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.