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. . . . . "My boy, where are you?" . . . . "Lost, in the deepest darkness." . . . . "Where is your lamp?" . . . . "It is out." . . . . "And the stream?" . . . . "Disappeared." . . . . "Axel, Axel, take courage!" . . . . "Wait! I am exhausted! I can't answer. Speak to me!" . . . . "Courage," resumed my uncle. "Don't speak. Listen to me. We have looked for you up the gallery and down the gallery. Could not find you. I wept for you, my poor boy. At last, supposing you were still on the Hansbach, we fired our guns. Our voices are audible to each other, but our hands cannot touch. But don't despair, Axel! It is a great thing that we can hear each other." . . . . During this time I had been reflecting. A vague hope was returning to my heart. There was one thing I must know to begin with. I placed my lips close to the wall, saying: "My uncle!" . . . . "My boy!" came to me after a few seconds. . . . . "We must know how far we are apart." . . . . "That is easy." . . . . "You have your chronometer?" . . . "Yes." . . . . "Well, take it. Pronounce my name, noting exactly the second when you speak. I will repeat it as soon as it shall come to me, and you will observe the exact moment when you get my answer." "Yes; and half the time between my call and your answer will exactly indicate that which my voice will take in coming to you." . . . . "Just so, my uncle." . . . . "Are you ready?" . . . . "Yes." . . . . . . "Now, attention. I am going to call your name." . . . . 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?" . . . .