Down on the sheltered porch of the Third Gate control house, young Tavi stood before a three-foot wooden blackboard that he had lashed to an iron bollard with tarred hemp. In his right hand he held an iron carpenter’s pencil. Around his neck hung a small brass ship’s whistle on a braided cord. Despite the freezing rain pelting his bare wrists, his handwriting was neat, clear, and perfectly disciplined.
Every two minutes, he recorded the physical water mark on the stone cutwater opposite his post, cross-referencing it against the yellow signal flags flying from the turning basin tower. At twelve-thirty-nine, his eyes widened behind his wet fringe of hair. The water level in the lower basin had dropped two inches. Meanwhile, the upstream head at High Tor had climbed four inches in ninety seconds.
“Pressure back-and-forth swing!” Tavi shouted, seizing the clapper of his hand-cranked bell and striking three rapid notes. Ding-ding! He snatched his slate and ran fifty paces along the sheltered stonework arcade to Ada’s testing pipe hub. “Mistress Morland! Look at the slope! The basin dropped, but the gorge is stacking water behind the cutwater! There’s an eight-inch standing wave coming down the tailrace in four minutes!”
