On the morning of July 4, 1826, the leading residents of Quincy, Mass., and Charlottesville, Va., began their last celebration of the nation’s birth – and their last day on Earth. They faced eternity as friends. High on his small mountain in the foothills of the Blue Ridge, the master of Monticello lay asleep. Throughout the spring, Thomas Jefferson had become increasingly feeble. By mid-June, the daily horseback rides were over. In Quincy, John Adams’ health had also declined during the late winter and spring. On sunny days, he was able to take short carriage rides, but even they had...