Elevation of Bourges Cathedral

      The archaeology of the Middle Ages should not be confused with the philosophy of art that is sometimes taken as a model and sometimes proposed as an end.

History and its methods

In the preceding pages, we've looked at floor plans, but these are only part of the problem. They are but the shadow of immense structures stretching skywards. They are both the indirect consequence and the primary cause. In Aristotle's terms, the two are mutually architectonic. So let's be curious and raise our heads to the elevations, which are unquestionably part of the purest of geometries.

The first cathedral on this site is Bourges. Now's the time to discover its secrets and put into practice what we've learned in Chartres (the Chartres study is included in the book). The plan in figure 1 is a laser scan, an elevated section of the nave of Bourges. You can see that the floor plan enclosures have been projected vertically. These lines determine the layout of the cathedral's volumes: central nave, aisles and side aisles.

Fig. 1 - Elevation of Bourges Cathedral

Let's start by determining the origin of the vault. To do this, I superimposed two equilateral triangles (fig. 2). As you can see, the procedure is identical to that used to find the Chartres profile. The fact that the same layout can be used to determine both structure and profile is instructive. It teaches us that design procedures were common to all Gothic builders, and that they were universal in scope. There's no doubt that medieval master builders belonged to the same school, the same initiation.

Fig. 2 - Elevation of Bourges Cathedral (stage 1)

Let's move on to an analysis of the interior profiles. As can be seen in figure 3, the design of the central nave is a long square (double square), as in many cathedrals. The vault is traced by a line divided into five parts. We studied this process earlier.

 

Fig. 3 - Elevation of Bourges Cathedral (stage 2)

We now have the cathedral's interior volumes and layout. The following plate brings these systems together (fig. 4). We can see that they are the two complementary aspects of the monument's design: the structure and the interior profiles. The lines of the polygons used juxtapose and merge. This layout is a true masterpiece.

Fig. 4 - Elevation of Bourges Cathedral (stage 3)

Without the reference points provided by the triple enclosure, we wouldn't know how to place the volumes in relation to each other. The role of the segments is to simplify this construction. But in practice, how did the architect switch from the first system to the second? There has to be a common value, a link that unites them. We have seen that the height of the central vessel plays this role. Once determined by the procedure in fig. 2, this height is divided by two to form the two superimposed squares of the central vessel (fig. 3).

The side aisles are made up of three superimposed squares, topped by a new 2/3 ratio. The vault is traced in tiers-point (on an equilateral triangle), as are those of the aisles. The height of the aisles is aligned with the first square of the side aisles.

It's important to understand that the horizontal difference between the green line (the layout) and the yellow line (the profile) is equal to half the width of the fixtures, the stacks. Nothing could be easier than to determine the latter. This relationship is represented by the white circle. The elevation is now traced. The rest of the structures, buttresses and other fixtures are calculated on a point-by-point basis.

Credits: The scan in figure 1 comes from a document supplied by Andrew Tallon to Professor Robert Bork for his article The Geometry of Bourges Cathedral under CC BY 3.0 license. Here, the scan has been turned negative to enhance legibility.