Nearly 10 million people attended the Centennial International Exhibition of 1876 in Philadelphia. —RockingStock—Getty ImagesAs we get ready to celebrate the 250th anniversary of the nation’s founding—in an America stressed by authoritarian overreach, economic anxiety, and partisan trench warfare—we might look back for a lesson or two to 1876, when the United States threw itself its most lavish birthday party, on its Centennial. For six months, …