Many a U-Boat commander has thought it possible with a certain amount of excess weight to submerge without difficulty and to keep his boat at a fixed depth. Suddenly, however, the pressure gauge registers a greater depth and the boat drops in the water, like an aeroplane which has fallen into an air-pocket, until a test of the specific weight and temperature of the water gives the clue to her behaviour. It will therefore be seen that such measurements are necessary before the commander can count with certainty on being able suddenly to submerge and as suddenly to reappear above the waters. In the meantime we have finished our submerging test satisfactorily. All has gone well and each part fully performed its functions. We are in complete control of our complicated apparatus. Now the command to reappear is given, the diving rudders are set to "up," and immediately I am able by the depth gauge to follow their working and that of our stout pumps. After assuring myself that there is no noise of propellers to be heard anywhere in our vicinity, and that on all sides there is no likelihood of collision with any steamer, we pass through the dangerous "blind moment." By this I mean that space of time during which the boat has risen so high that she could be rammed; while, on the other hand, she is still too deep under the water to get the periscope above the surface and take a look round. This lasts a few minutes. I stand at the periscope and watch. Already the field of vision is lighter. Silvery air bubbles rise up glittering; a blinking and twinkling appears on the glass. Then it is day. A picture arises, clear and shining. The North Sea sways before my eyes with an empty and endless horizon. Now we are rising to the surface. By the use of the rudder the boat pushes forward faster and faster to the surface of the water. In order to accelerate the ascent compressed air is forced into one of the submersion tanks. Now she moves very rapidly: the tower is already free. The deck rises dripping out of the water, the conning-tower hatch is opened, fresh air streams in, and I give the command: "Blow out ballast tanks!" A wild howling and screeching comes in reply from the control-room, while the powerful turbine engine presses the water out from the submerging tanks. This does not take long. As soon as a tank is empty the excess air rushes out with a pleasant sound at the side of the boat, and we are soon in normal floating trim again. We are still using electricity. Now comes last of all the starting up of the heavy Diesel engines by the electric motors. I have already climbed into the conning-tower and can see nothing of all this, except by the communications from the control-room. Those who are in the engine-room, however, will have an exciting spectacle.