Pb ACTIO AL Questions in Plows and Plowing. 153 



avowedly formed on mathematical principles, as Jefferson's, Ste- 

 phens', Rhani's and Knox's. Indeed, their object in invoking the 

 aid of mathematical principles was to iusnre this perfect regu- 

 larity in the twiwt of the mould-board. 



If we are correct in the })rinciples laid down above, the makers 

 of plows have been in error with regard to the desirableness 

 of a uniform twist in the mould-board. We grant that where 

 economy of power is the sole object, the twist must be regular 

 and gentle; but where the primary object is to mellow the soil, 

 then irregularity in the lines becomes indispensable. 



We have shoAvn that, in order to secure thorough pulverization, 

 the surface of the mould-board must be adapted to fracture the 

 slice in three several directions. It is therefore plain that there 

 must be three points, at least, where the twist must be intensified 

 so as to insure the desired result, and at these points, at least, the 

 regularity of the curve must be interrupted, so as to overcome the 

 maximum elasticity of the soil. 



The conclusion to which these views lead us is, that the best 

 plow must have irregular lines of curvature to an extent which 

 will insure the fracture of the furrow slice in three directions; 

 but the irregularity should never be greater than will accomplish 

 this result, as all excess will be a waste of power. 



We have spoken repeatedly of the elasticity of the earth of 

 which the furrow slice is composed, especially when its sui-face is 

 covered with a tough sod. In order to measure the extent of 

 this elasticity we inserted two wooden wedges, a and b (Fig. 79), 

 at right angles to the land side of the furrow; when the plow, 

 which was running twelve inches deep, came up with them at the 

 zero line, the wedges ceased to be at right angles with the land 

 side, but made an acute angle with it, and the wedge a' was seven 

 inches in one instance in advance of the ^vedge b' , and this w^as 

 the measure of the elasticity of the slice, as the two wedges 

 resumed their former relative positions when the furrow was 

 completely turned over. There is considerable difliculty in 

 making the experiment, as the slice is generally bi-oken before it 

 arrives at the zero line. We repeated it many times unsuccess- 

 fully. We found the stretch of the land side of the furrow twice 

 to be six inches, and only once did it cohere until it was stretched 

 seven inches. 



