And low bridge it was as the party entered the access tube. Like an oblique ladder leading downward the tube stretched away into the sea. The steel piping was less than four feet in diameter, and the only way to negotiate it was to duck down almost on all fours and make your way along laboriously like a telephone repairman in a conduit. Electric lights were stationed at intervals along the way to light up the submarine tunnel. "Keep your head down, Fritzie boy, or you'll get an awful bump on the cranium," cautioned the ship's captain. "Now we are going into the air-lock chamber, boys," he told them. "We are down below the surface of the Sound something like eighty-five feet. When we get on the deck of the _Nautilus_ we will be down an even hundred feet. Follow me right through." In response to the captain's tapping on a huge port immediately to his right it had swung open like the fire door of a huge locomotive. There, in the encircling frame, was the face of Superintendent Brown. "Welcome, boys. 'Will you step into my parlor?' said the spider to the fly." The yard official was all smiles as he greeted the boys. Captain Austin set his foot through the aperture and crawled through into the adjoining chamber alongside the superintendent. The boys followed suit as rapidly as they could. They found themselves now in a narrow little prison not more than four feet high, six or seven feet long, and about two feet wide. With difficulty the five men distributed themselves in the place. Crouched closely together, shoulders touching each other, they filled the whole compartment like so many sardines in a can. "This is the air-lock chamber, boys," announced Superintendent Brown. "From your submarine experience to date you can easily understand the function of this chamber. We have just stepped in here from the access tube where there maintains the air pressure of the surface. We want to go from here into the _Nautilus_, where we can roll back the open hatch from the bottom of the craft and gaze upon the very sea itself held in abeyance. How would you go about it, Mr. Monaghan?" asked the superintendent, knowing of Dick's predilection for mechanical problems and his desire to pursue his education through college. Just for an instant Dick hesitated, and then answered: "I should say you would have to equalize the air pressure, sir." "And you are right," answered the Bridgeford official. "That is exactly what we have to do here. It is out of the question to go directly from the pressure of the surface to a pressure of one hundred feet below the surface. We simply come into this air chamber, shut ourselves off completely from the world above us, and then step ourselves up to the required air pressure for one hundred feet." So saying, the superintendent slammed shut the door of the port through which the party had entered the air-lock from the access tube, and made it doubly secure with a stout pin that slid into place behind a reinforcing bar.