60 THE MASSACHUSETTS SOCIETY 



succeeding portion of earth which the plough raises 

 pushes off that which is on the transverse straight line 

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

 made up mathematically speaking) of an infinite num- 

 ber of such transverse straight lines. 



The angle which the straight line should form with 

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

 discovered 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 length of mould board. 



Col. Pickering does not give the date of his "learn- 

 ing 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 be- 

 gan 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. Jefferson'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 v/hich 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 



