FOll PROMOTING AGRICULTURE. 51 



correct. In a word, the curvature will be a portionof a 

 spiral screw. Take a large screw augur for an exemplifi- 

 cation. No earth can be left on such a mould-board ; for 

 every succeeding portion of earth which the plough raises 

 pushes off that which is on the transverse straight line be- 

 hind it, and the face of the mould-board consists (is made 

 up mathematically speaking) of an infinite number of such 

 tranverse straight lines. 



The angle which the straight line should form with the 

 sole of the plough is another material point, to be discov- 

 ered by experiments, and experiments are also necessary to 

 learn the proper angle of the essential straight line with 

 the land side of the plough, or to decide where lies the just 

 medium of breadth, of angle and x length of mould board. 



Col. Pickering does not give the date of his " learning to 

 hold the plough," other than by saying that it was soon 

 after the close of the war. His experience must have been 

 nearly contemporaneous with that of Mr. Jefferson in 

 Lorraine. It is noticeable that the two observers reached a 

 like conclusion by starting, mentally, from opposite positions. 

 Mr. Jefferson began with the thought that in the plough 

 there were two diversely acting wedges, one to lift and the 

 other to thrust. To blend these two into a properly hollowed 

 or curved surface was his problem. Mr. Pickering began 

 with the thought of the straight line in which the receding 

 earth moved over the mould-board, and, in imagination, on 

 that basis, shaped an ideal mould-board. When Mr. Jeffer- 

 son's model was called to his attention he saw his ideal 

 realized, and with reference to the straight line, exclaimed, 

 " Here is the principle on which this mould-board is 

 formed." But it was not so. Mr. Jefferson did not begin, 

 with a straight line and around it form the proper curvature, 

 but began with the outsides of his two co-working wedges, 

 and by mathematics, proceeded inward until the two were 

 blended ; and that blending proved to be the straight line 

 with which Mr. Pickering began. The coincidence was 

 inevitable, since the reasoning in each case had reference 

 to mathematical or geometrical laws. 



This view of the matter is confirmed by a competent 



