The Axis of this wooden Spindle ought to fall into a Line with the Axis of this Brass Spindle; but, unless Care be taken to prevent it, the wooden Spindle will so much wear the Hole thro’ which it passes, and be worn by it, as to have Room in the Hole to deviate from this Exactness, and may descend so low, that the Crank may come out of the Ends of the Fork; and for this Reason it is, that the Fork is made so long as it is; but when this wooden Spindle does, by the Contrivances hereafter shewn, keep its Axis in a Line with the Axis of the Brass Spindle, or very nearly so, then the Legs of the Fork need be no longer than half an Inch; and in that Case, the Joint of the Crank, which is perpendicular to the Spindle, must be shorter, or else descend deeper into the Wood, so that its End, which turns the Fork, may be in the Middle betwixt its Bottom and the End of its Legs.
The Use of the other End of the Spindle is this: When we have a mind that it should be turned by the left Wheel instead of the right, we screw in the Fork into the Hole I, and place a short Screw in the room of the Fork, to hold down the Slider.
Note, It is not absolutely necessary, that the hollow Cylinder, which appears on the Sides of the Seed-box, should both, or either of them, project farther than the Flanches; but I think it better that it should do so, at least, on that Side which is next to the Fork.
This Cylinder should be bored as true, and as even, as the Barrel of a Fusil is bored: and the Edges and Surfaces of its Ends must be smooth, and without Jaggs, to the end that the Wreaths may turn glibly against them.
The Figure or Shape of all Sorts of Seeds disposes them, more or less, to form an Arch, when they are pressed from above, and confined on all Sides.
The most effectual Way to prevent this is, to take care, whenever many Seeds are to descend together by their own Gravity thro’ a narrow Passage, that such Passage be never narrower downwards than upwards; but, on the contrary, that it be wider downwards, on some or one of its Sides; in which Case, if the Surfaces of all the Sides of this Passage be smooth, it is impossible, that Seeds should of themselves form an Arch therein.
On this Maxim depends the infallible Performance of a Drill, and from hence are derived the Uses of the Bevel of the Mortise: What I mean by the Word Bevel, in general, has been already defined.
The Bevel of the Mortise of the Seed-box is that Inclination of its Sides, whereby it is wider downwards, and narrower upwards; by which means the Seed is prevented from arching in the Mortise before it descends to the Notches of the Spindle. And this is the First Use of our Bevel; for this Arching might happen in the Mortise, if the Planes of its Sides were parallel to each other; and would be unavoidable, if their Inclination were downwards, as it is upwards; but these Planes opening downwards, the lower the Seed descends, the more Room it has to expand; so that the very Weight, which would otherwise cause it to arch and stop, does by means of this Bevel force it to descend to the Notches, and then it is safe from all manner of Danger of stopping. The Ends of the Mortise are at such a great Distance from each other, and the Cover so very thin, as to lie almost even with the upper Part of the Spindle, that the Seed can never form an Arch that way; or, if it did, the continual Motion of the Tongue would immediately break it down at the Fore-end of the Mortise.
The Second Use of this Bevel is, that it gives room for the Tongue to be in the same manner bevel, tho’ in a less Degree: By this means, the Seed cannot by any Impediment be stopped in its oblique Descent to the Notches, from the Fore-end, and all that other Length of the Mortise, along and upon the Surface of the Tongue.
But if the Mortise had not this Bevel, the Tongue could not have it; for then either the upper Surface of the Tongue must have no Bevel at all, which would destroy the Two empty Triangles which ought to be on its Sides; or else it must have a Bevel the contrary Way (i. e. a Bevel reversed), and be narrower downwards than upwards, which would cause the Seed to arch thereon, and hinder its free Descent to the Notches.