So, drawing a mental line from the dead criminals at the city gate, to the powder-house on the Common, to the witch-gallows, and finally to the harbor defenses, he has got one Cartesian number-line – what Leibniz would call the Ordinate – plotted out: he understands what people are afraid of in Boston, and how the churchmen and the generals keep the place in hand. But it remains to be seen what can be plotted in the space above and below. The hills of Boston are skirted by endless flat marshes that fade, slow as twilight, into Harbor or River, providing blank empty planes on which men with ropes and rulers can construct whatever strange curves they phant'sy.