by a small external float which can move up and down with the surface of the liquid element the falling motion of the float is limited by a stop when occupying its lowest position the little float is situated above the lower level of the external surface and then the opening of the valve is not allowed as the float begins to move up with the external surface the valve can fly open and let enter a swift flow that valve or the valve 15 of the communicating opening which is between the two floors of a compartment might be fitted out with a magnetic closing device fig5 and fig6 illustrate a third embodiment see sheet 3 according to that embodiment of the invention the device is a floating barrage which has the capability of sucking up oil slicks from rough seas also separating oil from the water which is in the gathered mixture and carrying the pollutant to a storage enclosure some collecting compartments 1 a 1 b etc are disposed in line from one end to the other end of the barrage each compartment comprises two superposed floors 11 and 12 which are separated one from the other by a horizontal partition 13 the shape and the buoyancy of the barrage are ensured by an inflatable structure comprising a longitudinal float 22 which is partitioned and placed in the upper part of the barrage.