compressed air, which is used in the
torpedo tubes of the Government submarines, we use a current of
electricity. It forces the cannon ball out with great energy."

"I wonder what they will do next?" observed the captain, peering
through a bull'seye.

"We can soon tell," replied the youth. "We'll go ahead, and if they try
to follow I'm going to fire on them."

"Suppose you sink them?"

"I won't fire to do that; only to disable them. They brought it on
themselves. We can't risk having them damage us. Help me with the
cannon, will you please, captain?"

The electric cannon was a long, steel tube in the after part of the
submarine. It projected a slight distance from the sides of the ship,
and by an ingenious arrangement could be swung around in a ball and
socket joint, thus enabling it to shoot in almost any direction.

It was the work of but a few minutes to get it ready and, with the
muzzle pointing toward the Wonder, Tom adjusted the electric wires and
inserted the solid shot.

"Now we're prepared for them!" he cried. "I think a good plan will be
to start ahead, and if they try to follow to fire on them. They've
brought it on themselves."

"Correct," spoke Captain Weston.

Tom hurried forward to tell his father of this plan.

"We'll do it!" cried Mr. Swift. "Go ahead, Mr. Sharp, and we'll see if
those scoundrels will follow."

The young inventor returned on the run to the electric cannon. There
was a whir of machinery, and the Advance moved forward. She increased
her speed, and the two watchers in the stern looked anxiously out of
the windows to see what their rivals would do.

For a moment no movement was noticeable on the part of the Wonder.
Then, as those aboard her appeared to realize that the craft on which
they depended to pilot them to the sunken treasure was slipping away,
word was given to follow. The ship of Berg and his employers shot after
the Advance.

"Here they come!" cried Captain Weston. "They're going to ram us again!"

"Then I'm going to fire on them!" declared Tom savagely.

On came the Wonder, nearer and nearer. Her speed was rapidly
increasing. Suddenly she bumped the Advance, and then, as if it was an
unavoidable accident, the rear submarine sheered off to one side.

"They're certainly at it again!" cried Tom, and peering from the
bull's-eye he saw the Wonder shoot past the mouth of the electric
cannon. "Here it goes!" he added.

He shoved over the lever, making the proper