Chapter 3

“Yes,” said the professor. “Very bad, because you see if that took
place, then, a few centuries later—I have the exact figures written
down—Neptune would so disturb Uranus and Saturn that they would cause
Halley’s comet to collide with Mars.”

“Another collision,” cried Tubby with concern. “That’s _very_ bad. Ain’t
I right?”

“Yes,” said the professor. “Because, you see, if Mars is thus deflected
from its normal orbit, it will affect both the earth and Venus very
materially. Worse than that, within a few million years Venus will
collide with—”

“Gosh,” ejaculated Tubby. “Things _will_ be on the fritz then, won’t
they?”

“Yes. Listen. When Venus does this, then, a million or so years later,
the earth—”

“The earth has a collision, too,” Tubby interrupted. “Ain’t I right?”

The professor nodded. “But you see, we moved that ball. Its attractive
force altered the position of Neptune to-night. Very little, of course,
but enough to avoid that collision nine hundred and eleven years from
now. I figured it all out—and that danger’s past.”

“Are there any others?” asked Tubby in alarm.

“Millions,” answered the professor. “I have them all listed and I’m
watching them. With that ball—and a thousand others I have in the next
room—I can take care of them all.”

“Fine,” said Tubby. “We’re all right then. That’s fine.”

A short pause.

“You ought to get a man to help roll the balls,” Tubby added. “Is _that_
what you wanted to see me about?”

The professor had fallen again into abstraction. Tubby’s question
brought him back with a start.

“No,” he said. “Not exactly. It was about specific gravity I wanted to
see you.”

“What kind of gravity?”

“Specific gravity deals with solid objects immersed in a liquid,” the
professor explained grandly. “A man in water—for instance.”

“I can’t swim very well,” said Tubby. “But I can float.”

“Of course you can. That’s what I wanted to see you about—to test out
my great invention.”

“What is it?” Tubby asked.

The professor’s little chest expanded proudly. He sat up so straight on
the stool Tubby feared he was about to tumble forward into the tank.

“My invention makes water so that nobody can sink in it,” the professor
stated slowly and impressively.

“Great!” exclaimed Tubby. “Then nobody can’t never get drownded. Ain’t I
right?”

“Yes,” answered the professor. “You are a man of perspicacity, I see. I
like men of perspicacity. Also ships will float better. Also—”

Another thought suddenly came to Tubby. “I got a question I want to ask,
professor. Why can I float when Jake sinks, and he’s so much lighter’n
me?”

“I’ll explain it all,” said the professor. “The law of floating bodies
is this: ‘The weight of a body immersed in a fluid is equal to its own
weight minus the weight of the fluid it displaces.’”

“Oh,” said Tubby.

“That works like this,” the professor went on. “Take water, for
instance.”

“Take me and Jake, and water,” suggested Tubby.

“Who is Jake?”

“A skinny guy.”

“All right. We’ll take you and him, and water. Now water weighs about
sixty-two and a half pounds per cubic foot. It differs according to what
kind of water it is—salt water weighs much more. Now about the water
you displace—do you understand that?”

“Yes—no,” said Tubby.

“Take a bathtub exactly filled with water and get in—head and all. The
water that flows over the edge of the tub is the water you displace.”

Tubby nodded.

“Now if you weigh that water you get the weight of the water you
displace. Do you see?”

“Yes,” said Tubby.

“Now to find out what you will weigh in water, we take your weight—”

“Hund’ed and ninety pounds—naked,” Tubby prompted.

“And the weight of the water you displace—is, say, a hundred and
ninety-five pounds. Thus you would float by a margin of five pounds.”

“Yes,” said Tubby. “How about Jake? He weighs hund’ed and five.”

“He might displace only a hundred pounds of water. Then he would sink by
a margin of five pounds. Thin men generally sink. That’s because they
are _heavier_ in proportion to their bulk than fat men—because the
skeleton—the bone—weighs more for its size than fat does. Do you see?”

“Yes,” said Tubby. “What’s your invention?”

The professor rose to his feet. “I’ll show you.”

He got down on his hands and knees at the edge of the tank. Tubby sat
beside him laboriously.

Two wires, connected with little cylindrical cases, led down into the
tank. Tubby could see that the water around the wires was boiling
sluggishly. Two other wires stuck up into the air; like a little
wireless outfit, Tubby thought.

“A molecule of water is composed of two atoms of hydrogen and one of
oxygen,” explained the professor. “H-O is its chemical symbol—you see I
am a professor of chemistry as well as physics.”

“Yes,” said Tubby. “Go on.”

“Now it is well known that an electrical current will change water into
oxygen and hydrogen—that’s called electrolisis of water. See?”

Tubby nodded.

“My invention is something like that—only different. _My_ current takes
oxygen from the air and adds it to the water. Now as oxygen is heavier
than hydrogen it makes the water heavier. And yet it does not change the
water in any other way—that’s the peculiar part.”

“That _is_ peculiar,” said Tubby.

“What _I’m_ going to do,” the professor continued, “is build a lot of
big plants like this little one here and make the water of all the
oceans heavier so that nothing can sink in them.”

Tubby nodded again. “What do you want me to—”

“I want you to test my tank,” said the professor. “I calculate you
should float with head and shoulders out. Try it and see. Jump in.”

Tubby drew back in sudden fear. “Suppose I sink? You might ’a’ made a
mistake, professor.” He climbed to his feet hastily.

“Nonsense,” said the professor. “Jump in.” He stood up also.

Tubby shook his head. “I ain’t interested in jumpin’ in,” he said
decidedly. “_You_ jump in—I’m goin’ home. I—”

He got no further, for the professor suddenly gave him a violent shove.
Tubby staggered and plopped down into the tank, sinking like a lead
plummet. A roaring filled his ears. He scrambled about on the bottom of
the tank, feeling as though he were glued down.

What was it Jake had said? “If you’re heavier than water you sink.” Jake
was right, then—and the professor was wrong.

A dim, distant voice came down to him through the water—the professor’s
voice. It was muffled and blurred, yet distinct enough for Tubby to hear
the words.

“Oh, my goodness,” it wailed. “I’ve made a mistake!”