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."