Sentence:  The wedge is simply two inclined planes put base to base, and the same reasoning is true of it—that is, the thinner the wedge or more gradual the slope, the more easily it is driven. Applying this to the plough, we find that the coulter, share, <WSD>wrest</WSD>, cheek-plates, and sole-shoe, all form more or less continuous parts of a large wedge or moving inclined plane. 
Meaning of the word marked with <WSD> tags:  A metal (formerly wooden) piece of some ploughs attached under the mouldboard (the curved blade that turns over the furrow) for clearing out the furrow; the mouldboard itself.