[Luc]

> This is, by the way, the genius in Wren's consruction of St. Pauls in London: he hid a chain line dome that carries most of the load between the visible outer and inner half spheres.

I think that's the wrong way around. The inner and outer dome follow catenary curves (they only need to support their own weight), and between them is a cone shape, supporting the heavy ornament on the roof. A 'chain line dome' is not a good way of supporting a heavy load on the roof (just imagine how the shape of the chain changes as you hang a weight in the middle of it).

http://en.wikipedia.org/wiki/St_Paul%27s_Cathedral#Structura... , third paragraph.