The swath of property that Andrew Carnegie purchased on Fifth Avenue at 90th Street was considered the boonies at that time – too far uptown from the acceptable social center. But, of course, where Carnegie goes, others follow. Visit Carnegie’s house, now the Cooper Hewitt Museum, as well as other turn-of-the-century mansions built by the Whitney, Vanderbilt, Post, Huntington, and Rockefeller dynasties, with a stop at the Warburg Mansion, now home of the Jewish Museum.