ABSENCE OF CENTER POINTS IN THE
CONSTRUCTION OF CIRCLES
To construct a circle accurately, one must firmly anchor the
center leg of the compass in the material on which the
circle is to be drawn. The point of this leg has to penetrate
deeply enough to stay in place while the outer leg strikes
the circle. This is bound to leave a mark in the parchment,
and for this reason, on all medieval architectural drawings
on which circles have been drawn with the aid of a compass,
there is always a clearly visible depression or minute hole
in the skin, which reveals the point from which the circle
was struck. I refer once again to the drawing of the southwestern
tower of Cologne Cathedral as a typical case. It
contains two circular installations, a spiral stairwell built
into the masonry of the southwestern corner pier (fig. 9)
and a circular opening in the vault of one of the two inner
bays of the tower (the latter not reproduced here). In both
instances the hole that the center leg of the compass left in
the parchment as the circle was struck appears as a small
but clearly perceptible mark.
For purposes of comparison, five circular installations
shown on the Plan of St. Gall are reproduced in figures
10-13. Figures 10.A-B are the ambo and the baptismal font
in the nave of the church; figures 11.A-B are the two
circular towers that flank the Church to the west. Figure 12
is the enclosure for the hens to the south of the House of
the Fowlkeepers. No trace of the center point is found in
any of these drawings. Furthermore, these circles are too
inaccurate to have been constructed with the aid of a
compass. A circle drawn by compass forms a continuous
line of equal thickness, all points of which are equidistant
from the center point, as is well illustrated by the circle
that forms the outer boundary of the stairwell of Cologne
Cathedral. The circles reproduced in figures 12-13, on
the other hand, are drawn in successive motions of the
rotating hand, the beginning and end of which can still
be identified in many cases. Thus, for instance, close
inspection of the Plan of St. Gall shows that the outer
circle of the Henhouse (fig. 12) was drawn in five separate
strokes. The circle must have been started at the top
with a leftward motion and continued counter-clockwise
in four successive strokes, as I have indicated in figure 13.
As the draftsman passed through this course, he must
have rotated the two skins on which he worked in a
clockwise motion, a procedure which he repeated when
he entered the large explanatory title enclosed by the
circle that identifies this structure as the Henhouse. As he
approached the close of the circle, the draftsman discovered
that his terminal stroke was not in alignment with his
opening stroke, corrected this discrepancy with an additional
stroke which ran parallel to the first, but about 1 mm.
farther out.
While the circles are neither continuous nor accurate
enough to have been drawn by compass, they are far too
accurate to have been drawn without auxiliary construction
lines. As no such lines are to be discovered, we are once
more left with no alternative but to assume that the circles
were traced directly from an underlying original.
