Then I understood it all. To make myself heard, I must speak along this wall, which would conduct the sound of my voice just as wire conducts electricity.

But there was no time to lose. If my companions moved but a few steps away, the acoustic phenomenon would cease. I therefore approached the wall, and pronounced these words as clearly as possible:

“Uncle Liedenbrock!”

I waited with the deepest anxiety. Sound does not travel with great velocity. Even increased density air has no effect upon its rate of travelling; it merely augments its intensity. Seconds, which seemed ages, passed away, and at last these words reached me:

“Axel! Axel! is it you?”

…

“Yes, yes,” I replied.

…