e went on as he looked from one
of the aft observation windows, "they'll foul us, and--"

His remarks were interrupted by a jar to the Advance. She seemed to
shiver and careened to one side. Then came another bump.

"Slow down!" cried the captain, rushing toward the pilot house.

"What's the matter?" asked Tom, as he threw the engines and electrical
machines out of gear. "Have we hit anything?"

"No. Something has hit us," cried the captain. "Their submarine has
rammed us."

"Rammed us!" repeated Mr. Swift. "Tom, run out the electric cannon!
They're trying to sink us! We'll have to fight them. Run out the stern
electric gun and we'll make them wish they'd not followed us."

Chapter Eighteen

The Electric Gun

There was much excitement aboard the Advance. The submarine came to a
stop in the water, while the treasure-seekers waited anxiously for what
was to follow. Would they be rammed again? This time, stationary as
they were, and with the other boat coming swiftly on, a hole might be
stove through the Advance, in spite of her powerful sides.

They had not long to wait. Again there came a jar, and once more the
Swifts' boat careened. But the blow was a glancing one and,
fortunately, did little damage.

"They certainly must be trying to sink us," agreed Captain Weston.
"Come, Tom, we'll take a look from the stern and see what they're up
to."

"And get the stern electric gun ready to fire," repeated Mr. Swift. "We
must protect ourselves. Mr. Sharp and I will go to the bow. There is no
telling what they may do. They're desperate, and may ram us from in
front."

Tom and the captain hurried aft. Through the thick plate-glass windows
they could see the blunt nose of the Wonder not far away, the rival
submarine having come to a halt. There she lay, black and silent, like
some monster fish waiting to devour its victim.

"There doesn't appear to be much damage done back here," observed Tom.
"No leaks. Guess they didn't puncture us."

"Perhaps it was due to an accident that they rammed us," suggested the
captain.

"Well, they wouldn't have done it if they hadn't followed us so close,"
was the opinion of the young inventor. "They're taking too many
chances. We've got to stop 'em."

"What is this electric gun your father speaks of?"

"Why, it's a regular electric cannon. It fires a solid ball, weighing
about twenty-five pounds, but instead of powder, which would hardly do
under water, and instead of