Northeast of the Bronx, on the placid waters of Croton Reservoir, the water police helicopter was carrying out its normal patrol to ensure that no power boats were churning up the surface. If left undisturbed, the water would be aerated through the action of the sun’s ultraviolet light, and once rid of impurities, would pass through the aqueducts and tunnels, built a hundred years before, and become part of the one and a half billion gallons of water that New Yorkers consumed every day. The chopper came down as it spotted the quality-control men on the only power boat allowed in the lake lowering the seki disc — which they could see was visible down to about three and a quarter meters, much deeper than the “two-meter clarity” required by law. When the chopper had gone, a man in his late fifties, taking his usual walk by the dam, sat down by the edge of the spillway, fascinated as always at how quickly the placid surface of the lake became a cascade of white water over the elegant beveled curves of the dam. He emptied the thermos into the water.