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.