“Funny we don’t hear any noises,” Estelle observed.  She had caught some of Arthur’s calmness.

“It passes so quickly that though our ears hear it, we don’t separate the sounds.  If you’ll notice, you do hear a sort of humming.  It’s very high-pitched, though.”

Estelle listened, but could hear nothing.

“No matter,” said Arthur.  “It’s probably a little higher than your ears will catch.  Lots of people can’t hear a bat squeak.”

“I never could,” said Estelle.  “Out in the country, where I come from, other people could hear them, but I couldn’t.”

They stood a while in silence, watching.

“When are we going to stop?” asked Estelle uneasily.  “It seems as if we’re going to keep on indefinitely.”

“I guess we’ll stop all right,” Arthur reassured her.  “It’s obvious that whatever it was, only affected our own building, or we’d see some other one with us.  It looks like a fault or a flaw in the rock the building rests on.  And that can only give so far.”

Estelle was silent for a moment.

“Oh, I can’t be sane!” she burst out semihysterically.  “This can’t be happening!”

“You aren’t crazy,” said Arthur sharply.  “You’re sane as I am.  Just something queer is happening.  Buck up.  Say your multiplication tables.  Say anything you know.  Say something sensible and you’ll know you’re all right.  But don’t get frightened now.  There’ll be plenty to get frightened about later.”

The grimness in his tone alarmed Estelle.

“What are you afraid of?” she asked quickly.

“Time enough to worry when it happens,” Arthur retorted briefly.

“You—­you aren’t afraid we’ll go back before the beginning of the world, are you?” asked Estelle in sudden access of fright.

Arthur shook his head.

“Tell me,” said Estelle more quietly, getting a grip on herself.  “I won’t mind.  But please tell me.”

Arthur glanced at her.  Her face was pale, but there was more resolution in it than he had expected to find.

“I’ll tell you, then,” he said reluctantly.  “We’re going back a little faster than we were, and the flaw seems to be a deeper one than I thought.  At the roughest kind of an estimate, we’re all of a thousand years before the discovery of America now, and I think nearer three or four.  And we’re gaining speed all the time.  So, though I am as sure as I can be sure of anything that we’ll stop this cave-in eventually, I don’t know where.  It’s like a crevasse in the earth opened by an earthquake which may be only a few feet deep, or it may be hundreds of yards, or even a mile or two.  We started off smoothly.  We’re going at a terrific rate. What will happen when we stop?”

Estelle caught her breath.

“What?” she asked quietly.

“I don’t know,” said Arthur in an irritated tone, to cover his apprehension.  “How could I know?”

Estelle turned from him to the window again.