He sat up and pulled out his watch. There was too much gloom to see the hands.
Even as he looked a brilliant beam of light flashed straight in through the window. It was a quarter to one. For a few seconds only the searchlight rays played upon the building; then all was darkness, rendered the more opaque by reason of the sudden change.
"A searchlight from an airship," exclaimed the Sub. He knew perfectly well that since from his window he could see nothing of the searchlight apparatus placed on the fortifications, it was conversely impossible for one of those searchlights to throw a direct beam through the window of his cell.
In a trice he was out of bed. Propping the partially dismembered stool against the wall, he climbed up to the window and looked out. He was just in time to see a large Zeppelin in the act of descending somewhere to the left of his prison. By the arrangement of the cars he knew that it was not the same craft that he had seen recharging her petrol tanks on the morning of his arrest.
"I wonder whether they are stationed here?" he asked himself. "An airship of that size must need an enormous shed, yet I'll swear I never saw one when I was being taken ashore. And they would never risk mooring a lubberly craft like that in the open."
As there was nothing more to be seen, the Sub clambered down from his insecure perch and prepared once more to turn in. Suddenly he felt inclined to put his theory to the test. It was not yet one o'clock, four precious hours yet remained ere dawn. He would make another attempt to squeeze through that baffling hole in the wall.
This time he succeeded with comparative ease, although his bruised hips gave him a bad time of it during the operation. In his sense of elation pain and discomfort were forgotten, for he was now underneath the cell occupied by his comrade, with only six inches of stone floor to separate them.
It puzzled Hamerton considerably to know how the stone flags were supported, since there was a cavity underneath the floor, but now he made a discovery that he had hitherto overlooked. The floor had been constructed at a fairly recent date, and was supported by slight iron girders, spaced about eighteen inches apart, which were in turn supported by small brick piers rising from the upper side of the vaulting. Fortunately the Sub had by chance decided to remove a stone that was only partly resting on a girder and held in position by a wedge-shaped stone—the second one that he had removed.
"It's about time I gave Detroit a hint," he thought. "I'll tap out a message as lightly as possible, in case the sound travels to another part of the building."
"Am crawling under the floor of your cell," he announced. "Will try and join you either to-night or to-morrow. Have you anything you can use to help shift one of the stones?"