Plough.—See Plane.
Plumb.—You can make a plumb-line by merely hanging any weight at the end of a cord, when the cord will of course be vertical as soon as it stops swinging (Fig. 644). For convenience in using hang the cord on a board as shown in Fig. 130. When the cord hangs exactly on the line or at the apex of the notch the edge of the board will be vertical.
A long board will give a more accurate test than a short one in most cases, just as a long plane will make a straighter edge than a short plane, for the long board will bridge over the irregularities of the surface to be plumbed. For example, to take an exaggerated case, the post plumbed as at a (Fig. 645) is vertical, taken as a whole; while the same post plumbed as at b leans over, because the short board happens to be placed where the surface of the post is not straight.
Fig. 644.
Fig. 645.
Fig. 646.
When the plumb-line is used to determine a point exactly over or under another point, as in surveying, the bob is shaped with a point like a top (Fig. 646). For making the plumb, see page 96. (See also Level.)