314 DESCRIPTION of a MOULD-BOARD. 



notice of the board, as from a zeal for improving the condi- 

 tion of human life, by an interchange of its comforts, and 

 of the information which may increafe them. 

 ********* 



In a former letter to you I mentioned the conftrudion 

 of the mould-board of a plough which had occurred to me, 

 as advantageous in its form, as certain and invariable in 

 the method of obtaining it with precifion. I remember 

 that Mr. Strickland of York, a member of your board, 

 was fo well fatisfied with the principles on which it was 

 formed that he took fome drawings of it ; and fome others 

 have confidered it with the fame approbation. An experi- 

 ence of five years has enabled me to fay, it anfwers in 

 pradice to what it promifes in theory. The mould-board 

 fhould be a continuation of the wing of the ploughfhare, 

 beginning at its hinder edge, and in the fame plane. Its 

 firft office is to receive the fod horizontally from the wing, 

 to raife it to a proper height for being turned over, and 

 to make, in its progrefs, the leajl rcfjlence pojjibk ; and 

 confequently to require a minimum in the moving power. 

 Were this its only office, the wedge would offer itfelf as 

 the moft * eligible form in pradtice. But the fod is to be 

 turned over alfo. To do this, the one edge of it is not 

 to be raifed at all ; for to raife this would be a wafte of 

 labour. The other edge is to be raifed till it pafTes the 



perpendicular, 



* I am aware that were the turf only to he raifed to a given height in a 

 given length of mould-board, and not to be turned over, tlie form of leaft 

 "rcfiftence would not be rigoroufly a wedge with botli faces ftraight, but with 

 die upper one curved according to the laws of the folid of leaft refiftence 

 defcribed by the mathematicians. But the difference between the effedt of 

 the curved and of the plain wedge, in the cafe of a mould-board, is fo 

 minute, and tlie difhculty in the execution which the former would fuperin- 

 duce on common workmen is fo great, that the plain wedge is the moft eli- 

 gible to be affumed in praftice for the firft element of our conftrudlion* 



