the building progressed until the whole formed an open shaft from ground floor to roof in the completed edifice.
The lower course having been built up in this manner to a height of six inches wooden arches were placed at intervals the lengths of the corridors which were now ceiled over by the simple expedient of fastening thin wooden strips lengthways of the corridors from arch to arch until the corridors were entirely roofed. The strips, or boards, which overlapped one another, were fastened in place by wooden dowels driven through them into the peripheries of the arches. As this work was progressing the walls of the various chambers and the outer wall of the building were raised to a height of twenty-four inches, bringing them to the level of the ceilings of the arched corridors, and the spaces between chambers and corridors were packed with bowlders, the interstices between which were filled with smaller stones and gravel. The ceiling beams were then placed across the other chambers, timbers six inches square hewn from a hard, tough wood being used, and in the larger chambers these were further supported, at intervals, by columns of the same dimensions and material. The ceiling beams being in place they were covered over with tight fitting boards, doweled to place. The ceilings of the chambers now projected six inches above the surrounding course of the structure, and at
