Presumably this is Ruskin’s House 30, San Marco 2847, Nadali & Vianello (1999) Tav. 50, described at House Book 1 p.35. See also Notebook M pp.92-3 and Sheet No. 69. It is not the same as the Cavalli next the Post Office mentioned at Notebook M p.145L. It appears to be the building Ruskin has in mind whenever he refers to a Palazzo (or Casa) Cavalli without qualification. Similarly Selvatico (1847) pp.113-4 and 522 distinguishes between the Cavalli and the ‘Cavalli alle Poste’.
This building is described as next to the Palazzo Barbaro (see House Book 1 p.31L, House Book 1 p.71) and ‘opposite the Academy of Arts’ and the Accademia Bridge (see Works, 11.368). It was substantially rebuilt by Camillo Boito for Baron Franchetti, after whom it is now named and who bought the house in 1878, but it is possible to gather from the existing building some idea of what Ruskin saw.
For earlier images of the building and an outline of its history see here.
[Version 0.05: May 2008]