about midnight
`` I had a sheath knife , which I had managed to conceal inside my trousers , and immediately set to work , and wrenched up a stool fixed against the wall . There were several nails in it , which I cut out ; and then , making a couple of deep notches in one of the angles of the wall , I fixed the bench a certain height below the roof , which enabled me , by standing on it , to force up one of the slabs with my back . Knowing where the nails were driven in , I carefully cut around them , making as little noise as possible . It was , I calculated , about midnight when I had finished my preparations . The slab lifted even more easily than I had expected . I listened for some minutes , expecting to hear the tread of a sentry , but not a sound reached my ears . I had great hopes that he had fallen asleep .