a b and c show modified plummets of the invention the plummets of the invention may be provided with position controlling ribs or fins 17 fig3 a or projections 17 39 fig2 to prevent it from swinging the ribs of fins 17 and projections 17 39 are preferably made of metal fig4 is a longitudinal sectional view illustrating the arrangement of a plummet in another embodiment of the invention in the embodiment shown in fig4 a baffle 18 is provided to prevent the supplied water from approaching directly the plummet and also to prevent the plummet from swinging due to waves in the water tank 1 fig5 is a block diagram of an automatic system of the invention a change in buoyancy exerted on the plummet is detected by the detector 8 and input to the amplifier 11 as a voltage change signal 19 upon receipt of the signal 19 a control signal 20 is sent from the amplifier 11 through the controller 12 the water supply controlling valve 15 is designed to open or close automatically upon receipt of the control signal 20 the control signal 20 may be either of the proportional control type or of the on off control type next the method of operation of the invention is described below with reference to fig1 the oil material 3 is.