Wednesday, 4 September 2013

The medieval toolchest: the level and plumb bob

The level and the plumb bob are two instruments using the same principle - gravity and a weight - to establish straight horizontal and vertical lines, respectively. In medieval times, both tools were mostly used in construction activities by masons and carpenters. The level is known in Roman times as the 'libella'. Sometimes this tool has markings at the bottom to measure varying degrees.

Detail from the base of the 'Four crowned martyrs' at the Or San Michele church in Florence, Italy, showing an A-shaped level. The marble made around 1370-1421 by Nanni di Banco was commissioned by the guild of stonemasons and woodworkers.

An A-shaped level used to construct the Tower of Babel from the French Bible Historiee, c. 1250.  French Ms 5, 16 r, John Rylands Library, Manchester, UK.

Cunrad (Steinmetz), a stonemason from the Hausbuch of the Mendelschen Zwolfbruderstiftung in Nurenberg, Germany. Died before 1415. His tools of trade include the typical long A-like level used by stonemasons, as well a a square and a template. He sits on a one-legged chair.

Miniature of the apostle Andreas from the Book of Hours of John the Fearless, between 1406-1416. The illumination shows an A-style level and low-angle block plane, with smaller versions of both tools in the background. Planes were the personal symbol of John the Fearless of Burgundy and can be found in other illuminations on his clothing. Ms lat novv acc 3055, Bibilotheque Nationale, Paris, France.

Our level is a triangle made up of three strips of wood jointed together at angles of 45 and 90 degrees. We used European walnut for the body, with maple pins securing the different strips. The leaden weight is attached with a waxed linen cord to the top corner. The long side of the level measures 41 cm, and the short sides are 39.1 cm each.

The level in standing position.

The level and a (walnut) square lying flat.

Levels can also have an inverted T shape, here shown by a colleague medieval woodworker during the Limburg Brothers medieval festival in Nijmegen, the Netherlands.

Two types of levels, the A (figure a) and the inverted T (figure b), as well as a plumb bob with a wooden holder (figure c). Image from the book  'Das werkzeug des Zimmermans' by Hans-Tewes Schadwinkel and Gunther Heine.

While the level is used to set horizontal lines, the plumb bob is used to set straight vertical lines. It is sometimes used together with a wooden block, to check equal distances from the construction.

A plumb bob and a square used at the construction of the Babel tower. Illumination from the Hortus Deliciarum of Herrad von Landsberg, 1175-1185. Only copies of parts of the book remain.

The use of a plumb bob is shown here in an 15th century illumination of the construction of the tower of Babel from Le tresor des Histoires, Cotton Augustus V folio 22. British Library, London, UK.

The plumb bob lying flat on the table and hanging above the same table.

Our plumb bob is an antique one, bought at a brocante shop. We recently used the plumb bob to check our height measurements of the entrance tower at castle Doornenburg (Doornenburg, the Netherlands) during a medieval festival. This was our story behind it:
We were commissioned by a French knight who complained of a supposed bias of the referees at the Lage Landen Tournament. His complaint was not granted, so he wanted revenge. We were to build him a storm tower that should have an equal height as the entrance tower. Therefore we needed to know the height of the tower.
The measurement trick, today still used to measure the height of trees (with a yardstick) . H1 = height to eyesight, H2 = horizontal length from person to the wall. H3 = height of the wall equals H2 plus H1.

We first used a trick to measure the height using a wooden block with an 45 degree angle. From the 45 degree angle we moved to a distance where we just could see the top of the tower while holding wooden block horizontally flat. At this point, the height of the wall equals the height of the person to his eye (H1 in the figure above) plus the length from him to the wall (H2 in the figure). We asked several visitors to measure as well, which gave us a mean height of 33 feet. Then, our children went up to the wall and lowered the plumb bob for an exact measurement: 34 feet and 6 inch.

(Left) The children lower the plumb bob from the entrance tower. It can be seen just above the window. (Middle and Right) Bram grabs the plumb bob at the gate and checks when it has reached its lowest point.

Some measurements of the tower using the trick: 31.5 feet, 36.25 feet, 36.5 feet, 32.7 and 34.5 feet. 
The actual measurement with the plumb bob yielded 34 feet and 6 inch.

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