By Joseph A. McGeough
Plumb line, level, and square
A plumb line is a light line with a weight (plumb bob) at one end that, when suspended next to a workpiece, defines a vertical line. Plumb comes from the Latin plumbum, or “lead,” the material that replaced stone as the weight for the bob or plummet.
The spirit, or bubble, level, a sealed glass tube containing alcohol and an air bubble, was invented in 1661. It is used to find or create vertical and horizontal lines and surfaces.
While an end-weighted string defines the vertical, its direct use for plumbing walls (making them vertical) is awkward. The Egyptians devised a tool resembling the letter E, from which a plumb line was suspended from the upper outboard part of the E. When the tool was placed against a wall, the wall was determined to be vertical when the string just touched the lower outboard part of the E. Oddly, this useful tool was apparently forgotten for many centuries and reappeared only in modern times.
The tool for determining horizontal direction is called a level. The Egyptians used an A-frame, on which a plumb line was suspended from the vertex of the A. When the feet of the A were set on the surface to be checked, if the plumb line bisected the crossbar of the A, the surface was horizontal. The A-frame level was used in Europe until the middle of the 19th century. Sometimes a variation is shown in which the frame is an inverted T with a plumb line suspended from the top of the vertical stem.
Because the surface of a body of water is always horizontal, a trough or channel filled with water can serve as a reference in some situations. The hose level, first described in 1629, consisted of a length of hose fitted with a glass tube at each end. Water was added until it rose in both vertically held tubes; when the surfaces of the water in each tube were at the same height, the object was level. This idea was impractical with only leather hose, but the development of vulcanized rubber hose in 1831 led to a resurgence of the device in 1849. Because the hose could be carried through holes in the wall, around partitions, and so on, the instrument enabled levels to be established in awkward circumstances.
The spirit, or bubble, level, a sealed glass tube containing alcohol and an air bubble, was invented in 1661. It was first used on telescopes and later on surveying instruments, but it did not become a carpenter’s tool until the factory-made models were introduced in the mid-19th century. The circular level, in which a bubble floated under a circular glass to indicate level in all directions, was invented in 1777. It lacked the sensitivity of the conventional level.
The square appeared in the ancient Egyptian world as two perpendicular legs of wood braced with a diagonal member. In the following centuries many variations were designed for specific purposes, including a square with shoulders that allowed it also to cast a mitre of 45 degrees. Iron squares were rarely used before 1800, and factory-made metal squares did not appear until 1835. The adjustable, or bevel, square was used for angles other than 90 degrees beginning in the 17th century. In the earliest examples, the thin blade moved stiffly because it was riveted into a slot in the thick blade. Later models of the 19th century, however, were equipped with a thumbscrew that permitted the thin blade to be adjusted with respect to the thicker blade.