By the gauge I can follow the depths we are making. Through the flooding we have added several tons of dead weight to the boat. We have made the closed up ship's body heavier than the mass of water displaced--and our giant fish sinks--almost falls--into her element down below. At the same time we are moving with the electric engines, and the forward thrust of the propeller brings pressure and reaction upon the diving rudder, and transforms the sinking into a downward gliding movement. When the desired depth is reached, which I can tell at once from the depth gauge, further sinking is prevented by the simple means of making the boat lighter again through pumping out the superfluous water from the submersion tanks. The furious working of the pumps is thus always the sign that we are approaching the desired depth. Then it stops, only the electric engines continue humming, and from the control-room comes the announcement: "We are lying at eleven fathoms." "Boat is trimmed!" We are travelling at a depth of eleven fathoms. This means that we are practically blind and can only judge our way by the depth gauge and by the help of that carefully protected boat's treasure--the gyro-compass. No glimmer from outside now reaches us; the periscope has long been swallowed up, and the steel safety shutters in the windows of the conning-tower are tightly closed. We are entirely transformed into the character of a fish. Now the communications from all compartments, control-room, engine-room, stern-room, bow-room, holds, accumulator rooms, come through without a hitch. We can travel safely with our "Deutschland" in the deeps. It is not always, however, such an easy matter to bring a boat of this size down to a prescribed depth. The changes in the specific weight of the water, owing to varying temperatures or to the different proportions of salt held in solution, play a very important rôle. How strong an influence this can prove I will show in the difference between the Baltic and North Sea waters. The specific gravities of the two seas are in the proportions of 1·013 to 1·025. This difference in itself may not appear very considerable. With a boat, however, of the size of the "Deutschland," for the submersion of which a very heavy excess weight of many tons is necessary, very important consequences are bound to follow from this difference in specific gravity. Thus in order to submerge in the heavier waters of the North Sea it is necessary to make the boat at least seventeen tons heavier than in the Baltic, as otherwise we should not sink. Moreover, during sudden alterations in the temperature of the water in the bays and river mouths, where the lighter fresh water comes in, the most unpleasant surprises often occur.