The five-point Pitot cross assembly was an instrument of exquisite clockwork care, forged by the Cambridge Scientific Instrument Company for measuring boundary-layer flow in town water mains.
The assembly consisted of a hollow stainless steel stem, barely three-sixteenths of an inch in width, fitted with a micro-orifice at its forward tip and an integrated hair-width fine-scale barrel graduated in fiftieths of an inch. By turning the knurled bronze thimble at the top of the barrel, Ada could advance the sensing tip across the four-inch bore of the live bronze channel, positioning the orifice at five exactly chosen outward points: five percent of width, twenty percent, fifty percent (the exact centerline core), eighty percent, and ninety-five percent.
“A river warden measures flow with a timber chip,” Ada explained, her wrench clicking as she threaded the packing gland of the cross assembly into Eren’s backup port. “A chip gives you surface speed. It tells you nothing about the boundary layer where the water grinds against the pipe wall. That’s where energy is lost to friction. If your father’s relief water chute has smooth-flowing flow, the speed curve will form a clean, even parabola. If it’s churning, the curve will be flat and ragged, showing boundary shear that erodes the stonework.”
