Civil Engineering Reference
In-Depth Information
reliefs depicting the rite illustrate is not a single cord stretched out between the
rods but a cord looped around the rods so that there is a double strand between
the rods (and any intervening pegged out points on the line). h is practice records
the line very exactly, since the line is exactly between the two strands of the loop.
It also permits the line to be extended some distance, since any deviation from the
rectilinear is immediately noticeable. h is property, however, is not a vital one,
since the extension can be ranged in by visual means as previously noted. What
the looped cord does is to strengthen the line and provide the means whereby it
can be stretched very taut, as also to demarcate the line very exactly (Eisler JARCE
XXVI 1989 pp. 193-205).
Determi-
nation of
length
(b)
Length
h ere is ot en apparent in (archaeological) discussion a confusion between the path
of a line (direction) and its length (measure). To establish (mark out) a (straight)
line is one thing, to measure along it a certain length is another. h ey are two
dif erent operations performed by using two dif erent instruments. h ey are not
carried out at the same time by using one and the same item of equipment.
To lay out, set out, mark out on the ground (or other surface) a straight line
between any two points a long piece of cord (twine) is required which can be
stretched out tautly along the ground between the two points, and if the cord is
not long enough interim points must be established along the required line (by
visual means) which are within the reach of the cord. To measure out accurately
along the line so demarcated an exact length some instrument must be used which
is calibrated in the units of length to be employed, and is used in such a way
that the length measured out along the line exactly equals the arithmetical total
specii ed of the unit of length. h is instrument is not the same as the “cord” used
for marking out the path of the straight line.
h e essence of the “cord” is that it can be “stretched” taut so that it is follow-
ing the shortest distance between the two points. h us if this piece of cord or
twine is graduated in linear measure the graduations will be continually deformed
by “stretching”—i.e. the length indicated beween x feet on the line will become
greater than the true length of x feet on the ground. On the other hand the essential
of the measuring device is that the units marked on it remain invariable. If this
device is of l exible material so that it can be rolled up, then it must be of “non
stretch” material and not subjected to tension while being used—e.g. in modern
times chains, steel tapes and reinforced linen tapes. Such devices, however, are
not perfect and become deformed by stretching. h e accurate measuring device,
therefore, must be a rigid one: a rule/rod where the lengths marked out on it
remain unchanged by use, which is, in practical terms, a wooden rule of limited
length e.g. 5' or 10'.
34, 35
