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." * * * * * You are standing aloft, balancing yourself upon tiny planks at the steadily advancing end of the bridge, as it forces itself over a stream of formidable width. Overhead, a gigantic, ungainly traveller, equipped with steel derricks at every corner, is advancing foot by foot as the bridge advances foot by foot. Underneath, through the thin network of planks, of girder and of supporting false work, you can see the surface of the river a full hundred feet below. A steamboat is passing directly beneath you. From your perch she looks like a great yellow bird. Those fine black specks upon her back are the humans who are gathered upon her upper deck. Whistles call and the derricks groan as they swing the thousands of bridge-members, that are flying together at the beck of the engineer, into their final resting-places. There is the deafening racket of the riveters, here and there and everywhere. There are crude railroad tracks upon the temporary flooring of the bridge deck, and the calls of the dummy locomotives add to the racket. The railroad tracks lead to the shore, to temporary yards where the bridge materials are assembled as fast as they come from the shops in a city three hundred miles distant.