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.