The fourth watch arrived without bells. In the forward salient of Trench Line Four, iron rings and brass horns were wrapped in greasy wool rags to suppress any high-hum metal chime that might betray the guard post’s watch rotation.
Outside the sandbag stone retaining wall, the heat hovered at twenty-eight degrees below freezing. The gale had abated to a steady, dry drift that hissed against the frozen spruce timbering like spilled salt.
Pei Jing sat cross-legged on a pine bench inside the forward listening sap, ten yards north of the main firing step. His charcoal robe was pulled tight over his boiled-leather tunic. His eyes were closed. His bare hands rested palms-down upon the head of an iron-shod trench spade that had been driven six inches into the frozen ground floor.
The steel shaft of the spade was not a tool for digging tonight. It was a clockwork sound transducer.
Sound traveling through surrounding sub-zero air lost ninety percent of its energy to sky scattering and churning wind shear within fifty yards. Through dense, stone-rich frozen clay, however, crushing sound waves traveled at roughly thirty-four hundred meters per second. The crystal-clear lattice of frozen water and tightly packed gravel provided an unbending physical medium.
