SETTING OUT - Ancient Building Technology - page 31

h e simplest imaginable device of this nature is a length of rope with successive
divisions of 3, 4, 5 marked of on it. h e rope can be stretched along the base line
with the point where the right angle is to be erected reading 3 or 4 as a convenient,
and the right angle will be given by bringing the other two marks together to form
the triangle on the ground. Even more direct is to preform the triangle by tieing
or splicing the ends of the rope together and then adjusting it taut on the ground
(cf M. Sauvage, La Brique . . . en Mesopotamie, p. 75).
It is accepted that wooden set squares with long arms (e.g. 2 m long) for setting
out right angles on the ground were in use from the beginning of monumental
building in Egypt. Furthermore as a matter of course, if the designed building
included angles of 45°/135° (octagon), or 60° (hexagon) similar wooden devices
giving these angles could be fabricated to be used in like manner.
In short either ad hoc geometrical construction or the use of prefabricated
wooden squares would give an angle of sui cient accuracy for all practical build-
ing requirements. h us to set out accurately on the ground angles incorporated
in building was well within the capacity of any ancient builder.

Estab-
lishing
levels

32

4. Level

It would have been more logical to have begun these remarks with a consideration
of levelling since all measured setting out must be effected on a horizontal
plane.
When concerned with a coni ned local area the horizontal may be dei ned as
a plane at right angles to the vertical (= a line joining the station to the centre of
the earth). h ere are two ready indicators of this condition:

(i) h e vertical is indicated by a plumb line, thus the horizontal is a plane at right
angles to the plumb line.
(ii) h e horizontal is directly indicated by the plane of the surface of a liquid.

Both these indicia are used in modern precision levelling instruments (sighting
devices which may be rotated through 360° on a horizontal plane). In (ii) the
optical axis of the instrument is adjusted parellel to a horizontal tube of liquid
containing an air bubble (i.e. a spirit level). When the air bubble is central in the
tube then the surface of the liquid (and the optical axis) is horizontal. In (i) the
optical axis of the instrument is set at right angles to the plumb line. h e plumb
line automatically assumes the vertical and brings the optical axis into the hori-
zontal plane. In principle both devices were used during antiquity to indicate the
horizontal. However in antiquity the bubble mechanism was not available (it was
invented in the later 17th century AD), thus the surface of the liquid was observed
to coincide with a mark parallel to the optical axis.