h--they aren't sharks, either."

"Not sharks?" cried Mr. Hardley. "What are they, then?"

"Horse mackerel," Tom answered. "At least that is the common name for
the big fish. But they are far from being sharks, and we are in no
danger from them."

"Oh!" exclaimed Mr. Hardley, and he seemed a little ashamed of the
exhibition of fear he had manifested. "Well, they certainly seem
determined to follow us," he added.

The big fish were, indeed, following the submarine, and it required no
exertion on their part to maintain their speed, since below the surface
the M. N. 1 could not move very fast, as indeed no submarine can, due
to the resistance of the water.

"They do look as though they'd like to take a bite or two out of us,"
observed Ned. "Are they dangerous, Tom?"

"Not as a rule," was the answer. "I don't doubt, though, but if a lone
swimmer got in a school of horse mackerel he'd be badly bitten. In
fact, some years ago, when there was a shark scare along the New Jersey
coast, some fishermen declared that it was horse mackerel that were
responsible for the death and injury of several bathers. A number of
horse mackerel were caught and exhibited as sharks, but, as you can
easily see, their mouths lack the under-shot arrangement of the shark,
and they are not built at all as are the man-eaters."

"Bless my toothbrush!" exclaimed Mr. Damon. "Still, between a horse
mackerel and a shark there isn't much choice!"

Mr. Hardley, with a shudder, turned away from the glass windows, and
Tom glanced significantly at Ned. It was another exhibition of the
man's lack of nerve.

"We'll have trouble with him before this voyage is over," declared the
young inventor to his chum, a little later.

"What makes you think so?" asked Ned.

"Because he's yellow; that's why. I thought him that once before, and
then I revised my opinion. Now I'm back where I started. You
watch--we'll have trouble."

"Well, I guess we can handle him," observed the financial manager.

"I'm going a little deeper," announced Tom, toward evening on the first
day of the voyage on the open ocean. "I want to see how she stands the
pressure at five hundred feet. I feel certain she will, and even at a
greater depth. But if there's anything wrong we want to correct it
before we get too far away from home. We're going down again, deeper
than before."

A little later the submarine began the descent into the lower ocean
depths. From three hundred and fifty feet