Ada laid her deep-rock chart across the pine table, pinning the corners with Hal Rook’s brass measuring tongs and an iron tally weight. She pulled Hal’s thirty-year-old survey of the riverbed closer, comparing his measurements of the limestone strata with the shaking frequencies Imran was feeding through the audio link from Harrowby.
‘Here is the death blow, Pella,’ Ada said. Her pencil drew a precise sinusoidal wave across the deep-rock cross-section. ‘Lathmere sits on a shelf of fractured limestone forty feet thick, resting on a bed of soft shale. Every rock formation has a natural hum. A pitch at which it rings when struck, like a crystal wineglass.’
She tapped the paper. ‘Hal Rook measured the natural period of this limestone shelf during the quarry excavations thirty years ago. It rings at sixteen point two cycles per second. And what did we clock the Sluice Crown’s bubbling blast pulses at ten minutes ago?’
‘Sixteen point four,’ Pella answered, her throat going dry. She saw where the two lines intersected on the chart.
‘They match,’ Ada said, her voice dropping into a register of chilling, technical finality. ‘The Sluice Crown is driving the bedrock at its exact ringing hum. It’s the principle of the marching soldiers on a mid-air bridge: a thousand men walking in step will bring down an iron bridge that could carry twenty thousand tons of dead freight. Sorell isn’t just straining his pipes. He is shaking the entire rock shelf until the lime mortar between the stones…
