The day passed, and the next. He made another search of the bubble's interior with the same results as before. He felt almost sure, then, that there was nothing in the bubble with him. He established a routine of work, pastime and sleep that made the first week pass fairly comfortably but for the gnawing worry in his mind that something invisible was lurking just outside the windows. Then one day he accidentally kicked the wall with his metal shoe tip. It made a sound like that from kicking a tight-stretched section of tin and it seemed to him it gave a little from the impact, as tin would do. He realized for the first time how thin it was--how deadly, dangerously thin. According to the specifications he had read it was only one-sixteenth of an inch thick. It was as thin as cardboard. He sat down with pencil and paper and began calculating. The bubble had a surface area of 146,500 square inches and the internal air pressure was fourteen pounds to the square inch. Which meant that the thin metal skin contained a total pressure of 2,051,000 pounds. Two million pounds. The bubble in which he sat was a bomb, waiting to explode the instant any section of the thin metal weakened. It was supposed to be an alloy so extremely strong that it had a high safety factor but he could not believe that any metal so thin could be so strong. It was all right for engineers sitting safely on Earth to speak of high safety factors but his life depended upon the fragile wall not cracking. It made a lot of difference. * * * * * The next day he thought he felt the hook to which the exerciser spring was attached crack loose from where it was welded to the wall. He inspected the base of the hook closely and there seemed to be a fine, hairline fracture appearing around it. * * * * * He held his ear to it, listening for any sound of a leak. It was not leaking yet but it could commence doing so at any time. He looked out the windows at the illimitable void that was waiting to absorb his pitiful little supply of air and he thought of the days he had hauled and jerked at the springs with all his strength, not realizing the damage he was doing. There was a sick feeling in his stomach for the rest of the day and he returned again and again to examine the hairline around the hook. The next day he discovered an even more serious threat: the thin skin of the bubble had been spot-welded to the outside reinforcing girders. Such welding often created hard, brittle spots that would soon crystallize from continued movement--and there was a slight temperature difference in the bubble between his working and sleeping hours that would daily produce a contraction and expansion of the skin. Especially when he used the little cooking burner.