Of this Ellipse the longest Diameter (or Axis transversus) b c is at right Angles with the upper and lower Edges of the Sides of the Mortise.
Its shortest Diameter (or Axis rectus) d e, is the Diameter of the Cylinder, bisecting the Axis transversus at right Angles in the Centre a; and is in this Figure one Inch and an half.
This Ellipse being concentric with the Circle, the Letter a is the Centre of both.
The Semi-ellipsis c e b is the Part of chief Use; and therefore the Edge must of Necessity be smooth, and without Flaws, as must the Surfaces of the Sides of the Mortise betwixt the Ellipse and the Fore-end.
The Tongue of the Seed-box ([Plate 3. Fig. 1.]) differs from that in the Sound-board of an Organ (from which I took the Idea of it) in Shape, in Situation, and in the Manner of its being fix’d to the Mortise.
The Tongue, in the Organ, is on its Surface a long Square, or rectangled Parallelogram, a little broader and longer than the Mortise (or Grove) it shuts against; but this Tongue on its upper Surface, which is here turned downwards, being a Plane, is a Trapezium, of the same Shape with the Fore-end of the Mortise just now described, except that the Tongue has a less Bevel.
The Situation of that in the Organ is on the Outside of the Mortise, which it shuts by its Spring behind it, and opens immediately by the Finger of the Organist pressing down the Key to let in the compressed Air to its Pipes; but this Tongue is situate within the Mortise of the Seed-box, and placed almost, in a manner, diagonally; for, had it been placed like the other, the Seed getting betwixt it and the Edges of the Mortise, would not have given Way to its Shutting (as the Air does to the other), but have kept it always open; which would have render’d it useless for sowing of Seeds.
