For, while the labourers were working in front, the bricklayers behind built up the sides and roof, and formed the floor of the tunnel, the soil at the roof being supported by the shield until the masons had completed their task. For nine feet, the tunnel proceeded through clay, but then came an unwelcome change. Wet, loose sand prevailed, and the work progressed with peril for thirty-two days, when firmer ground was reached. Six months passed and substantial headway was made, the tunnel being completed to the extent of 260 feet. Then, on the 14th of September, the startling intelligence came that the engineer feared the river would burst in at the next tide. He had found a cavity over the shield. Sure enough, at high tide, when the river was brimming full, the workmen heard the ominous rattle of earth falling on their shield, while gushes of water followed. So excellent were the precautions, however, that no disastrous effects followed, and Father Thames himself rolled earth or clay into the hole and stopped it up. It was a warning, and emphasised the fear that haunted the men’s minds all through the hazardous undertaking—the fear that the river would break through and drown the tunnel. In October, another small irruption took place, and was successfully combated. Then, in the following January (1827), some clay fell, but still no overwhelming catastrophe occurred. The ground grew so moist, however, that it was examined on the other side. That is, the river bed was inspected by the agency of a diving-bell, and some ominous depressions were found. These were promptly filled by bags of clay. It may be asked, Why had Brunel not gone deeper? Why had he not placed a greater thickness of earth or clay between his work and the waters of the Thames? The answer is this—He had been informed by geologists that quicksand prevailed lower down, and the shaft that he sank for drainage below the level of the proposed tunnel, indicated that this view might be correct. In fact, when he got down 80 feet, the soil gave way, and water and sand rushed upwards. He was therefore apparently between the Thames and the quicksand. The Tower Subway, constructed in 1869, and driven through the solid London clay, is, however, 60 feet deep where it commences at Tower Hill. Work went steadily forward at Brunel’s tunnel until the 18th of May. Mr. Beamish, the assistant engineer, was in the cutting on that day, and as the tide rose he observed the water increase about the shield; clay showed itself and gravel appeared. He had the clay closed up, and went to encourage the pumpers. Suddenly, before he could get into the cells, a great rush of sludge and water drove the men out of the cells, extinguished the lights, floated the cement casks and boxes, and poured forward and ever forward, filling the tunnel with the roaring of the flood. The Thames had broken in with a vengeance this time, and drowned the tunnel. CHAPTER II. UNDER THE RIVER.