Its Shape must conform to that of the Brass Tongue already described.
The Degree of its Stiffness is known by weighing, as has been directed for the other Spring; and being laid with its Face downwards over a Chink, with a small Piece of Wood of the Thickness of a Barley-corn at Each end, and a String taking hold of its Middle, and descending thro’ the Chink, the Weight of Five Pounds, tied to the End of the String, will just bend the Spring, till it touch the Edges of the Chink; and this is the Stiffness of a Spring that has performed well, for many Years, in drilling of Turnep-seed.
The Setting-Screw.
[Fig. 6.] is the Iron Setting-screw, which passes thro’ the Hole in the Fore-end of the Mortise, [Fig. 2.] and passes up to the Middle of the Spring by the prick’d Line p q in the same Figure. The Use of this Setting-screw is, to increase or diminish the Proportion of Seed to be turned out by the Notches; and this it does by forcing up the Spring and Tongue (where there is one) nearer to, or farther from the Spindle, whereby the Seed-passage is made wider or narrower, as is shewn by the Three prick’d Lines in [Fig. 2.] and [Fig. 3.]
Observe, that the prick’d Line p q, [Fig. 2.] (being the Mortise of the Turnep-box) stands higher than the same Line doth in [Fig. 3.] which is the Mortise of the Wheat-box. The Reason of this Difference is, because the Spring in the Wheat-box bears at its lower End against the Tongue below the Seed-passage, and at its upper End below the Axis of the Tongue, whereby the Middle of that Spring is lower than the Spring of the Turnep-box, which, being both Spring and Tongue, bears against its Axis above, and against the Seed-passage below; therefore its Middle is higher. This Setting-screw should be placed perpendicular to the Tongue when at its mean or middle Distance from the Spindle, which may be supposed to be the middlemost of the Three mention’d prick’d Lines. This Setting-screw ought to be smooth and round at its End, which bears against the Spring; for, if it should have sharp Corners or Edges, the Spring might be wounded by them, and in time might break there, being press’d by every Notch that turns against it; and, as I have computed it, a Spring undergoes One hundred thousand of these Pressures in one Day’s Work; and yet, in my whole Practice, I have had only one Spring broken, and that was in drilling a large Sort of Peas with a Wheat-drill, and was occasioned by a jagged End of the Setting-screw, which was not placed perpendicular to the Spring, by which means the rough End of the Screw made Scratches against it a Quarter of an Inch long, and so deep, that the Spring broke off there: Let not this Setting-screw be any longer than just to force the Tongue up to the Spindle; for, if it should be longer, an ignorant Driller might happen, by the Force of the Screw, to break the Tongue, or its Axis; but in the Turnep-drill, which has only a Spring instead of a Tongue, the Setting-screw may be a Thread or Two longer; because the Spring will yield a little to it, after it touches the Spindle, and is sometimes of Use in that respect, when the Notches are too large. This Screw must be of such a Bigness, that it may not be in Danger of bending; for if it should be bent, it could not be screw’d up with any Certainty, because its End, being crooked, would be below its Place at one Half-turn, and above it at the other Half-turn, and so the Spring might be set farther from the Spindle instead of nearer, and nearer instead of farther, by the Crookedness of the Setting-screw. Its Head may be made with a Notch in it, to be screw’d in with a Knife, or else with a Head like a T, to be turn’d with the Fingers, which I think is best, especially for a Wheat-drill; because as the Brine and Lime, which stick on the Wheat, grow drier, it will run faster; and therefore the Setting-screw must be frequently screw’d in to lessen the Seed-passage.
The Seed-passage, or Place of Pressure, is where the Seed passes down betwixt the Spindle and the Tongue; and is in that Part where they are nearest together; for there the Seed is press’d hardest by the Force of the Notches, which carry it down: And this Passage is higher or lower, as the Tongue stands nearer or farther from the Spindle; for as it stands wider, it becomes nearer to perpendicular to the Top of the Mortise, and then the Seed-passage is higher; and when it stands nearest to the Spindle, then the Seed-passage is lowest. This appears in [Fig. 3.] by the Three prick’d Lines a n, a o, and a p.
The Spindle, with its Notches, is best shewn where it is large, and made of Wood, as that of the Wheat Seed-box; it is a solid Cylinder that passes thro’, and fills the great Hole, or hollow Cylinder, of the Seed-box; it is of various Lengths, according to the Distance its Wheels go asunder; it is always in large Boxes the Axis of Two Wheels, and turns round with them, as the Axis of the One Wheel of a Wheelbarrow does with that: These Wheels, by their Circumferences, measure out the Ground over which they carry the Seed-box, and, by the Notches in their Axis, deliver down the Seed equally, whether they move swift or slow; because an equal Number of Notchfuls of Seed will be deliver’d thro’ the Seed-passage at each Revolution of the Wheels.
The Notches resemble those in the Hinder-Cylinder of a Cyder-mill, which break the Apples by turning against the Notches of the Fore-cylinder, as our Notches turn against the Tongue; and bruise the Apples which come betwixt them, as our Notches might sometimes bruise soft Seeds, if the Tongue stood close to the Notches, without any Spring behind it to give Way to their Pressure, and return the Tongue again to its Place, at every Interval betwixt Notch and Notch.
