…

I put my ear to the wall, and as soon as the name “Axel” came I immediately replied “Axel,” then waited.

…

“Forty seconds,” said my uncle. “Forty seconds between the two words; so the sound takes twenty seconds in coming. Now, at the rate of 1,120 feet in a second, this is 22,400 feet, or four miles and a quarter, nearly.”

…

“Four miles and a quarter!” I murmured.

…

“It will soon be over, Axel.”

…

“Must I go up or down?”