No use of
leaving him here to give an alarm."

"We'll gag him!" declared the young captain. And without loss of time
the three cadets fixed up a gag such as they sometimes used when they
were initiating a new member of one of the secret societies of the
military academy. Then the German's hands were bound tightly behind
him, and he was ordered to get up and march.

"Wait a minute," said Fred. "We'll have to fix things here so it won't
look suspicious. We'll make it look as if this chap had just stepped
out for some fresh air."

This was done, and a few minutes later the whole party left the
submarine, Gif going ahead and the Rovers following the prisoner, each
with a pistol ready for use. In this fashion they passed over the
gangplank, and then made their way alongside of the underground pond
until they came to the spot where the cadets had first discovered the
Germans.

"I don't believe we ought to leave him here," declared Jack. "That gag
might slip and he might have a chance to make considerable noise, and
if he did that the others might take the alarm and sail away before we
could get help."

"We made a mistake!" cried Gif. "I thought we were going to injure
some of the machinery, so that they couldn't use the U-boat."

"You're right, Gif! I got so excited I forgot all about that,"
declared Jack.

"You can run back now and do it if you want to," said Fred. "I'll
watch the prisoner."

The young captain and Gif did as had been suggested. On the submarine
they looked over the intricate machinery with care, and presently
found some things which they could disarrange and which would probably
not be noticed immediately. They went to work with vigor, and came
away again in less than ten minutes.

"I guess she's fixed now," declared Jack to his cousin, when they had
rejoined Fred and the prisoner. "If they start up those engines, that
submarine will perform stunts they never dreamed of."

With the flashlight shining ahead and at times on the prisoner, the
Rovers and Gif compelled the German to move along the passageway until
they gained the opening near the gully.

"Look out there, will you?" cried Gif suddenly, pointing between the
bushes to the bay. "There is a big motor boat cruising up and down!
Maybe they can give us aid."

"It looks to me like a revenue boat," declared Jack, after a hurried
inspection. "Say, maybe they are patrolling the bay!"

"That's right!"

"I am going to signal to them and fin