Oil Agricultural Mechanics. 115 



I do not know what portion of the draught of the plough is due to the 

 mere cutting of the furrow- slice; but it is evident that the tearing and 

 breaking of the shoe occasioned by the abrupt rise of the mould-board 

 in many ploughs, before it is properly cut, must greatly increase it. 



The mould-board, besides being most gradual in its rise from the 

 share, must terminate in such a manner that the furrow-slice, even sup- 

 posing it to be cut of the proper dimensions, shall not merely drop off 

 it into its ultimate position, but that it shall be pressed into its proper 

 place.* It should be twisted over so as to incline from the plough at 

 an angle of 45° where it terminates, 



In the construction of the mould-board there are two points to attend 

 to, the length and the nature of the curve. 



Both of these have hitherto been determined merely by experience. 

 It is, however, evidently a subject capable of mathematical solution. 



With regard to the first of them, the mould-board may be divided 

 into two parts, — first, that between the share and backwards, till by 

 its action the sod becomes vertical ; and that between this point and 

 backwards, till the turf by its action assumes its ultimate position. Now, 

 as the proper length of the mould-board is that Vv^hich receives least 

 friction in turning the turf, it is obvious ihat the circumstances on which 

 that length depends vary in the two portions into which the mould- 

 board is divided. 



There are two different descriptions of pressure on the mould-board, 

 by each of which, of course, friction is produced : that arising from the 

 mere weight, and that arising from the twist of the turf. It is obvious 

 that it is the first portion only of the mould-board on which both these 

 forces act. The hinder part of it cannot be acted on by the weight of 

 the turf. It is only the force required to twist the turf that will produce 

 friction on this part of the mould-board ; and here, after the turf has 

 been forced over to a certain degree, its own weight will tend to twist it, 

 and so assist the action of the mould-board. 



If only the force of torsion in the sod acted on the mould-board, the 

 more gradual the twist, and the longer the mould-board, the less force 

 would be exerted to overcome it. The longer, then, the hinder part of 

 the mould-board is, the less force will be required to overcome the force 

 which acts on it. The only limit to the length of the hinder part of the 

 mould-board, then, is that of convenience in the form of the implement. 

 There is, however, a limit to the length of the first and more important 

 portion of this part of the plough, which is imposed in another way. 

 It has been said, that if torsion were the only force acting on it, the 

 longer the mould-board was made, the more easily would it overcome 

 this force. There is, however, besides this, the force of gravity acting 

 on the mass of the furrow lying on the mould-board, and the less the 

 weight of the turf the more easily, of course, will this be overcome. The 

 shorter, then, the first part of the mould-board is, according to this 

 view, the less force will be required to work it, since there will be a less 



* This is particularly necessary in ploughing turf, when it is difficult to 

 bury the sward, unless the mould-board, by a pressure on the upper edge 

 of the sod, tend perfectly to turn it over. 



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