drum. The text underneath is given under No. 788.

MS. B, 3--three slight sketches of plans in connexion with the
preceding ones._

_Pl. XCIX, No.1 (MS. Tr. 15) contains several small sketches of
sections and exterior views of the Dome; some of them show
buttress-walls shaped as inverted arches. Respecting these Leonardo
notes:_

758.

L'arco rivescio e migliore per fare spalla che l'ordinario, perche
il rovescio trova sotto se muro resistete alla sua debolezza, e
l'ordinario no trova nel suo debole se non aria

The inverted arch is better for giving a shoulder than the ordinary
one, because the former finds below it a wall resisting its
weakness, whilst the latter finds in its weak part nothing but air.

[Footnote: _Three slight sketches of sections on the same
leaf--above those reproduced here--are more closely connected with
the large drawing in the centre of Pl. C, No. 4 (M.S, Tr. 41) which
shows a section of a very elevated dome, with double vaults,
connected by ribs and buttresses ingeniously disposed, so as to
bring the weight of the lantern to bear on the base of the dome.

A sketch underneath it shows a round pillar on which is indicated
which part of its summit is to bear the weight: "il pilastro sara
charicho in . a . b." (The column will bear the weight at a b.)