The Spring must bear against the Back of the Tongue at each End, and lie hollow in the Middle: But the Degree of Hollowness of the Spring is very material; for thereon depends the Distance of the Tongue’s Motion towards the Spindle by Force of the Spring, and back again quite to the Setting-screw, by the Seed that is pressed against it by Force of the Notches, when they are moved by the Wheels; because the more the Spring is curved, the farther will it thrust the Tongue from its Middle, if its Strength be superior to the Force that resists it, as it ought to be when a Notch is passed and before the next: This Motion of the Tongue is called its Play.
In order to measure the Distance (or Quantity) of this Motion, we must consider, that the Tongue, moving on its Axis above, describes with its lower End the Arch of a Circle, the Chord of which Arch is the Measure required.
To measure this by the Angle the Tongue makes at its Centre, would be no Rule for making Boxes; because some Tongues are longer, some shorter, in proportion to the different Diameters of the Spindles they move against; and yet the Play of the shortest must be as much as that of the longest, that is, it must describe as great an Arch at the Place of Pressure (described in [Fig. 3. Plate 2.]); and therefore the shortest Tongue would make the greatest Angle.
A short and easy Way, then, for a Mechanic to measure, is thus: Screw in the Setting-screw until the Tongue come within a quarter of an Inch of touching the Spindle; then take out the Spindle, and from the Centre of the Hole draw a Line on the Side of the Mortise, perpendicular to the Tongue, and at the Tongue’s Edge make a Mark with the Compasses, or a Pen; then force back the Tongue against the Setting-screw as far as it will go (that is, until the Spring touch the whole Back of the Tongue); produce the said Line to the same Edge of the Tongue, or set the End of the Rule thereon, and draw another Line, by the Rule, from the Mark to the Edge of the Tongue, when farthest back, and there make the second Mark. The Ruler used this Way will shew both the Perpendicular, and the Measure.
But yet a quicker Way is, to set the Tongue, by the Setting-screw up to the Edge of the Hole; and, when it is forced back, measure from the Tongue to the nearest Part of the Hole, which will ever be a perpendicular Line drawn from the Centre of the Hole to the Place of Pressure above-mentioned, and make another Mark there: Now the Distance between these Two Marks is the Measure (near enough) of the Tongue’s Play at the Place of Pressure. Tho’ this Line drawn on the Side of the Mortise be not exactly perpendicular to the Surface of the Tongue, but only to its Edge; yet the Difference is next to nothing, and not to be regarded.
If its Measure be a quarter of an Inch, it is what Experience shews to be of a good Size for all Corn and Peas; a little less is no Harm, but greater is the most fatal Error, into which most of the Pretenders to the making of this Machine have fallen; they give the Tongue half an Inch, sometimes Three quarters of an Inch Play. The Mischief of this Error is yet farther increased, if the Spring be weak, if the Mortise have too great a Bevel, or if the Angle made by the Tongue at the upper Edge of the Mortise be too acute.
When the Tongue has too great Play, the Seed is apt to be turned out too fast, or else too slow, in spite of the Driller. For when the Tongue is set at its due Distance from the Spindle, and is thrust quite back by the Seed pressed against it by the Turning of the Notches; but the Spring being unable to return the Tongue to its former Place at such a Distance, at the time of passing the Intervals which are betwixt the Notches; then the Space between the Spindle and the Tongue being too open, the Seed is sent down too fast.
To prevent that, they set up the Tongue to the Spindle; and then, as often as the Spring happens to overcome the Force of the Seed’s Pressure (as sometimes it will), it is sent out too slowly.
The Inequality of the Running of the Seed makes such Boxes useless, which the Expence of Two-pence (for another Spring, or new Seasoning of that) at most would rectify, if the Maker understood how to mend his own Work. If time did permit, more should be said on this Point, because I find it is the Pons Asini of a Workman. Sometimes it may be prevented, when the Spring is too hollow, and gives too much Play. Screw the Screw, that holds it on the Tongue, down closer, so that the lower Part of the Screw’s Head press against the Spring, and thereby force its Middle nearer to the Tongue, until you find its Play lessened to its just Distance.
The Spring, remaining in this compressed State, has lost the weakest, and retains only the strongest, Part of its elastic Force. Therefore, if you find it then too stiff, make it weaker by Filing or Grinding, or else put another into its Place, which is honestly worth no more than Two-pence.