150 



Report on Trials of Plows. 



flexure, the edge 5, touched the dotted line at e; it is now sep- 

 arated from it by the space b e, and each successive sheet is seen 

 to be in advance of the one beneath it. To produce this successive 

 advancement of the upper sheets, each must have moved over the 

 surface of the other. Every plowman * knows that the furrow 

 slice has a certain amount of elasticity, that is, the parts may be 

 displaced to a certain extent, and then, on the removal of the 

 displacing forces, they will return to their former position. This 

 elasticity varies with the nature of soil. In some the slightest 

 displacement will cause a permanent disruption; in others it will 

 stretch considerably before disruption takes place. The bending 

 A n upward, as shown in the figure, is pre- 



cisely what is done by the plow, the 

 point of the plow being directly beneath 

 cZ, and the point, b, resting on the 

 mould-board a little to the right of the 

 shin. Let us suppose that the soil of 

 the furrow slice, E ^s the most elastic, 

 and that the distance, b e, is just sufii- 

 J^iff. 74.' cient to overcome that elasticity, then 



it is evident that every soil whatever will be split horizontally 

 into flakes at its lower edge by a plow having that elevation. We 

 say at its lower edge because, as the upper edge does not move 

 through a space equal to b e, and as that is the precise point 

 where the elasticity of the slice is overcome, any less motion will 

 fail to split the layers. It follows, from this reasoning, that every 

 kind of soil should have a mould-board specially adapted to it, if 

 we would obtain the greatest economy of power, because the 

 more blunt is the entering wedge, the greater will be the draught; 

 and it would be uneconomical to use an entering wedge sufficiently 

 blunt to flake up a stiff" clay for a light loam where a much moi-e 

 acute angle will suflice. Still, if economy of power is not an 

 object, a plow which will separate the most tenacious soils longi- 

 tudinally will be sufficient for all soils that are less so. 



Our object is, as we have said, to pulverize the soil; but split- 

 ting it longitudinally is not pulverizing it. To accomplish this 

 object we must split it in other directions. Jethro TuU, as we 

 have stated, invented a plow with four coulters, figured in Plate 

 I, which made four vertical sections through the slice. This can 



*See Tull's remarks, page 18. 



