12 Axa mses A

eafy to perceive, how thefe changes in fi- gure produce their correfpondent effects. For, as this board is, or ought to be, ufed in land fo loofe that it does not rife whole, the curve at the fore-end turns over fud- denly the earth of the furrow, and, in fo doing, feparates and bruifes it; and the pro- jetion on the other end lays it floping

backwards, to prevent its falling again into~

its former place. The hinder part of this board has no elevation; for, if it had, as the earth it moves through has not much cohe- fion, and does not rife whole, a great part of it would not be ftirred at all; which it is plain would happen to all the earth that thould lie below the elevation.

It is alfo eafy to perceive, from the de- {cription, how a few not very confiderable changes in the inclined fide of the half- wedge, would reprefent the figure of this

mould-board*.. Another

* Suppofe BC, fig. 7. a thick plain block, out of which a curved mould-board is to be formed; it mutt rife by a fmall projection above the plane from CtoD, and by a ftill greater projection from D tg B, onthe upper fide, but not on the lowes.