The starting of the heavy machinery was sufficient indication that night
had come. They were now going up and at an angle which was very
perceptible. The boys had become quite expert in detecting certain
activities, as they tried in every way to understand the use of the
signals. One thing was certain; two sets of bells were brought into play
as the signal for changing the motive power. The first signal, three
bells followed by two more, was invariably the necessary preparation for
this event.

A highly pitched bell next gave the signal to stop the gasoline engines
and a deep-toned bell indicated the coupling of the electric motor.
Occasionally a new set of signals would resound, which they tried to
figure out. During the night Alfred thought he had found the key.

"Did you notice the big hand wheel on the side of the upright tank,
which we pass as we go into the dining room?" he remarked.

"That is connected with a large valve," said the captain. "What did you
observe?"

"Well, did you ever notice that before they rang the shrill bell four
times we always have heard a whistle?" asked Alfred.

"Why, I have heard the bell ring four times on several occasions without
the whistle," contended Ralph.

"So you have, but it was always after the four rings that followed the
whistle. A little while ago I was near the tank, and I heard the
whistle. The attendant sprang to the wheel, and when the four rings came
he turned the wheel around twice. When the four next rings came (without
the whistle, of course), he quickly turned it back again," said Alfred.

"That is the submerging tank," said the captain. "I see you are rapidly
learning how to handle a submarine," and he laughed at the eagerness of
the boys trying to conquer the details of signaling.

During that night there was hardly a half-hour but some movement or
other was indicated by the bells. They submerged, halted, rose to the
surface, steamed at full speed, and in one or two instances it was
evident from the sudden stopping that the submarine had to reverse.

This constantly kept them alert, and while engaged in conversation late
in the morning, they were thrown forward on their seats with a motion
that indicated a collision with something which was not very rigid, for
there was no concussion such as usually accompanies the contact of the
hull of a vessel with a hard object.

The boys looked at the captain in astonishment. They could now feel the
propeller pul