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Ferromagnetic liquid, also called ferrofluid, is used extensively at the present time as a leakproof fluid seal between regions of different pressure. Ferrofluid "plugs" have been proposed to facilitate blood flow stasis in arteries during surgery as an alternative to present methods which cause arterial wall damage. The hydrostatic loading of ferrofluid is investigated theoretically and experimentally for liquid and air pressure loading. It is found that the Bernoulli equation, modified to account for magnetic field-ferrofluid coupling, adequately predicts the sealing capacity of the hydrostatic plug when the ferrofluid surfaces are in stable equilibrium. General criteria are outlined for the determination of the stability of various liquid-liquid and gas-liquid configurations. Loading measurements are independently correlated by vibrating magnetometer measurements.