of example may be nine and a half inches the cross beams 16 and 18 are permanently attached to the supporting box members 12 and 14 by conventional means such as welding referring to fig1 and 3 on the ground between the supporting box members 12 and 14 and beneath the lowermost cross beam 18 is the base track 20 the base track 20 is comprised of track supports 22 and 24 which support thereon a gear and rack system which will be described in greater later on track support 22 further comprises a track 23 on which a pair of rollers may roll track support 24 further comprises a track 25 on which a pair of rollers may roll the track supports 22 and 24 are aligned parallel to each other and are attached by means of transverse spacing members 26 and 28 which also run parallel to each other thereby forming a generally square base which rests on the ground resting immediately above the base track 20 is a first movable support member track 30 the first movable support member track 30 is comprised of a pair of parallel x direction beams 32 and 34 and a pair of y direction beams 36 and 38 which are connected together to form a generally rectangular frame the frame comprised of members 32 34 36 and 38 of first movable support members 30 supports transverse roller member 40 and 42 transverse roller member 40 is supported between y direction beams 36 and 38 and is generally parallel to x direction beams 32.