"Never," the other said. "The next step was to drive it down into the hard sand at the bottom of the bay. Father had made borings and found a true sea-bottom sand fifteen feet and a half below the level of the shoal. It was to that depth that the whole caisson had to be sunk.
"You remember that I told you there was an air-shaft in the middle of the caisson?"
"Yes."
"Well, on the top of this air-shaft an air-lock was built. The water in the air-shaft was forced out by compressed air and the men entered the caisson."
"Into the compressed air?"
"Yes. It takes a special kind of worker for the job. In the air-lock, you know, the men have to stay for a while before they enter the chamber, so as to get used to the compressed air gradually. Lots of people can't stand it."
"Did you try it?"
"Yes. I asked Father and he wouldn't let me. But I slipped into the air-lock once and tried it, anyhow."
"Well?"
"Not for me!" said his friend. "I got out in less than five minutes. My head seemed bursting, and I was bleeding from the ears as well as the nose. But some of them, especially an old chap called Griffin, the foreman, didn't seem to mind it at all.