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A pressure sensor, located for 4 months in the middle of a 1275-m long tautdeep-sea mooring in 2380 m water-depth above a seamount with sub-surfacetop-buoys and seafloor anchor-weight, demonstrates deterministic spectral peaksat sub-harmonics of the local near-inertial frequency. None of thesefrequencies can be associated with oceanographic motions. No correspondingpeaks are found in spectra of other observables like water-flow (differences),temperature, and pressure in the top-buoys of the mooring. The mid-cablepressure sensor was mounted on a nearly 1-kN weighing non-swiveled frame. Itsdata seem to reflect a resonant mechanical oscillation of the tensioned mooringcable under repeated short-scale Strouhal vibrations induced by water-flow.Equivalent vertical motions of 0.5-m amplitudes are found dominant at frequencyf*/4, with monotonically decreasing peaks at f*/2, 3f*/4 and f* = 1.083f, wheref denotes the local inertial frequency of Earth rotation. The observationsprovide, physically, proof of the Earth rotation and, oceanographically, of therelative vortex-rotation dzeta = 0.083f induced by (sub-)mesoscale eddiesassociated with a nearby seamount, so that effective near-inertial frequency f*= f + dzeta. As instrumentation only shows mid-cable resonant signals it is notconsidered as a Foucault pendulum, but more likely an Earth-rotation equivalentof the Wheatstone device with the mooring cable acting as elastic springs.While the f*/2 motion reflects the Foucault principle of linear orbits fixed inan inertial frame of reference, the dominant sub-harmonic f*/4 suggestsresemblance with the precession of a spherical oscillator at half the angularspeed of rotation.

Foucault-Wheatstone device in the deep-sea