A comparison made between two published ground-plans of the cathedral, one in Banister Fletcher (1943) and the other in Lisa. A. Reilly (1997), showed marked disparities, particularly with regard to the dimensions of the west façade. It was therefore essential to make a detailed personal investigation of the true dimensions on site. This was carried out in May 1998.
A 100-foot fibre-glass tape-measure, a six-foot metal tape-measure and an ultra-sound measure were used. A large set-square was also made use of. Essential assistance in the use of the tape-measures was readily available. In addition photographs of the façade were taken at right–angles to the façade, at nine-foot intervals and at a distance of 125 feet using a telephoto lens. The camera lens’s horizontality was checked by spirit level.
Most of the measurements were easily obtained with the measuring tapes, but one important dimension presented special difficulties. This concerned the width of the central portal opening. From ground-level to a height of thirty-two feet the portal, c.1220, is totally enclosed, buried in the masonry of the Perpendicular porch c.1380, and a further ten feet of the inner width of the opening is also hidden by the new porch (Fig.46a). Access to the roof of the porch is impossible without scaffolding.
Photographs taken at nine-foot intervals across the width of the central portal, which obviated any distortion from the effects of perspective, (Figs. 46b, 47), proved that the distance between axes of the inner colonettes of the splays appeared to be related exactly to the width between the buttresses on either side of the porch, which measured 18 feet 4 inches. But the application of this dimension to the opening of the c.1220 portal was judged to be insufficiently precise for the analysis of the geometry.
Nevertheless an accurate confirmation of the measurement of the central opening was obtained by the following means:
Inside the porch, close to the eastern corners of the vestibule two remnants of the Early English portal are to be found, one in each corner, projecting from the side walls (Figs. 48 , 49 , 50 , 51). Careful measurements of these remnants established the fact that they related exactly and in every respect, (including the level above ground-level ), to the mouldings of the bases of the colonettes to be found on the outer corners of the great central piers where they define the inner limits of the north and south portals (Fig.52). These openly displayed bases were carefully measured and drawn, and their measurements applied to the remnants. Thus one arrived at the exact profiles of the hidden shafts of the central portal and of their bases (Figs. 53, 54, 55, 56). The distance between the mouldings on the remnants was measured, and from this the distance between the shafts was obtained. This is 18 feet 4 inches (5.59m.) between their central axes (Fig. 57). The only problem remaining was to decide whether the distance between the edges of the shafts should be recorded for the purposes of the geometrical analysis, or the distance between their centres. One’s earlier decision in entering on the whole project was that the geometry must lie in the central axes of columns, and this principle was followed here.
The achievement of the measurement of the central opening, led to two important results. The first was the confirmation that the c.1220 opening of 18 feet 4 inches (5.59m.) was exactly defined by the space between the buttresses on the front of the c.1380 Perpendicular porch. The second was that the ratio between the central portal opening and the north and south portal openings was not two to three, as Webb had supposed, and which led to the development of a theory of ‘musical’ proportions that has been widely accepted. The true ratio of the 18.33 (5.59m.) feet of the central portal to the 26.5 feet (8.08m.) of the north and south portals, ( measured again between the central axes of the colonettes ), is 1.44, which is very close to the square-root of 2, (1.414). If, however, the 18.33 feet of the opening between the central colonette axes, which is also the opening between the c.1380 buttresses, is related to the actual 26 foot (7.92m.) opening between the edges of the colonettes of the north portal, the ratio is 1.418, which is within half an inch of the exact Ö2 ratio. This ratio, unlike the ‘musical’ ratios, is created geometrically with rule and compass and in fact derives, as will be shown, from the geometry of the complete façade.
Soon after the measurement of the façade had
been completed a plan of the cathedral, measured and drawn by C.R.Peers in
1905, was obtained (Fig. 42), and a photocopy
provided by the
Vertical dimensions were achieved by comparing confirmed measurements of horizontal features with vertical features as displayed on photographs, in particular the Pitkin photograph of the façade (Fig.44), as well as the author’s photographs taken on site. They were all found to be free of any vertical proportional distortion. At the same time a drawing was made by perspective projection from the Fig.44 photograph in order to eliminate the lateral perspective distortions created by the camera (Fig. 45), for in order to include the whole width of the façade it is necessary to position the camera about eighteen feet to the north of the central axis.
The spires and pinnacles were measured and presented as they appear today, reference being made to photographs taken from a high level (Fig. 59b), in order to avoid the perspective distortions implicit in photographs taken near the building from ground level. In addition, calculations were made to assess the perspective corrections required to arrive at the true height of the spires from those presented by the Fig.44 photograph. However, as will be seen in the seventeenth century engraving (Fig.59a), there have been considerable depredations and replacements in this area, and dimensions of the pinnacles in particular cannot be taken into consideration in any geometrical analysis. The dimensions of the spires would appear to be more reliable.
From the dimensions thus acquired the drawing (Fig.60) was produced, at a scale of 2 millimetres to one English foot. Throughout the present investigation all measurements are presented in English feet and inches, for the modern Imperial foot was the measure used by the builders of this part of the cathedral, although the inch would appear not to have been used by stonemasons until the late medieval period. In all of Salzman’s medieval documents the first mention of an inch occurs in 1340 and even then only in carpentry and in French (Salzman, 1992 ). Conversions to Metrical dimensions are however provided in the text and drawings where desirable. Here it should be pointed out that the English foot measure of the present day is exactly the same as that laid down by William I in the eleventh century (Zupko 1977). However, the English foot of William I did not become standard throughout the country until the reign of Henry I, 1100 – 1135. Until then the Norman foot of 11¾ modern inches continued to be widely in use.
One problem that arises in the presentation of correctly dimensioned plans and elevations for geometrical analysis is created by the distortions produced in the process of photocopying. Length can be distorted in relation to width by one per cent. Various photocopiers available were tested for distortion, and one was found that had no measurable distortion and was used for all important copying. Nevertheless, with repeated photo-copyings and reductions in scale distortions do occur. It was necessary to check all copies for exact reproduction to scale. It was also found necessary to check all published scales against measurements taken on site. Errors are common and can create many problems, and many published scales needed to be replaced with scales recalculated from measurements taken on site.
 English laws on weights and measures: King Edgar 959-75: The measure of