engaged to retain water in the sink if the water rises above a predetermined level in the sink it overflows via the pipes 14 and 15 and the overflow device 10 to drain beneath the sealing disc 24 as will now be explained more fully referring to fig3 a 3b 3c 3d 3e and 3f the lower body portion 34 and the upper body portion 36 together form three passages a 60 b 62 and c 64 passages a 60 and b 62 are separated by a passage member 63 and together with an interior volume of the cap 38 form a siphon passage b 62 and passage c 64 are separated by a further passage member 65 passage c 64 together with the remainder of the cap 38 form a non mechanical air valve i e a valve with no moving parts having an air entry port 40 and an inverted weir 42 the inverted weir 42 extends downwardly from an upper end 66 of the passage c 64 such that its lowest point 68 in use is below an upper end 70 of the further passage member 65 the sequence of events shown in fig3 a 3b 3c 3d 3e and 3f will now be described first the water level in passages b 62 and c 64 mirrors the level in the sink bowl fig3 a in fig3 b the water level reaches the predetermined overflow level water begins to drain into passage a 60 the level in passage c 64 continues to match the level in the sink bowl as the level in the sink bowl increases seen in tube c 64 the air valve 40 is shut off drainage speed now increases due to the syphonic effect on the water.