History of the Plow. 25 



of the rear end of the moukl-boarcl beyond tlie perpendicidar 

 which Mr. Jelt'crson thought the most convenient length in prac- 

 tice. 



At the point d erect a straight stick twelve inches long, which 

 is twice the depth of the proposed furrow; then from the point 

 b stretch a string tightly to the top of the stick at d. When this 

 is done, take a straight edge twelve inches long, and placing it on h c, 

 where it will rest horizontally and will coincide with the plane 

 of the parallelogram, move it backwards towards the line a d, 

 keeping the point of the edge which was at first coincident with 

 the point c along the line c e, and always preserving the paral- 

 lelism of the edge with the vertical plane of b c, it is evident that 

 as the edge resting on the diagonal b d moves backwards, the end 

 which at first coincided with b will rise upward, and every suc- 

 cessive removal towards the line a d it will assume a larger angle 

 with the horizontal plane. When the end which was at c arrives 

 at the point gr, and the corresponding part of the edge is on the 

 string at h, the line gr li will form an angle of forty-five degrees 

 with the horizontal plane. When the end of the stick stands on 

 the point f^ and the edge coincides with/* on the string, the stick 

 will be exactly perpendicular to the plane: passing on in the line 

 / e, towards a cZ, the angles will assume an opposite direction, so 

 that when the end of the moving stick rests upon e, and the edge 

 rests on the string at tZ, it will make an angle with the plane of 

 110^ degrees. When the path of the straight edge from b c to 

 e d is attentively considered, it is evident that it will have des- 

 cribed a curved surface, which is the ideal of what we seek to 

 reproduce in wood. 



Let us assume that the depth of the furrow is six inches, its 

 width nine inches, and the length of the mould-board two feet, 

 these fio;ures will decide the size of the block from which the 

 mould-board must be cut. 



The transverse section of the block. Fig. 



— —, ^ 19, will then be nine inches at the base b c, 



thirteen and a half inches at its summit a d, 

 and twelve inches at the side a b. The line 

 b c must be nine inches, because hat is the 

 width of the furrow. The line a b is placed 

 at twelve inches, because Mr. Jefferson found 

 that unless the height of the mould-board 

 jF///. /O. was tAvice as gieat as the furrow (which we 



