The best Way, that I can think of, to shew the making of these Notches, is by a Section of the Spindle at right Angles, in the Middle of the Notches, as in [Fig. 4. of Plate 3.] which is a Circle whose Circumference is cut off by Six Notches; which shew the different Sort of Notches, that increase or diminish the Proportion of Seed to be carried thro’ the Seed-passage by them: The Length of the Notches we never alter; but make them always parallel to the Axis of the Spindle, and of the Length of the Distance there is between the lower Ends of the opposite Axes transversi of the Ellipses, or great Holes, of the Mortise; for if any Part of the Surface of the Spindle should be betwixt the End of a Notch and the Hole, one or more Seeds coming betwixt that Surface and the Tongue, might hold it open, and prevent its pressing against the Notch, to hold the Seed therein from falling without the Turning of the Wheels.
This Proportion of Seed is alter’d by the Number of Notches, and by their Depth or Breadth, or by both. b c is the Depth of a Notch, which we call its Side; and is that which takes hold of the Seed, and carries it down thro’ the Seed-passage. The Manner of cutting this is seen by its being a Portion of the Radius A c. The Bottom of a Notch is made in different Forms[264]: As, first, it may be convex, as is shewn by the curve Line b d. We may enlarge the Capacity of this Notch, by taking off the Convexity of its Bottom, as in the Bottom of the Notch shewn by the Line e f; and if we would increase it more, we make it concave, as g h.
[264]The convex Form is best for turning out a great Proportion of Seed; because such a Bottom may be broader than one of any other Form, in a Notch of the same Depth and Capacity; and such a Notch, having its Capacity more in Breadth than Depth, will be less liable to let fall any Seed without the Turning of the Wheels, than a Notch that is deeper and narrower, except it be very narrow, which it cannot be for throwing out a large Proportion of Seed; for a great Number of Notches cannot have altogether the same Capacity as a lesser Number of the same Depth may. The concave Notch, if it were as broad as the convex may be, would make the Interstice, that is before it, liable to be broken out, and so Two Notches would become One; but the Convexity of the other supports the Interstice like an Arch, and for that Reason may be made to reach almost quite to the Notch that is before it, without that Danger.
But of whatever Sort or Dimensions one Notch is made, all the rest should be the same exactly; and consequently, the Interstices (or Intervals) between Notch and Notch, of which the Line f c, being an Arch of the Circle, is the Breadth, must be equal[265], and cannot be otherwise, if the Notches are all equal and equidistant, as they appear in the adjoining [Fig. 5.] which is a Section like the former, and shews Six Intervals, with their Six Notches, of the Size wherewith we drill St. Foin with high Wheels; but when we would drill very thin, it is better to have but Four or Five Notches instead of Six.
[265]But these cannot be equal, unless the Notches are all of equal Breadth, and equidistant from one another; and if they are otherwise, the Seed will not be equally delivered to the Ground.
[Fig. 6.] shews a Notch of the Spindle. a b is the upper Edge of the Side of the Notch, being always an acute solid Angle. c d is the Edge of its Bottom, being always an obtuse Angle. e f is the Angle made by the Side and Bottom, and is always shorter than the aforesaid Two Edges, by reason of the Obliquity of the Two Ends; this Angle is never obtuse, except when the Bottom of the Notch is concave. These Three Lines must be parallel to the Axis of the Spindle.
[Fig. 7.] is one End of the afore-described Notch; the Line a b being joined to the Line f d of [Fig. 6.] and the Line a c, being joined to the Line b f in [Fig. 6.] would be the End of that Notch in its proper Posture; and then the Line b c, being an Arch of the cylindrical Spindle, would be the Edge of the upper End of the Notch. a b c, being the Area of this End, is a Plane, and, when in its Place, makes an Angle of Forty-five Degrees with the Axis of the Spindle. The other End is the same with this in all respects, except that, being opposite to it, it is inclined to it in an Angle of Ninety Degrees, at the bottom Angle of the Notch, at the Line e f in [Fig. 6.]
