Note 261 — CHAP. XX. Of the Drill-Boxes.

Take care that these opposite Sides be sure to be true Planes, especially all that Part of their Areas, that is before the transverse Axes of their Ellipses herein after described; for should they be otherwise, the Bevel of the Mortise would be spoiled, and so would the Ellipses, and the acute Triangles, on the Sides of the Tongue; which how necessary they are to be true, is shewn in the proper Place. Workmen are very apt to fail in this when they file by Hand, and make these Sides of the Mortise convex instead of plane. Therefore this might be done with less Difficulty, and more Exactness, with a File placed in a Frame, whereby it might move upon a true Level without rising or sinking of either End.