FOR PROMOTING AGRICULTURE 57 



by him to the French Academy and to the president of 

 the British Board of Agriculture in 1798. Mr. Jeffer- 

 son's thoughts were first drawn to the subject in 1788. 

 Travelling that year in Lorraine, in France, he fre- 

 quently alighted from his carriage to watch the opera- 

 tion of the ploughs in use in the fields, and, as a result 

 of his observations, entered, at the time, the following 

 in his diary: "The offices of a mould-board are to 

 receive the sod after the share has cut under it, to 

 raise it gradually and to reverse it. It should be as 

 wide as the furrow and of a length suited to the 

 construction of the plough." 



In his letter to the president of the British Board he 

 elaborates this idea in description, and compares the 

 action of the mould-board to the movement of two 

 wedges, so combined as to present a curved surface. 

 The function of one wedge, he says, is to lift so much 

 of the sod, or slice of earth, as is necessary to the full 

 height required, and the function of the other is to 

 exert force laterally and obliquely to carry the sod so 

 that its upper edge shall go beyond the perpendicular, 

 that it may be inverted by its own weight. And he 

 adds that the form of the mould-board must be such 

 as to present in its passage the least possible resistance, 

 and so require the minimum of moving power. His 

 further proposition is that a mould-board of this com- 

 pound-wedge sort can be constructed according to a 

 mathematical formula and by a process so exact, that, 

 in the hands of "any common workman its form will 

 not vary the thickness of a hair." He gives in detail 

 a mathematical analysis of the problem and a descrip- 

 tion of the method of manufacture. He has in mind in 

 this description a wooden mould-board, and says that 

 in practice it works well and that he has several such 

 ploughs in use on his farms. In his communication to 



