Washington DC is a capital that is spread out lavishly and built in grand style. But it isn’t a vibrant human confluence like New York. It is distant and withdrawn. Perhaps it is so because it is the hub of power. But Washington DC holds out to you some of the finest repositories of knowledge the world has: the Smithsonian museums. You can immerse yourself in them for days together, and all 17 of them, big and small, give you free entry. Each one is a gem, whether Natural History, African American History, American Indian History or Air and Space....