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.
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.