26 Of the Coulter.

the plough becomes the director of its motion, and therefore, in the perfect ſtate of the inſtrument, ſhould perform no other part of the work. This con- ſideration ſhows at once that the land ſide of a plough ſhould be a flat or plane ſurface. If it has any other form, being, ſtrongly preſſed to the firn- ground, ts proje¿(ng; parts muſt ſink into it, and the plough cannot advance without continually tearing of the earth with îts projeding parts; which earth muít tumble in between the coulter and ſheath, or into any hollow in the land ſide of the plough, and, colle&ing gra- dually there, will both obſtruct its mo- tion, and turn it from the land. Here, without any farther argument, appears the great“ imperfe@ion of the Scotch.

plough