You have expressed your interest in the sand-hog, given vent to a desire to go down into their underworld. You wonder what three pressures is going to feel like. Permission is given and a physician begins examining you. You cannot go into the caisson unless you are sound of heart and stout of body. This is no joking matter. The sand-hogs' rules read like the training instructions for a college football team. No drink, regular hours, simple diet, the donning of heavy clothes after they leave the pressure, constant reëxamination--these rules are inflexible when the caissons go to far depths. By their observance the difficult foundation construction of this new bridge has been kept free from accident--there have been few cases of the "bends" brought to the specially constructed hospital in the bottom of the cavity. The "bends" sounds complicated, and is, in reality, almost the simplest of human ailments in its diagnosis. A "bubble" of high pressure air works its way into the human structure while a man is in the caisson. When he comes out into the normal atmosphere the bubble is caught and remains. If it is caught near any vital organ that bubble is apt to spell death. Generally the bubbles are caught in the joints--frequently the elbow or the knee--where they cause excruciating pain. Then the specially constructed hospital crowded on the narrow platform formed by the top of the pier, comes into full play. Its sick room is incased in an air-tight cylinder. The man suffering from the "bends," together with physicians and nurses, is put under a pressure that gradually increases until it reaches that of the caisson. After that it is a comparatively simple matter to relieve the bubble and bring the air in the hospital back to a normal pressure. The path is clear for us to go down into the caisson. A party of sand-hogs, hot and exhausted after forty minutes of work within, come out of the little manhole at the top of the air-lock. We step through the little manhole and into a tiny steel bucket that rests within the air-lock there at the top of the shaft. A word of command--farewell to the bright blue sky overhead--the black manhole cover is replaced. It is suddenly very dark. A single faint incandescent gives a dim glow in the tiny place. [Illustration: CONCRETE AFFORDS WONDERFUL OPPORTUNITIES FOR THE BRIDGE-BUILDERS] [Illustration: THE LACKAWANNA IS BUILDING THE LARGEST CONCRETE BRIDGE IN THE WORLD ACROSS THE DELAWARE RIVER AT SLATEFORD, PA.] [Illustration: THE BRIDGE-BUILDER LAYS OUT AN ASSEMBLYING-YARD FOR GATHERING TOGETHER THE DIFFERENT PARTS OF HIS NEW CONSTRUCTION] [Illustration: THE NEW BRANDYWINE VIADUCT OF THE BALTIMORE & OHIO, AT WILMINGTON, DEL.] You are not thinking of that. They are putting the pressure on. You can feel it. Your eardrums feel as if they would break; they vibrate. You must show your distress. "Pinch your nose and swallow hard," says the man who stands beside you in the bucket.