“You should have been down in one of the boxes when they had to burn torches, before they got the electric light,” says one of the bridge engineers. “I worked in one of those that we left under a stone tower of the Brooklyn Bridge. Now we’re almost in clover. They even cool and dry the compressed air before we breathe it.”
An order goes aloft over an electric wire, the engineer who sits smoking his pipe on the sun-baked platform of the traveller derrick pulls a lever, and we go slipping up the shaft toward fresh air and freedom only a little less rapidly than we descended it. We do not reach it too quickly. There is a long wait in the air-lock after the lower manhole has closed, while the pressure is being reduced. You begin to worry and you ask your guide as to the delay. Nothing wrong?
He smiles at your timorous question and explains. It would be dangerous to come out from the caisson pressure quickly. He does not want to have to send you to that air-tight hospital with a bad case of the “bends.”
“How long in the air-lock?” you ask.
“Fifty minutes,” he answers.
Then he explains in more detail. You have been under a pressure of 50 pounds to the square inch—that’s your three atmospheres, and under the rules you must spend fifty minutes in the tiny air-lock. Up to a pressure of 36 pounds you must spend two minutes there for every three pounds of pressure. When you get above that “law of 36” it is a minute to the pound.
When that manhole cover overhead finally slides open you feel blinded by the light, even though the sun is hidden behind a passing cloud. The air-lock tender reaches down with his arms and gives you a lift up onto his narrow perch.
“Want to be a sand-hog?” he smiles.
“Not yet a while,” you answer, in all truth. “Not until every other job is gone.”