C. Julius Caesar, Commentaries on the Civil War
William Duncan, Ed.

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When, by means of this tower, they thought they had sufficiently provided for the security of the works around it, they resolved to build a gallery sixty feet long, of wood, two feet in thickness, to extend from the brick tower to the tower of the enemy, and the very walls of the town. The form of the gallery was this: First, two beams of equal length were laid upon the ground, at the distance of four feet from one another; and in these were fixed little pillars five feet high, joined at the top by beams designed to suppirt the roof of the gallery. Over these were laid rafters, two feet square, fastened strongly with nails and plates of iron. The upper part of the roof was composed of square laths, four inches thick, which were placed at a small distance one from another, to bear the tiles that were to be laid upon them. Thus was the whole finished with a sloping roof, which being partly composed of tiles and mortar, was proof against fire, and had besides a covering of hides, to hinder the mortar from being washed away by spouts of water. Over all we threw strong mattresses, to screen the hides from fire and stones. This work was finished close by the brick tower, under cover of four mantles, and immediately carried forward upon rollers, in the manner ships are launched, till it unexpectedly reached the very tower of the enemy.