enemy and discharges its torpedo. Yet the description
which these pages have already given of its operations
shows the falsity of this idea. It is important that we
should keep constantly in mind the fact that the sub-
marine is only occasionally a submarine ; and that for
the greater part of its career it is a surface boat. In the
long journeys which the German U-boats made from the
Heligoland Bight around Scotland and Ireland to those
great hunting grounds which lay in the Atlantic trade
routes, they travelled practically all the time on the surface
of the water. The weary weeks during which they cruised
around, looking for their victims, they also spent almost
entirely on the surface. There were virtually only two
circumstances which compelled them to disappear beneath
the waves. The first of these was the occasion on which
the submarine detected a merchant ship ; in this case
it submerged, for the success of its attempt to torpedo
depended entirely upon its operating unseen. The second
occasion which made it necessary to submerge was when
it spied a destroyer or other dangerous patrolling craft;
the submarine, as has been said, could not fight a vessel
of this type with much chance of success. Thus the
ability to submerge was merely a quality that was utilized
only in those crises when the submarine either had to
escape a vessel which was stronger than itself or planned
to attack one which was weaker.
The time taken up by these disappearances amounted to only a fraction of the total period consumed in a cruise. Yet the fact that the submarine had to keep itself momentarily ready to make these disappearances is precisely the reason why it was obliged to spend the larger part of its time on the surface. The submarine has two sets of engines, one for surface travel and the other for subsurface travel. An oil-engine propels it on the top of the water, but this consumes a large amount of air, and, for this reason, it cannot be used when travelling under the surface. As soon as the vessel dives, therefore, it changes its motive power to an electric motor, which makes no inroads on the oxygen needed for sustaining the life of its crew. But the physical limitation of size prevents the submarine from carrying large storage batteries, which is only another way of saying that its cruising radius under the water is extremely small, not more than fifty
