At Jones Point Park, I seek out the Boundary Stone in its niche in the retaining wall. When the Potomac River recedes enough to detect the eroding words, rimmed in green algae, I picture Banneker camping here that cold damp Spring of 1791. He is 60 years old. He lies supine each night on the ground. This is base camp for calculating the ten-mile square of Washington, DC. In the dark, he notes the exact moment of each star’s transit over the zenith. He gathers all the light of the stars to his thin chest. He is master of the theodolite. In letters, he argues with Thomas Jefferson to wean yourself from narrow prejudices that keep slaves, so numerous a part of my brethen under groaning captivity and cruel oppression. Now, in the shadow of a massive steel bridge named for Woodrow Wilson, who declared segregation not humiliation but a benefit, the water makes a hollow pong and I look across the wide dark river where for four months Banneker looked, where, among all surveyors, he completed the most difficult math.