Chapter 2

The laboratory into which Tubby was ushered a few moments later was a
gigantic room completely full of a variety of mechanical contrivances.
Nothing was familiar to Tubby but a big pool of very black, very
ominous-looking water that occupied the center, and around which the
various pieces of apparatus—complicated affairs of pulleys, weights,
pendulums and little incline tracks with leaden balls to roll down
them—were clustered.

Tubby’s host took off his hat and coat and laid them on a stool. A huge
lead ball as tall as Tubby’s knees stood near by. The little man looked
at his watch anxiously. Then he got behind the ball and, exerting all
his strength, tried to roll it forward on the floor. After a moment of
ineffectual effort he stopped and stood panting before Tubby.

“You might help me,” he said.

“Sure,” said Tubby. He rolled the ball forward. It was an
extraordinarily heavy ball; it took all his strength to move it.

“Wait! Oh, my goodness—wait!” The little man gripped his arm in terror.

Tubby stopped rolling the ball; and the little man dropped to his hands
and knees on the floor. Tubby saw that the boards were marked with chalk
lines like a ruler.

“You’ve moved it too far,” wailed his host. “Quick! Move it back. Two
inches—no, wait.” He whipped out a magnifying glass and examined the
chalk marks where the ball lay.

“One inch and a half. A little less. Roll it back!”

His agonized tone alarmed Tubby. He shoved the ball back carefully.

“There! Now—all right!” The little man stood up trembling, and mopped
the sweat from his forehead with a torn black-silk handkerchief.

“We nearly did it wrong,” he said, smiling weakly. He looked at his
watch again, and sighed with relief. “But it’s all right now. I guess
the danger’s past. It’s all right now.”

“I’m glad,” said Tubby. “We’re all right now, ain’t we?” He paused,
suddenly embarrassed. “Say, how about me and you gettin’ acquainted?”

“I _beg_ your pardon,” said the little man contritely. “My name’s
Graves—Dr. Graves, P. G.”

“Pleased to meet you,” said Tubby, shaking hands. “Mine’s Tubby. What’s
the P. G. for?”

“Why P. G.—Professor of Gravity,” said the little man.

“Oh,” said Tubby.

“Professor of Gravity,” repeated his host. “I see you don’t know what
that means?”

“No—yes,” said Tubby. “You’re sad. Ain’t I right?”

The professor smiled. “You will have your little joke, I see. You are a
clever man.”

“Yes,” assented Tubby. “What did you want to see me about? Why did you
roll that ball? What’s a Professor of Gravity?”

The professor looked at him with undisguised admiration. “You are a
precise man, too, I see. You don’t forget anything.”

“No,” said Tubby. “What—”

“I can answer all your questions at once,” said the professor. “I am a
Professor of Gravity—I know all about the laws of gravitation.”

“That’s fine,” said Tubby. “What’s gravitation?”

“Gravitation? Why—why—gravitation is what makes the weight of things.”

“Oh,” said Tubby. “Is that what you wanted to see me about?”

“Partly,” said the professor. “I’ve made a wonderful discovery—I’ve
been working at it eighty-two years, ever since I was a little boy—and
now it’s all ready.”

“Fine,” said Tubby.

“And I want you to help me test it out,” the professor added. “I picked
you because of your extreme weight.”

“Hund’ed and ninety pounds,” announced Tubby proudly. “Naked.”

The professor beamed. “Wonderful! Marvelous, for so short a man.”

“Yes, ain’t it?” said Tubby. “Why did we roll that ball?”

The professor’s face clouded again with anxiety. “To save the world,” he
said. His voice trembled with emotion. “In seven million nine hundred
thousand and two years, one month, six days, four hours, thirteen
minutes, eight and one-quarter seconds, the world would have been
destroyed if we had not moved that ball just where we did.”

“Countin’ from when?” Tubby asked anxiously.

“From eight thirty-four this evening—the moment at which you moved it,”
the professor answered.

“But it won’t be destroyed because we _did_ move it,” said Tubby.
“That’s fine. Ain’t I right?”

The professor nodded abstractedly. After a moment a thought occurred to
Tubby. He wrinkled his forehead. “Say, professor, you ain’t tellin’ me
nothin’. Why did we have to roll that ball to save the world?”

“‘Thou canst not touch a flower without troubling of a star,’” the
professor quoted. “You have heard that, haven’t you?”

“No,” said Tubby. “What’s that got to do with—”

“It’s a poetical quotation,” explained the professor. “It’s based on the
laws of gravity. Shall I explain?”

“Yes,” said Tubby.

The professor sat down on a little stool with his feet dangling
perilously over the black pool. Tubby sat beside him, a little further
away from the water.

“The law of gravitation was formulated by Sir Isaac Newton,” the
professor began. “He saw an apple fall, and worked it out from that.”

“He must ’a’ been a clever guy,” said Tubby.

“Yes,” answered the professor. “Here it is: ‘Everybody in the universe
attracts every other body with a force that varies directly as the mass
and inversely as the square of the distance.’”

“Ah,” said Tubby.

“That means,” the professor went on enthusiastically, “that if an
object—a person—or a world, is twice as big, it exerts twice as much
force—that’s ‘directly as the mass.’ Do you see?”

“Yes—no,” said Tubby.

“And if it’s twice as far away it exerts one-quarter the force—that’s
‘inversely as the square of the distance.’ That’s plain, isn’t it?”

“No—yes,” said Tubby. “Why did we roll—”

“I’m coming to that. Now you see, every particle of matter in the
universe attracts every other particle. That ball, therefore, attracts
the sun, the moon, and all the stars, just as they do it. It attracts
the earth also—as the earth attracts it.”

“But the earth is so much bigger it holds it down tight,” said Tubby.
“Ain’t I right?”

“You are indeed,” beamed the professor. “The reason we had to place the
ball in a certain position at eight thirty-four to-night is this: I have
calculated—I am a very wonderful man, you know—I can calculate
anything—my figures show that in nine hundred and eleven years and a
few odd days, minutes and seconds, the orbit of Neptune will bring that
planet into collision with a new comet which has just been discovered.”

“A collision?” repeated Tubby. “That’s bad.”