“Turn a tumbler upside down and force it down into a basin of water,” he replied, “and you trap some air in the top of the tumbler, which prevents the water from rising beyond a certain point. The same thing takes place in our tanks, and to fill them we have to force in the water with powerful pumps that compress the air in the tanks to a very small part of its original bulk. This compressed air acts as a powerful spring to drive the water out of the tanks again when we wish to rise. By blowing out the tanks, a submarine can come to the surface in twenty seconds or one sixth the time it takes to submerge.” “When are we going under?” I asked him. The lieutenant looked at his watch and answered, “We have been submerged for the last four minutes.” I experienced a feeling of the most profound disappointment. Ever since I had been a very small boy I had been looking forward to the time when I should go down in a submarine boat, and now that time had passed without my realizing it. “But why didn’t I feel the boat tilt when she dived?” I demanded. “Because she went down a very gentle slope, between two and three degrees at the steepest. The only way you could have noticed it would have been to watch these gages.” Large dials on the wall of the turret indicated that the _X-4_ was running on what was practically an even keel at a depth of sixteen feet and under a consequent water-pressure of 1024 pounds on every square foot of her hull. “How deep could she go?” “One hundred and fifty feet--if she had to. The strong inner hull of a modern submarine is built up of three quarter inch plates of the best mild steel and well braced and strengthened from within. But as a rule there is no need of our diving below sixty feet at the deepest, or far enough to clear the keel of the largest ship. You will notice how the depth-control man is holding her steady by manipulating the forward horizontal rudders, just as an aviator steadies his aeroplane.” [Illustration: Courtesy of the Electric Boat Company. Depth-control Station, U. S. Submarine. Wheel governing horizontal rudders, gages showing depth, trim, etc.] “He must be a strong man to handle those two big horizontal rudders.” “He has an electric motor to do the hard work for him, as has the quartermaster steering the course here with the vertical rudder.” The same red-headed petty officer that I had noticed on the bridge now grasped the spokes of a smaller steering-wheel inside the conning-tower. “What is that queer-looking thing whirling round and round in front of him?” I asked.