The defect of this plough, which is roughly represented below, is in its weight and clumsiness more than in the shape of the mould-board; for when it is made lighter, it is not so bad a construction as appears at first sight, and it does its work very neatly in heavy learns with a dry subsoil. If, instead of one mould-board, two were used alternately, of a better shape for turning over the furrow-slice, this plough would be much improved ; and this plan is adopted in many other forms of the turn-wrest plough, as in that made by Cousins, of Southmolton.
The form of the turn-furrow is of material importance, for on this depends not only the perfection of the work, but also the lightness of the draught. When wo follow a plough working in a mellow soil which slightly adheres to the plough, we often perceive that, instead of being turned aside, the earth is carried forward, and only falls off when the accumulation of it becomes heavy enough to overcome the adhesion. It does not slide off from the mould-board itself, but separates from the earth which adheres to the latter; thus showing that the shape is defective, and giving good hints for its improvement. But as the same plough will sometimes turn over the same earth better when it is either drier or moister, it is very difficult to determine, by experiment only, what may, on the whole, be the best shape. A. little reflection and tho application of scientific principles may greatly assist us here. It is not sufficient however to find the curve which will make the plough go through the ground with the least force. The plough must also perform its work perfectly, and if anything is to be sacrificed, it is better to employ more power than to plough the ground badly. After having ascertained the mechanical principles which bear on tho working of the plough, we must observe its action carefully, follow the plough day after day, in different soils and different weather, and thus we may be led to observe all the circumstances which attend its operation, and correct any mistakes which an erroneous theory might have led to.
Many attempts have been made to ascertain the exact curve which the turn-furrow should have to perform the work well and at the same time to produce the least resistance. The difficulty of the problem lies in determining the data, or principles en which tho investigation is founded ; and these are so various, that it is not surprising that no very satisfactory conclusion has yet been obtained. We will make an
attempt at a solution from a simple examination of the motion to be produced in the portion of earth to be turned, which wo will call the furrow-slice. We shall suppose this separated from the adjacent soil by the vertical cut of the coulter, and at the same time from tho subsoil by the horizontal cut of the share ; a section of the slice, by a plane at right angles to the line of the ploughing, will be a parallelogram ADD0 (Ky. 5), the depth, A O, being the thickness of the slice, and A B its width. Confining our attention to this section of tho slice, the object is to move it from its position A D D c, as cut off by the coulter and share, to that of lid' c' a', where it is inclined at an angle of 45" to the horizontal lino ; the surface, A it (h' a'), being laid on the slice previously turned over, so as to bury the grass or weeds which might bo rooted there, exposing the roots to the sun and air. The more uniformly this motion is produced, and the more regularly the successive sections follow each other, the less power will be required to turn over the whole slice. The motion of o n round the point n must therefore be uniform. If the turn-furrow is horizontal at the point where it joins the share, and of the same width as the furrow-slice, it will slide under the slice ; and if the vertical sections of its upper surface, at equal distances from the share, are inclined at angles regularly increasing with this distance till it arrives at the perpendicular, the turn-furrow will, as it advances, turn the slice from a horizontal to a perpendicular position : the section of it will then be Dcab. The inclination of the section of the turn-furrow must now be to the other side, forming an obtuse angle with the section of the sole, until it has pushed the slice over at the required inclination of 45°, which theory and experience have shown to be the best adapted to .expose the greatest surface to the action of the atmosphere, and likewise to form the most regular fun-ows for the reception of the seed, which the harrow can then meet readily bury.