One of the keys to raising the dome was the precise calculation and
measuremenTofTiich horizontal layeFoFbrick or stone as it was added in a
gradually contracting sequence. But how would these measurements be
taken? How could the curvature of the eight mdividual walls be controlled
during the process of construction? The difficulty was made even more
acute by the fact that each wall had to incorporate two shells rising in
tandem, as well as their supporting ribs. A deviation of only several inches
in one o{ these ribs — each of which was over loo feet in length — meant
that the connection, like that at Saldae, would not be achieved.