The wooden float gauge was an eight-foot pine lath that ran vertically through an iron collar bolted to the corner of the cistern tank.
At its lower end, inside the dark, echoing cistern where four thousand cubic feet of river water rested in pitch-sealed stone, floated a hollow cedar block weighted with two pounds of pig-lead. At its upper end, extending six feet into the cellar air beside the intake pipe, the pine lath was notched with twenty-four horizontal grooves, each groove painted with red lead and numbered in plain carpenter’s strokes. When the cistern was brim-full, the top notch sat level with the iron collar; as the cold-water siphons drank the head to feed the subterranean cooling jacket, the wooden lath slid downward through the iron collar with a soft, scraping rustle.
Tobin tapped the iron collar with the flat of his brass wrench. The pine lath gave a slight shudder, bobbed half an inch in the grease, and settled with the seventh painted notch resting flush against the upper rim of the iron plate.
“Seven minutes of head, Master Hale,” the boy called out, his voice hushed in the cavernous room. “The indicator peg is sitting true. The water is up to the top intake sill in the reservoir.”
