History of the Flow. 



31 



eiit points of the edge h /, those by which it comes out will form, 

 on the face b I o c, a curve j s q; and if, through these points and 

 those corresponding to them in the lines e p and jp q, there will 

 be diaMm straight lines, such as that which passes through z and 

 w, which we have taken as an example, the surface touching these 

 straight lines, and, uncovered by means of any sharp instrument, 

 will form the remainder oH j h o I q t' of the plane bloc, being 

 the opposite side of the mould-board. 



It is fixed to the plow by mortising the fore part, o Z, Fig. 25, 

 into the posterior end of the share, which must be made double, 

 like the handle of a pocket comb, that it may receive and secure 

 this fore part of the mould-l>oard. A screw nail is then made to 

 pass through the mould-board and the handle of the share at the 

 place of their contact, and two other screw nails pass through the 

 tail of the mould-board and the right handle of the plow. The 

 part of the tail which passes be3'ond the handle must be cut 

 diagonally, and the work will be finished. 



^ We have thus given the plan of forming 

 the mould-board in full detail, so that the 

 principle of its construction maybe clearly 

 understood; })ut Mr. Jefferson found, in 

 actual practice, that some modifications of 

 it might be usefully made. Thus, he says: 

 " Instead of beginning to form the block 

 as represented hy a b c d, Fig. 26, where 

 a b is twelve inches in length, and the 



111 



J^i^. 26. 

 an^le at b is a riorht one, I cut off towards the 

 bottom and along the whole length, b c, of the 

 block a Avedge, b c e, the line e b being equal to 

 the thickness of the bar of the share (which I sup- 

 pose to be one and a half inches); for, as the face 

 of the wing inclines from the bar to the ground, if 

 the block were placed on the share, without taking 

 into the account this inclination, the side a b would i | 

 lose its perpendicular direction, and the side a d 

 would cease to be horizontal. Besides, instead of 

 leaving at the top of the block a breadth of thir- 

 teen and a half inches from m to w, I remove from 

 the right side a kind of wedge, n d z c p, of one 

 and a half inches in thickness, Fig. 27; because experience has 



