This section is from the book "Spons' Mechanics' Own Book: A Manual For Handicraftsmen And Amateurs", by Edward Spon. Also available from Amazon: Spons' Mechanics' Own Book.
This consists of a straight-edge to which is attached a cord having a weight suspended from the end, as shown in Fig. 251. The top end a of the straightedge has 3 saw-cuts made in it, one being exactly in the centre. From this centre cut a line is drawn perfectly straight to the other end b. On this line at c a pear-shaped hole is cut out of the straight-edge. A piece of supple cord is next weighted by attaching a pear-shaped lump of lead, and then fastened to the top a of the straight-edge by passing it first through the central saw-cut, and then through the others to make it fast, just so that the leaden weight is free to swing in and out of the hole. The law of gravity forces the cord to hang (when free) in a truly upright (perpendicular) position; on placing the side d of the straight-edge against a surface e, whose perpendicularity is to be tested, if there is any disagreement between the cord and the line marked on the straight-edge, then the surface is not upright, and it must be altered until the cord exactly corresponds to and covers the line marked down the centre of the straightedge.