The removal of this one log served to make the removal of the others possible. By diligent efforts the four logs which composed the lower tier were detached. The side logs were too long for the pit, and therefore had to be placed erect, and leaned against the side. The end logs could lie down easily. The second tier then followed, and was removed more easily than the first. Then the third tier was detached, and the fourth. In each case the logs of the side had to be stood erect, while the end logs were laid on the ground at the bottom. A serious difficulty now appeared before the boys, and one, too, for which they had not been prepared. The length of the side logs was a very embarrassing circumstance. They were too long to be placed at the bottom, and had to be stood up. But this took up space, and infringed very seriously upon the narrow area in which their operations were carried on. In passing from one side to remove the logs on the other, they had to lift these backward and forward so as to get them out of the way--a work which was most exhaustive, and at the same time hindered them in their proper efforts. Still they kept on, until at length about eight tiers of logs had been removed, and the longer ones filled up so much space, that it was quite impossible to do any more. They still worked away at those which were within reach, and managed to remove a dozen logs more; but after this they could do nothing, for the bottom of the pit was completely filled, and the staying was now a compact mass from which nothing further could be detached until the logs were removed which were covered up by those piled against it. Bart and Pat were now compelled to desist for a time, and as they felt quite exhausted, they raised themselves to the top of the pile of upright logs, and there sat down. Scarcely had they done this, when they were aware of a trembling all around, like an earthquake. In horror they sprang to their feet. The sides seemed to be moving; the logs separated, and descended, and through the crevices there protruded sand and clay. It was as though the whole mass of the casing was falling in. In an instant they knew what it was. In their thoughtlessness they had taken away the foundations of this structure, and it was all falling in. An involuntary cry of terror burst from both. They shrank together, clinging to the pile on which they stood, and awaited their last hour.