On a humid July day in Pennsylvania, hundreds of tourists, as millions have before them, are drifting among the simple gravestones and timeworn monuments of the national cemetery at Gettysburg. Several thousand soldiers are buried here. A few graves are decorated with flowers, suggesting some of the dead have relatives who still come here. There's a sign at the entrance, reminding people that this is a cemetery. It says: "SILENCE AND RESPECT." Most of the tourists are being reasonably respectful, for tourists, although many, apparently without noticing, walk on the graves, stand on the bones of the soldiers. Hardly anybody...