He paused a moment, his fingers on his pulse.

"Yes, it takes about fifteen seconds for all the four seasons to pass. That means we're going backward in time about four years a minute. If we go on at this rate another hour we'll be back in the time of the Northmen, and will be able to tell if they did discover America, after all."

"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. T