It is a peculiar quality of the district here called Old City that the proprietors of many of its galleries and shops complain about the floors. They slant. “It’s always been a problem for us,” said Robert Aibel, the owner of Moderne, a vintage furniture gallery at 111 North Third Street. But “most people don’t notice, unless a piece of furniture looks crooked.” At the Roche Bobois showroom, at 313 Arch Street, Natalie Suresch, the manager, said the floors tilt four feet from back to front. “You feel like you have vertigo,” she said. Such are the wages of historicism. Old City, the home of Benjamin Franklin and Betsy Ross and the temporary residence of George Washington before he moved south, was the kernel from which the rest of Philadelphia grew.