perspective view of part of the height adjustment elements; FIG. 5 is a side view, partly broken away, of the column of FIG. 1 with its height adjustment and fastening means; FIG. 6 shows on a larger scale and in perspective a detail of the column shown in FIG. 5; FIG. 7 is a view in section taken along the line VII--VII in FIG. 5; and FIG. 8 is a view of one form of horizontal conduit which form