CHAPTER VIII.
THE ACTION.
This important subject will be prefaced by a few definitions, superfluous, perhaps, for some readers, necessary for others.
Backfall. A lever of any clean wood, ⅜ inch or less in thickness, 1 inch or 2 inches in width, and seldom more than 1 or 2 feet in length, turning upon a wire as its axis or fulcrum.
Fig. 27.
Bridge. Backfalls occur in sets, corresponding to the number of keys in a manual or of pallets in a wind-chest. They are arranged side by side in notches formed by taking out the wood between saw-cuts in a balk of mahogany or oak 2, 3, or more inches square. This balk is called a bridge. Fig. 27 shows part of a set of backfalls and their bridge.
Square. Squares are now usually of metal, but may be easily made of wood, and consist of two arms, 2 or 3 inches long, united at a right angle to each other, or cut at once from a single piece, and turning on a wire as an axis passing through a hole at the intersection of the arms. Like the backfalls, they may be arranged side by side in a bridge, but the modern metal squares are screwed separately in their places (Fig. 28).