OPERATIONS OF TILLAGE. 



127 



surface will be exposed to the atmosphere, and the suc- 

 cessive furrow-slices laid resting upon each other, thus : — 

 Fig. 18. 



" An essential property of the plough is, that it shall 

 move in the earth with a steady motion ; and the giving 

 to it the force and combination of parts necessary for that 

 purpose is one of the main difficulties attending its con- 

 struction. 



'' Were it ascertained, by experiment on the plough 



when at work, at a given depth of furrow, and in soil of 



a given texture, that a cord attached to any point A, fig. 



19, and drawn in the oblique direction A B, would so 



Fig. 19. 



pull forward the plough, that it should press uniformly 

 upon the earth at all points, from C to D, so that the 

 share should neither tend to point upwards or downwards, 

 but should move horizontally forward, then it is to some 

 part of this line that the moving power should be applied ; 

 and further, it is known from the principles of mechanics, 

 that it matters not, in so far as regards the force exerted, 



