d make out nothing save the swirl and boiling of the
sea, caused by the progress of the Advance through it. But suddenly, as
he looked up, he was aware of some great, black body a little to the
rear and about ten feet above his craft.

"A shark!" he exclaimed aloud. "An immense one, too."

But the closer he looked the less it seemed like a shark. The position
of the black object changed. It appeared to settle down, to be
approaching the top of the conning tower. Then, with a suddenness that
unnerved him for the time being, Tom recognized what it was; it was the
underside of a ship. He could see the plates riveted together, and
then, as he noted the rounded, cylindrical shape, he knew that it was a
submarine. It was the Wonder. She was close at hand and was creeping up
on the Advance. But, what was more dangerous, she seemed to be slowly
settling in the water. Another moment and her great screws might crash
into the Conning tower of the Swifts' boat and shave it off. Then the
water would rush in, drowning the treasure-seekers like rats in a trap.

With a quick motion Tom yanked over the lever that allowed more water
to flow into the ballast tanks. The effect was at once apparent. The
Advance shot down toward the bottom of the sea. At the same time the
young inventor signaled to Captain Weston to notify those in the
engine-room to put on a little more speed. The Advance fairly leaped
ahead, and the lad, looking up through the bull's-eye in the roof of
the conning tower, had the satisfaction of seeing the rival submarine
left behind.

The youth hurried down into the interior of the ship to tell what he
had seen, and explain the reason for opening the ballast tanks. He
found his father and Mr. Sharp somewhat excited over the unexpected
maneuver of the craft.

"So they're still following us," murmured Mr. Swift. "I don't see why
we can't shake them off."

"It's on account of this luminous water," explained Captain Weston.
"Once we are clear of that it will be easy, I think, to give them the
slip. That is, if we can get out of their sight long enough. Of course,
if they keep close after us, they can pick us up with their
searchlight, for I suppose they carry one."

"Yes," admitted the aged inventor, "they have as strong a one as we
have. In fact, their ship is second only to this one in speed and
power. I know, for Bentley & Eagert showed me some of the plans before
they started it, and asked my opinion. This was b