Eight-sided plan and three sections of which the dome of one is labelled Semi / circle and with a note: I say ^think it is impossible / to raise an ^^entire semicircular / dome on an octangular / Plan damme John SoanDr David Yeomans wrote (letter, 3.1.2007): 'Soane does not say why he thinks this is impossible. It cannot be he thought the geometry was impossible so we must presume that he is referring to the structure. Of course, a thin hemispherical dome (which is what he means by semicircular) is itself impossible without some kind of tensile reinforcement in the lower part to contain the tensile loop stresses that are generated. Without that the dome would simply crack into a number of separate radial arches. If such tensile reinforcement is included then this would be sufficient to contain the outward thrust of the dome at the top of the pendentives, just as a dome can be raised upon a drum with tensile reinforcing around its base - a ring beam in common parlance. Soane must surely have been aware of this''Below the base of the hemisphere the dome is supported on pendentives, which are therefore in compression and so must themselves produce some outward thrust. The question then is how is this outward thrust to be contained? It would be possible to provide tensile reinforcement in the walls of the octagon so that what Soane says is not strictly correct. However, in this case the walls of the octagon are pierced with large openings that rise above the springing of the pendentives, into the zone where the tensile reinforcement is needed. This particular arrangement does not look sound. The only alternative to (this now impossible) reinforcement would be to buttress the corners.'

Drawing

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