Part 30

Its Two opposite Sides are equal, but not parallel, by reason of their Inclination to each other upwards, which is the Bevel hereafter to be described.

The Two Ends are neither parallel nor equal, because the Hinder-end D is perpendicular to the Top and Bottom, and the Fore-end oblique, and therefore longer.

When Two opposite Sides, or Surfaces, are inclined to each other upwards, I call that Inclination a Bevel; but when they are inclined downwards, I call it a Bevel revers’d.

The Line _a e_, being the Bottom, or Base, of the Hinder-end D, by being longer than the Line _l m_, shews that the Mortise is bevel.

The Two prick’d Lines _m n_ and _l o_, with the Line _l m_, and Part of the Line _a e_, make a rectangled Parallelogram, which shews the exact Depth of the Mortise, and forms on each Side of it a rectangled Triangle, the one _m e n_, and the other _l o a_; which Triangles being similar and equal, and their acute Angles at _l_ and _m_ being each of Four Degrees, make the whole Bevel, or Inclination of the Sides of the Mortise, to be of Eight Degrees, their Hypothenuses being the same with the Sides of the Mortise.

This End D, being raised up to its Place, will be at right Angles with the Plane of the Top and of the Bottom of the Mortise; which, being both rectangled Parallelograms, prove that Bevel, or Angle of Inclination, to be the same from one End to the other of the Sides, which Sides are the Hypothenuses of those Two Triangles: But this could not be proved by the Triangles in the opposite End C; because the Bases being the same with the other, and having their Legs longer, the vertical Angles at _k_ and _i_ are more acute. The Legs are longer; because the End C, when in its Place, is not at right Angles with the Top and Bottom of the Mortise, as the End D is.

The next thing to be described in the Mortise, is the Bore, great Hole, or Perforation; which is best shewn in the Side of a Mortise of a Wheat-drill, being larger, as in _Fig._ 3. wherein _c e b d_ is the great Hole, and is a Section of an hollow Cylinder, that passes through the Mortise, with its Axis parallel to the Edges of the Ends of the Mortise: This Cylinder, being cut by the Side of the Mortise obliquely, and not parallel to its Base, is an Ellipse.

The prick’d curve Line is a Circle parallel to the Base of the Cylinder, and the curve Line _b d c e_ is the Ellipsis; and this Curve is more or less elliptical (or oval) in proportion to the Angle of Inclination, or Bevel, of the Sides of the Mortise.

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.

The Manner of fastening the Organ-tongue to its Mortise is by Parchment and Leather glu’d to its Surface, and also to the Sound-board, at its End which is opposite to that pressed open by the Key, and shut by the Spring; but this our Tongue is held within the Mortise, and moves on an Axis, which passes thro’ its upper and narrower End, which Axis is the Pin A (which must be exactly parallel to the Edge of the End of the Mortise), and also thro’ the Hole _f_ in _Fig._ 3. in _Plate_ 2. which is seen in its Place at A in _Fig._ 3. _Plate_ 3. and likewise through both Sides of the Mortise near their upper Edges, and as near the Fore-end of the Mortise as may be, without the Tongue’s rubbing against the said Fore-end.

The Breadth of the Tongue must be conformed to the Breadth and Bevel of the Mortise, and when it is on its Axis, it being raised tight up as far as the short prick’d Line _l m_ in _Fig._ 3. _Plate_ 2. being One-eighth Part of the great Hole, and being there, you see its upper Edges touch both Sides of the Mortise by their whole Length: Then it is rightly made: and by this touching both Sides of the Mortise tightly and closely, when raised up to that Degree, it appears, that the Two upper Edges of the Sides of the Tongue are inclined to each other in an Angle that is more acute, by about One-third, than is the Angle of Inclination of the Sides of the Mortise.

Hence, when the Tongue is let down to its Place, there will be on each Side of it an empty Space, betwixt it and the Mortise, of the Form of a very acute Triangle, whose vertical Angle is more or less acute, according as the Tongue approaches nearer to, or recedes farther from the Spindle.

This _Fig._ 1. _Plate_ 3. is the brass Tongue with its Back-side upwards. The Two outer Lines _a b_ and _c d_ are the Edges of the upper Surface (tho’ turned downwards in this Figure), which are inclined to each other, as afore-mentioned; but the Two inner Lines _e f_ and _g h_ are nearer to each other, whereby this under Surface is narrower than the upper: Both must be plain Surfaces, but the upper and its Two Edges very free from Flaws, and smooth, or polished.

The Reason why the under Surface is narrower than the upper, is to preserve the Bevel of the empty Triangle: For though the Bevel of the Sides of the Mortise would be sufficient for this, if both Sides of the Tongue were sure to keep equally distant from the Spindle; yet as the Tongue never is so tight on its Axis, but that sometimes one Corner of it may be nearer to the Spindle than the other, in this Case, that Side which is nearest to the Spindle would reverse that Bevel, so as to make the small empty Space that is betwixt the Mortise and the Tongue, wider above than underneath.

C C are the Two little Knobs that prevent the Spring from slipping to either Side, and are at the Distance from one another of the Breadth of the Spring.

_Fig._ 2. shews one Side, and the Thickness of the Tongue the other Side, being the same. _a b_ shews the polished Surface (being a true Plane), whereon the Seed runs down to the Spindle. _c d_ the Back-side, which lies turned uppermost in _Fig._ 1. _b e d_ shews one End of the hollow Cylinder of the Tongue, thro’ which its Axis passes.

The Length of the Tongue must be such, as will reach lower than just to touch the Bottom of the great Hole as a Tangent: for, if it be not longer than that, it might happen, that when the Mortise is empty of Seed, and the Tongue set up close, a Wheel might, in Turning, or otherwise, go a little backwards, and cause a Notch of the Spindle to take hold of the End of the Tongue, and tear it out of the Mortise: Therefore let the Tongue reach a little below the Spindle, as the pricked Line _g h_, in _Fig._ 3. of _Plate_ 2. doth.

As for the Posture in which the Tongue ought to stand in the Mortise, it is shewn by the Three pricked Lines in _Fig._ 3. _Plate_ 2. where the pricked Line _g h_ makes an Angle of Forty-five Degrees, being the nearest that it can stand to the Spindle; the pricked Line _i h_ makes a somewhat greater Angle, and it is a mean (or middle) Distance from the Spindle; and the pricked Line _k h_ is supposed to be its greatest Distance, where the Tongue makes its greatest Angle with the Top and Bottom of the Mortise. If the Tongue stood so obliquely as to make an Angle much less than Forty-Five, the Tongue would rise too much against the Bevel of the Mortise, and the Spring would have the greater Difficulty in returning it to its Place, when driven back by the Force of the Notches.

And beside, when the Tongue stood wide from the Spindle, there might be so much Room betwixt it and the Sides of the Mortise, that some Seeds might fall thro’ there.

The Steel Spring is D, properly placed upon the Back of the Tongue, in _Fig._ 1. _Plate_ 3.

At first, I made the Spring double, _i. e._ with Two Legs, in Imitation of that in the Organ, and fastened into its Tongue, much after the same manner as the Spring of the Organ is into its Tongue or Flap, which prevents the compressed Air from passing out of the Sound-board, except whilst the Key is thrust down by the Finger of the Player; but the Drill-spring requiring to be of a vastly greater Strength than that, I made it of Steel, of the Breadth of half an Inch, instead of Brass Wire: This performed very well, and several Drills are yet extant, that have only this Sort of Springs: Yet I found there was great Difficulty to set the Legs at their due Distance from each other; for their Seasoning would alter them from what they were, whilst the Steel was soft: They also took up too much Room in the upper Part of the Mortise. Then, to remedy these Inconveniencies, I made it single, with only one Leg, which by full Experience is found to be much better than the double one; it does not contain a Fourth Part of the Metal, and is most easily made, requiring none of that Trouble and Nicety that the double Spring doth. I shall therefore give a Description of the single Spring only.

B, the End of the Screw, which holds the Spring to the Tongue, thro’ a Hole near the upper End of the Spring; D, the Middle, against which the End of the Setting-screw bears.

Its Length is almost the whole Length of the Tongue; the End E reaching very near to the lower End of the Tongue, and the End B is as near the upper End of the Tongue; as it can be placed without touching the Cylinder of the Tongue.

The Breadth is usually about half an Inch; the Thickness must be in proportion to its other Dimensions, and according to the Degree of Stiffness required.

The longer it is, the thicker it must be, to have the same Stiffness; but the broader it is, the thinner it must be of the same Length; so that it is hard to determine its Thickness. It is made stiffer or stronger by being cut shorter; it is made weaker, or less stiff, by filing or grinding it either thinner or narrower.

The common Thickness is about that of a Shilling[262].

[262] Not quite so thick as a milled Shilling, but rather of an old broad stamped Shilling, which is a little thinner.

The Degrees of Stiffness are measured in this manner; _viz._ Fix Two Boards together, leaving a Chink betwixt them, in one Place of an Inch long; lay the Spring (when seasoned across this Chink) with its Middle exactly over it; then put a String over the Spring, which may pass with both Ends thro’ the Chink, and tie so much Weight to the Ends of the String under the Boards, that will pull down the Middle of the Spring, till it touch the Chink, and is strait with both its Ends; This will shew the Degree of Stiffness. But note, That the Spring must be crooked, and bear only upon its Ends, with the hollow Side upwards.

If ten or a dozen Pounds Weight pull it down to the Board, it is a good Degree of Stiffness, for a large Box: We are not confined to be very nice or exact in the Degree of Stiffness; for by our Fingers pressing it, we that are practised in it, know well enough, whether a Spring be of a sufficient Degree of Stiffness, without weighing it; but for such who are unacquainted with them, it is best not to trust to Guess, but Weights; and to adjust the Stiffness to that of a Spring, that has been known to perform well.

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.

This Holding-screw has a pretty broad Head, and is screwed in by a Notch, like the Screw-pin of a Gun-lock.

The Hole in the Spring must be somewhat bigger than the Holding-screw, because the Spring must have room to move and play thereon.

If the Middle of the Spring were against the Middle of that Part of the Tongue, that is betwixt its Axis and the Place of Pressure, the Distance of the Spring’s Hollowness would be just half the Distance of the Spring’s Play, to wit, the One-eighth Part of an Inch; but as the Spring does not quite reach up to the Axis, and reaches much below the Place of Pressure, the Hollowness at the Place where the Setting-screw bears against the Middle of the Spring at D, is considerably nearer to the Place of Pressure than to the Axis of the Tongue; this Hollowness of the Spring at the Setting-screw may be something more than the One-eighth Part of an Inch, to give the Spring a Quarter of an Inch Play: but it seldom has so much.

_Fig._ 4. in _Plate_ 2. shews the Length and Thickness of the Steel Spring of a Turnep Seed-box: This serves both for a Tongue and Spring: It is made first strait, and then the narrowest End of it is turned round, till it reach to _a_, and forms the Cylinder A, thro’ which its Axis passes; but is not welded or joined to the other Part of the Spring at _a_: It is placed in the Box with the Cylinder Part underneath. The Face of this Spring is seen upon its Axis, mark’d K. in _Fig._ 5. Its Axis is to pass thro’ the Hole E, and screw into the Hole F, in _Fig._ 2. as is seen more plainly at _a_ in _Fig._ 9.

As the Top of every Tongue ought to be even with the upper Edges of the Mortise, the Thickness of the Cylinder of the Brass Tongue causes the Hole in the Sides of the Mortise, into which it is held by its Axis, to be far enough from the Edges of the Mortise, to be bored and screwed without Danger of breaking the said Edges; but the Spring of the Turnep-drill being so very thin, there is some Difficulty in making the Hole so high, and near the Edges: To prevent which Danger, _Fig._ 7. shews the End of a small hollow Cylinder of Iron or Brass, of the Thickness of the Mortise; which, being put into the Cylinder A, in _Fig._ 4. raises the Spring higher above the Hole; so that it may be made as low in a Turnep Mortise, as that is which holds the Brass Tongue in the Wheat-drill. But we do not always use this inner Cylinder[263]; but must then take the more Care in boring the Hole, or else it will burst out at the Edges of the Mortise.

[263] For, instead of this, we may use a Bit of Woolen Cloth of the Breadth of the Mortise, glued on to the Bottom of the Hopper, which, filling the Vacuity above the Steel Tongue, prevents any Seed from running over it, though the Holes are bored as low in the Mortise as if the Cylinder _Fig._ 7. were to be used.

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.