Those who have read the history of the first Thames tunnel will recall the early unsuccessful attempts at construction which had discouraged English engineers. Five years after Brunel’s patent was secured a company was formed to undertake the task again, the plan being to use the shield system, under the personal direction of its inventor as chief engineer. For this work Brunel selected the cellular shield mentioned as an alternative construction in his original patent. He also chose to make this shield rectangular in form. This choice is commonly accounted for by the fact that the strata to be penetrated by the tunnel were practically horizontal, and that it was assumed by the engineer that a rectangular shield would for some reason best resist the pressures which would be developed. Whatever the reason may have been for the choice, the fact remains that a rectangular shield was adopted. The tunnel as designed consisted of two parallel horseshoe tunnels, 13 ft. 9 ins. wide and 16 ft. 4 ins. high and 1200 ft. long, separated from each other by a wall 4 ft. thick, pierced by 64 arched openings of 4 ft. span, the whole being surrounded with massive brickwork built to a rectangular section measuring over all 38 ft. wide and 22 ft. high. The first shield designed by Brunel for the work proved inadequate to resist the pressures, and it was replaced by another somewhat larger shield of substantially the same design, but of improved construction. This last shield was 22 ft. 3 ins. high and 37 ft. 6 ins. wide. It was divided vertically into twelve separate cast-iron frames placed close side by side, and each frame was divided horizontally into three cells capable of separate movement, but connected by a peculiar articulated construction, which is indicated in a general way by Fig. 124. To close or cover the face of the excavation, poling-boards held in place by numerous small screw-jacks were employed. Each cell or each frame could be advanced independently of the others, the power for this operation being obtained by means of screw-jacks abutting against the completed masonry lining. Briefly described, the mode of procedure was to remove the poling-boards in front of the top cell of one frame, and excavate the material ahead for about 6 ins. This being done, the top cell was advanced 6 ins. by means of the screw-jacks, and the poling-boards were replaced. The middle cell of the frame was then advanced 6 ins. by repeating the same process, and finally the operation was duplicated for the bottom cell. With the advance of the bottom cell one frame had been pushed ahead 6 ins., and by a succession of such operations the other eleven frames were advanced a distance of 6 ins., one after the other, until the whole shield occupied a position 6 ins. in advance of that at which work was begun. The next step was to fill the 6-in. space behind the shield with a ring of brickwork.