e moment the commander gives the order
to submerge.

All this may sound very simple and yet there are a great many things
to consider. In the same manner in which an airplane is carefully
balanced before taking wing into the high regions of the sky, a
submarine must be accurately weighed and measured before it descends
into the watery depths of the ocean. The briny water of the North Sea
weighs far more than the less salty water of the Baltic Sea, whose
western basin is composed of practically fresh water. A boat floats
higher in the heavily salted waters of the North Sea and lies deeper
and plunges farther down in the waters of the Baltic. The same U-boat,
therefore, must take into its tanks a greater quantity of water
ballast in the North Sea, to be properly weighted, than when diving
into fresher waters. Even with small submarines of 400 tons
displacement, there is the enormous difference of 10 tons between
1.025 specific weight in the intake of North Sea water and 1.000
specific weight of fresh water. On the other hand, if too much water
is admitted into the tanks, the submarine may plunge with great
velocity deeper and deeper beyond its appointed depth, and in such a
case it might even happen that the hull of the boat could not
withstand the overpowering pressure and would be crushed beneath the
mass of water. And yet again if too small a quantity of water ballast
is admitted into the tanks, the boat may not sink sufficiently below
the surface, and thus we could not obtain an invisible attack which is
positively necessary for our success.

How much water then must we take in? The answer to this question is a
matter of instinct, education, and experience and we must also depend
on the cleverly devised apparatus made for this purpose.

The submarine like the airplane must be always maintained at the
proper level. The weight of the boat varies continually during a
prolonged voyage. Food is devoured and the diving material of the
machinery is consumed. The water in which the boat swims continually
changes weight and the boat is imperceptibly raised or lowered in a
way very difficult to ascertain. The officer responsible for the
flooding of the submarine must painstakingly keep its weight under
control during the entire navigation. The weight of a meal eaten by
each man of the crew, the remains of the food and the boxes in which
it was contained, which have been thrown overboard, must be calculated
as well as the we