62 so that cylinder 64 is pivotable about pin 62 cylinder 64 is smaller in diameter than casing 48 to provide adequate clearance for arcuate movement of cylinder 64 in casing 48 cylinder 64 has a bracket 70 affixed as by welding to the lower surface thereof and supports a bracket pin 72 a bracket 74 is affixed as by welding to wall 56 of casing slot 58 and carries a bracket pin 76 a shock absorber 78 of conventional construction and commercially available has its housing 80 pivoted to pin 76 and piston rod 82 pivoted to pin 72 rod 82 is connected to the shock absorber piston which is mounted for reciprocable movement in housing 80 and resiliently supported therein in the usual manner of conventional shock absorbers thusly cylinder 64 is supported in a substantially horizontal position in casing 48 but is free to move in either an upward or downward arcuate path for purposes later explained placed in cylinder 64 is a resilient plug 84 made of rubber or other resilient material and has a diameter corresponding to the inner diameter of cylinder 64 and a axial length approximately one half that of cylinder 64 a piston 86 is reciprocably mounted in cylinder 64 and is affixed as by welding to a piston shaft 88 which has at its opposite end a peripheral flange 90 cylinder 64 has a peripheral flange 92 at the right end.