To a Drill that plants upon the Level, Marking-wheels are necessary, to the End that every Row may be at its due Distance: As in a Drill with Five Shares, for planting Rows Eight Inches asunder, Four of the Five cannot err, because Four equal Spaces are included betwixt the Five Shares; but the Fifth (which we call the parting Space) being on the Outside unconfin’d, would scarce ever be equal, were it not kept equal by the Help of the Marking-wheels. The Rule for setting of these is thus: We compute altogether the Five Spaces belonging to the Five Rows; which being in all Forty Inches, we set the Marking-wheels Eighty Inches asunder, that is, double the Distance of all the Spaces, each Wheel being equidistant to the Middle of the Drill, which Middle being exactly over the Horse-path, when the Drill is turn’d, the Horse goes back upon the Track of one of these Wheels, making his Path exactly Forty Inches distant from his last Path: By this means also the Rows of the whole Field may be kept equidistant, and parallel to one another; so that it would be difficult for an Eye to distinguish the parting Rows from the rest. But when Two different Sorts of Seed are planted, suppose a Row of St. Foin betwixt every Row of Barley, the Rows of which being Eight Inches asunder, and the Barley drill’d by the fore Hopper into the Chanels made by the five Shares, and the St. Foin drill’d from the hinder Hopper into the Chanels made by Six Shares, the Marking-wheels must be at no greater Distance than those above-mention’d, where there are only Five Shares; because one of the Six, which are for the St. Foin, must always return in the same Chanel, going twice therein; for One Row of Barley would be missing, in case the parting Space should be made by this Sixth Share; and that parting Space would have no Barley in it. Therefore it is a Rule, that whensoever Two Sorts of Seeds are drill’d, the Rows of one Sort betwixt the Rows of the other there must be an odd Share in the Drill, which must go twice in one Chanel, and the Distance of the Marking-wheels must be accounted from that Rank of Shares which are the fewest: It must also be contriv’d in this Case, that each outermost Seed-box must deliver but half the Quantity of Seed that each of the inner Seed-boxes do; because the outer ones going twice in a Place, their Chanels would otherwise have a Quantity of Seed double to the rest. In a Drill that has Two Spindles, we place the Marking-wheels on the foremost, which upon their Account is the longest; but if we should use the Wheels of the hinder Spindle as Marking-wheels, then that must be the longest, and so the fore Wheels (their Semidiameters being much longer than the Semidiameters of the hinder Wheels, and their Spindles shorter) would strike against the hinder Spindle, unless it were set farther back than is convenient.