a larger type, with a displacement of 1,200 tons and an operating
distance of 5,000 miles.

The ordinary submarine in service at the beginning of the war could
remain below the surface for twenty-four hours at least. Reserve
amounts of air for breathing were carried in tanks under pressure,
and in the German type there were also chemical improvements for
regenerating air. Contrary to the opinion of laymen, submerging
was accomplished both by letting water into ballast tanks, and
also by properly deflecting a set of rudders; every submarine had
two sets of rudders, one of which worked in vertical planes and
pointed the prow of the ship either to the left or the right; the
other pair worked in horizontal planes and turned the prow either
upward or downward. A pair of fins on the sides of the hull assisted
action in both rising and diving. The action of water against the
fins and rudders when the ship was in motion was exactly the same
as that of the air against the planes of a kite; to submerge one
of the craft it was necessary to have it in motion and to have
its horizontal rudders so placed that the resistance of the water
would drive the ship downward; the reverse operation drove it upward.
And here lay a danger, for if the engines of a diving submarine
stopped she was bound to come to the surface. Her presence, while
moving entirely submerged could be detected by a peculiar swell
which traveled on the water above; if submerged only so much as
to leave the tip of her periscope still showing, the latter left
an easily discernible wake.

The periscope was merely a tube in which there were arranged mirrors
so that anything reflected in the first mirror, the one above the
surface of the water, was again reflected till it showed in a mirror
at the bottom of the tube, within the hull of the vessel, where
its commander could observe it safely. A crew of about twenty-five
men was necessary to operate one of these crafts, and theirs was
an unpleasant duty, first because of the danger that accompanied
each submergence of their vessel; second because of the discomforts
abroad. The explosive engines which drove the craft, whether burning
oil or the lighter refinements such as gasoline, gave off gases
that caused headaches and throbbing across the forehead; and it
was almost impossible to heat the interior of the craft.

Though merchantmen had gone to the bottom as victims of German
submarines before, the proclamation of a "w