ry grateful to
the boys after they had been using the vitiated atmosphere of their
craft.

In fact, the lads were much nearer the complete exhaustion of their
supply of usable atmosphere than they really comprehended.

"Um-m-m!" exclaimed Jimmie, inhaling great draughts of the incoming
current. "Smell that, will you? It's just like a posy bed!"

"That's quite remarkable!" declared Ned, as he, too, sniffed the new
atmosphere. "It does really seem to carry the odor of flowers!"

"Maybe it's a sort of gas that he's unloading on us to render us
unconscious, so he can capture the whole outfit!" conjectured Jack.

"I don't believe it!" protested Ned. "I'm quite convinced that this is
pure air. He seems to have quite a lot of it stored up!"

"Let's pump out some of this foul air and change with the new!"

"Go ahead!" consented Ned. "It's a good move, I'm sure!"

In a short time the boys began to feel the effects of the inflow of
vitalized atmosphere. They were livelier, with less depression.

Directly their attention was attracted to the porthole again by a
tapping. The stranger was once more trying to convey some information by
signs. He repeated the motions of a short time before.

"I got you!" cried Harry, holding up a hand as a sign of understanding.
"He wants us to shut the valve off. Perhaps he's given us all the nice
fresh air that he feels it possible to spare!"

"Shut the valve, then," directed Ned.

"What's next?" spoke up Jimmie, listening to a slight hammering outside
of the hull. "He's disconnecting the pipe now!"

"Better wait a bit and see what he wants us to do," cautioned Ned. "Maybe
he's going to cut the line out of our propeller."

The lad's prediction was correct. In a very few moments they could hear
the stranger working away at the encumbering line which held their
propeller in a vise-like grip.

Not many minutes passed before the stranger again appeared at the
porthole. Making a few signals easily comprehended by all, he repaired to
his own craft, entering and closing the door of the air lock.

Almost immediately the other craft began to ascend perpendicularly.

"Guess we may as well make a mooch!" stated Jimmie, as he watched the
other submarine rise out of their range of vision. "We're done here!"

"All right, let's get going!" agreed Harry, stepping toward the levers
and preparing to start the motors at the pilot's command.

At once Jimmie sprang to the wheel. He gave a pull at th