The Whispering Gallery
Harry stumbles backwards and forwards in the dark, completely lost. He dispairs for his very life. Utterly exhausted, he collapses against the side of the tunnel and passes out.
When he comes round some time later, he makes a discovery by accident. He can hear the voices of his friends, faint and far away, if he presses his ear against the tunnel wall! He calls out loud and times the reply. He hears his uncle answer after forty seconds. Knowing the speed of sound, he calculates he is seperated from the Professor and Hans by a distance of about a league and a half. Four and a half miles! His hopes, which had soared after hearing the others, are dashed once more.
The Professor offers Harry the encouragement he needs to go on and soon Harry sets off in the direction of the voices.