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You are an expert at summarizing long articles. Proceed to summarize the following text: 
The invention relates to a device for refilling and emptying lavatory flushing cisterns of the type which allows the user to choose either a weak or heavy flush. 
     BACKGROUND OF THE INVENTION 
     It is also of the type in which the flush valve and mechanism co-operate to form a homogeneous unit, the various parts of which are housed inside an inner tank. 
     The latter principle is known particularly from European Patent No. 01722778 A 3, but in the case of the present invention, the overflow device comprises arrangements which, when flushed, are aimed at providing a guide to prevent the water from being forced back into the lavatory cistern. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     To understand the invention fully, reference is made to the attached non-limiting drawings. 
     FIGS. 1, 2 and 3 are longitudinal sections which show the different phases of flush operation, i.e.: 
     1. the position when not in use, with the cistern filled 
     2. the open position for a weak flush 
     3. the open position for a heavy flush 
     FIG. 4 shows the flush valve and mechanism unit mounted in an inner tank with its overflow device. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The body of the flush valve along comprises a parallelepiped-shaped element (1) arranged horizontally which comprises a rectangular-section opening (2). The longitudinal axis (3) of the element (1) is perpendicular to the axis (5) of the opening (2). 
     On one side of the opening (2) along the axis (3) a horizontal cylindrical tube (6) is arranged which, on one side, discharges into the opening (2) and, on the other, comprises a shoulder (7) of greater diameter than that of (6) and which is extended by a cylindrical compartment (8) into which water enters under pressure. 
     A downward-directed open vertical tube (9) guided downwards is connected to the tube (6) in the proximity of (7). 
     A ball (10) of greater diameter than that of (6) is placed inside the compartment (8) in such a way that, under the effect of pressure, the ball abuts the shoulder (7) so as to be sealing-tight. 
     A valve (11) slides inside (6) and, on the side of (7), carries a nipple (12) of lesser diameter than that of (11). 
     A cam (13) slides vertically inside the opening (2) and comprises a ramp (14). The cam (13) is made rigid with a floater (18) by any means, such as for example a rod (19) arranged at right angles; 
     The unit is arranged in such a way that when the flush valve is in the closed position: 
     a) The base (15) of the ramp (14) is in contact with the end (16) of the valve (11). 
     b) The end (17) of the nipple (12) is in contact with the ball (10) abutting its seat (7). 
     Two horizontal elements (21) and (22) bear upon the upper horizontal side (20) of the cam (13), on the top of which elements vertical rods (23) and (24) are secured. 
     The elements (21) and (22) co-operate with any device for lifting the overflow tube (25), the said device may for example comprise a fork (27) which, by pivoting about a horizontal axis (28) moves a rocker bar (29), the unit being arranged in such a way that the tube (25) is lifted irrespective of which control rod is used. 
     The tube (25) which carries the emptying valve (30) at its base also carries, at its lower portion, a floater (31), then at its intermediate portion, a floater (32) and lastly, at its upper portion a horizontal projecting element (33). 
     An element (35) pivots freely about an axis (34) parallel to (28), whereas below either one of the control elements, for example (21) and rod (36) is fastened, the element (35) and the rod (36) being in the same vertical plane. 
     The element (35) is arranged and suspended in such a way that, in the normal gravity position, one of its sides comprises an inclined ramp (37) the lower end (38) of which is located in the vertical lifting space of the element (33). 
     The rod (36), in turn, then comes into contact with the ramp (37) at its end (39) in such a way that pushing down said rod (36) causes the element (35) to pivot, the end (38) of which exceeds the element (33) lift range. 
     The device operates as follows, the reference to FIG. 1; the cistern is full: 
     Pushing down either of the rods (23) or (24) lowers the cam (13) whose ramp (14) shifts the valve (11) which, via at its end (12) separates the ball (10) from its seat (7) 
     Water then circulates from the compartment (8) to the open tube (9) and fills the tank. 
     In the final filling phase, the floater (18) is lifted, which returns the cam (13) to its starting position. The valve (11) returns to its position and the ball (10) once again abuts its seat (7) thereby cutting off the flow and closes, the water inlet valve. 
     As depicted in FIG. 2, when the flush is triggered by the user by means of the rod (24) the element (21) stays in position with its rod (36), the same applies to the element (35). 
     The end (38) thus acts as a stop for the projecting element (33). 
     The lift amplitude thereby obtained corresponds to a situation in which the suction to which it is subjected would cause it to drop back down immediately onto its sealing seat, if the drop were not delayed by the action of the intermediate floater (32). 
     The latter is sized in such a way that the vertical thrust which it experiences is greater than the suction force exerted on the tube (25) which drops down again once the floater is no longer immersed. 
     This process corresponds to weak flushing the volume of which is set by the adjustable position of the floater (32) on the tube (25). 
     As depicted in FIG. 3, when the flush is triggered by the user by means of the rod (23), the element (35) retracts by the action of the rod (36) to allow free upward movement of the tube (25) which, being subjected to the accumulated thrust of the floaters (31) and (32), rises to the level which corresponds to a situation in which the valve (30) is outside the suction area. 
     The tube (25) is thus held in suspension until the floater (31) action is complete, this process corresponds to heavy flushing. 
     In FIG. 4, the flush valve and mechanism unit is housed in a tank (40) with a overall parallelepiped shape, which is in turn housed in the lavatory cistern (41). 
     The tank (40) comprises a round hole (43) arranged at the centre of its horizontal bottom (42). 
     On the underside of the bottom (42) a cylindrical element (44) is secured whose axis (49) coincides with that of the hole (43) and whose diameter is slightly greater than that of the former. 
     The element (44) comprises at its upper portion a series of horizontal slots (45a), (45b) and at its lower portion, a threaded portion. 
     The said element (44) passes through the hole (46) in the cistern (41) with which it is made rigid by means of a nut (47), shoulders (48a) and (48b) arranging a set space (50) between the outside of the tank (40) aid the inside of the cistern (41). 
     The hole (43) is provided--in a permanent manner--on its inner circumference with a downward-directed cylindrical element (51). 
     In the event of tank overflow (40), the excess water runs into the bowl through the slots (45), passing through the cylindrical space (52) which has been set between the concentric elements (44) and (51), the latter&#39;s function during flushing being to guide the flow by preventing the water from being driven back into the space (50).

Summary:
Device for a lavatory cistern including: a) a flush valve opened by the lowering of a horizontal cam (13) pushing on a horizontal valve (11) which separates a ball (10) from its sealing seat; and b) a dual flushing volume mechanism, with an emptying tube (25) carrying a base floater (31), an intermediate floater (32) and, at its upper portion, a horizontal projecting element (33) which is optionally locked in its ascending stroke according to whether a weak or heavy flushing is selected.