known when these external tie rods were installed. The technology would have been available at the time of the initial construction, but the extra support might not have been considered necessary until after the collapse in 1284, or even later. In the 1960s, the tie rods were removed; the thinking was that they were ungraceful and unnecessary. However, the oscillations created by the wind became amplified, and the choir partially disassociated itself from the transept. Subsequently, the tie rods were reinstalled, but this time with rods made of steel. Since steel is less ductile than iron, the structure became more rigid, possibly causing additional fissures. As the floor plan shows, the original design included a nave that was never built. Thus, the absence of shouldering support that would have otherwise been provided by the nave contributes to the structural weakness of the cathedral. With the passage of time, other problems surfaced, some requiring more drastic remedies. The north transept now has four large wood-and-steel lateral trusses at different heights, installed during the 1990s to keep the transept from collapsing (see photograph below). In addition, the main floor of the transept is interrupted by a much larger brace that rises out of the floor at a 45-degree angle. This brace was installed as an emergency measure to give additional support to the pillars that, until now, have held up the tallest vault in the world. These temporary measures will remain in place until more permanent solutions can be determined. Various studies are under way to determine with more assurance what can be done to preserve the structure. Columbia University is performing a study on a three-dimensional model constructed using laser scans of the building in an attempt to determine the weaknesses in the building and remedies. Starting in September 2022 the French government will begin a restoration of the cathedral starting with the roof and removal of the interior supports. Part of the roof was analyzed and restored in a project starting in 2010. The analysis done on the roof included a "multidisciplinary study" of the lead tiles which made up the roof. Interior Several of the chapels contain medieval stained glass windows made during the 13th through to the 15th centuries. In a chapel close to the northern entrance, there is a medieval clock (14th – 15th century), possibly the oldest fully preserved and functioning mechanical clock in Europe. In