Attention is called to this method of doing duty so that the reader may understand certain events which will be referred to later.
The personnel of the shifts was also changed at intervals so that while the lieutenant during one shift would have at work a certain machinist and petty officer, during the next or second shift thereafter another machinist or petty officer would be on duty. In this manner all became efficient, for they had the opportunity afforded of being drilled and handled by different combinations of men and assistants.
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.