axis of stem 32 this vertical vent passageway can intersect a horizontal passageway 140 having a diameter of approximately 0 090 inches which also provides a cross sectional flow area of approximately 0 006 square inches near the bottom of counterweight 30 valve 42 can be made of shore a 58 durometer polyisoprene elastomer by an injection molding process of the type well known in the art alternatively natural rubber can be compression molded to form valve 42 valve 42 is preferably assembled onto stem 32 by forcing the hole in its center over that portion of the stem to which the weight 30 is secured prior to assembly of the weight 30 onto the stem flange 44 on valve 42 can be about 0 035 inches thick and about 11 4 inches in diameter flange 46 which also acts as a piston within cylindrical wall 24 can be about 0 07 inches thick flange 46 preferably has a minimum outside diameter of about 1 19 32 inches as measured at its uppermost edge tapering to a maximum outside diameter of about 1 11 16 inches as measured at its lowermost edge in lieu of a taper a step like cross section could be employed to provide the desired degree of resilient deformability at the outermost periphery of flange 46 the tapered portion of flange 46 is resiliently deformable to form a movable piston type seal with the innermost surface of cylindrical wall 24 the ring 40 chain 38 and stem 32 including connecting member 34 are preferably molded as one unit using an acetal resin such as delrin via an injection molding process of the type well.