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 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's eye.

"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 connection. there was no corresponding report, for the cannon was noiseless, but there was a slight jar as the projectile left the muzzle. the wonder could be seen to heel over.

"you hit her! you hit her!" cried captain weston. "a good shot!"

"i was afraid she was past me when i pulled the lever," explained tom. "she went like a flash."

"no, you caught her on the rudder," declared the captain. "i think you've put her out of business. yes, they're rising to the surface."

the lad rapidly inserted another ball, and recharged the cannon. then he peered out into the water, illuminated by the light of day overhead, as they were not far down. he could see the wonder rising to the surface. clearly something had happened.

"maybe they're going to drop down on us from above, and try to sink us," suggested the youth, while he stood ready to fire again. "if they do—"

his words were interrupted by a slight jar throughout the submarine.

"what was that?" cried the captain.

"dad fired the bow gun at them, but i don't believe he hit them," answered the young inventor.

"i wonder what damage i did? guess we'll go to the surface to find out."

clearly the wonder had given up the fight for the time being. in fact, she had no weapon with which to respond to a fusillade from her rival. tom hastened forward and informed his father of what had happened.

"if her steering gear is out of order, we may have a chance to slip away," said mr. swift "we'll go up and see what we can learn."

a few minutes later tom, his father and captain weston stepped from the conning tower, which was out of water, on to the little flat deck a short distance away lay the wonder, and on her deck was berg and a number of men, evidently members of the crew.

"why did you fire on us?" shouted the agent angrily.

"why did you follow us?" retorted tom.

"well, you've broken our rudder and disabled us," went on berg, not answering the question. "you'll suffer for this! i'll have you arrested."

"you only got what you deserved," added mr. swift. "you were acting illegally, following us, and you tried to sink us by ramming my craft before we retaliated by firing on you."

"it was an accident, ramming you," said berg. "we couldn't help it. i now demand that you help us make repairs."

"well, you've got nerve!" cried captain weston, his eyes flashing. "i'd like to have a personal interview with you for about ten minutes. maybe something besides your ship would need repairs then."

berg turned away, scowling, but did not reply. he began directing the crew what to do about the broken rudder.

"come on," proposed tom in a low voice, for sounds carry very easily over water. "let's go below and skip out while we have a chance. they can't follow now, and we can get to the sunken treasure ahead of them."

"good advice," commented his father. "come, captain weston, we'll go below and close the conning tower."

five minutes later the advance sank from sight, the last glimpse tom had of berg and his men being a sight of them standing on the deck of their floating boat, gazing in the direction of their successful rival. the wonder was left behind, while tom and his friends were soon once more speeding toward the treasure wreck.