As to the permanent stability of this dome various opinions have been held by the experts among the older writers.[23] Its form is, as we have seen, as favourable to stability as it would be possible to make that of any vault which could be properly called a dome. It appears to the inexperienced eye as stable as a crest of the Apennines. Every precaution as to material and careful workmanship seems to have been taken to make it secure. The wall of the drum on which it rests is five metres in thickness, and the solid base of the dome itself is built, if the architect’s scheme was carried out as he had stated it before the Board of Works, of large blocks of hard stone, thoroughly bonded and clamped with iron. The lower system is sufficiently strong, and appears to rest on a solid foundation. But nevertheless there are ruptures in various parts of the structure which have caused apprehensions of danger,[24] and its future duration must be regarded as uncertain. The writers who have maintained that it is secure have argued on the assumption that the parts of a dome all tend toward the centre.[25] These writers overlook the fact that the force of gravity above, especially when the dome is heavily weighed by a lantern, neutralizes the inward tendency of the lower parts and causes a tendency in those parts to movement in the opposite direction. This neutralizing force is lessened by giving the dome a pointed form, as Brunelleschi has done, but, as before remarked (p. 22), it can hardly be overcome entirely so long as any real dome shape is preserved.[26] It may be thought that the object which Brunelleschi had in view, of producing a vast dome that should be an imposing feature of the cathedral externally, justifies the unsound method of construction to which he resorted (the only method by which the effect that he sought could be attained). But structural integrity is, I think, so fundamental a prerequisite of good architecture that in so far as this gifted Florentine was obliged to ignore sound principles of construction in order to attain an end not compatible with such principles, the result cannot be properly considered as an entirely noble and exemplary work of art, however much beauty and impressiveness it may have. The example set by Brunelleschi was, in point of construction, a pernicious one, and bore fruit of a still more objectionable character in the works of other gifted men less scrupulous than he, and less endowed with mechanical ingenuity, as we shall see farther on.