By the deep, by the mark
Somerset House, Terrace Galleries
By the deep, by the mark is a historic naval term for measuring the relative depths of the seabed, a process traditionally done with a plumb line by hand (see PLA). This line literally connected its thrower with water. Nowadays data historically obtained ‘by the deep, by the mark’ is integrated into complex algebraic and sim-ulation models (see Navier Stokes).