Daniel rose and, leaving his wig behind, so that the cool air bathed his stubbled scalp, climbed up toward the light, ascending the masons' ladders and ropes. Above his head, the gaps between planks were radiant, salmon-colored lines, tight and parallel as harpsichord-strings. He hoisted himself up through a hatch, rousting a couple of swallows, and found himself within the dome of the turret, sharing a hemispherical room with Robert Hooke. Dust made the air gently luminous. Hooke had spread out large drawings of wings and airscrews. Before the windows he had hung panes of glass, neatly scored with black Cartesian grids, plotted with foreshortened parabolae – the trajectories of actual cannonballs. Hooke liked to watch cannonballs fly from a stand-point next to the cannon, standing inside a contraption he had built, peering through these sheets of glass and tracing the balls' courses on them with a grease-pencil.