eh, Ilct us conceive at firlk that we have traccd in going irom the point c, the curve cup. parallc] to" k yn, then in going from ti:e point p, the curve p H is p-rallelto in. Let us then ſuppoſe thar fhe jaw cuts theedge al, ofthe face a b m 1, at the point D, nuated in the fame plan as X F and ts which plan has been taken for example