But his habit of silence gained upon him day by day, and was infecting us. External objects produce decided effects upon the brain. A man shut up between four walls soon loses the power to associate words and ideas together. How many prisoners in solitary confinement become idiots, if not mad, for want of exercise for the thinking faculty!

During the fortnight following our last conversation, no incident occurred worthy of being recorded. But I have good reason for remembering one very serious event which took place at this time, and of which I could scarcely now forget the smallest details.

By the 7th of August our successive descents had brought us to a depth of thirty leagues; that is, that for a space of thirty leagues there were over our heads solid beds of rock, ocean, continents, and towns. We must have been two hundred leagues from Iceland.

On that day the tunnel went down a gentle slope. I was ahead of the others. My uncle was carrying one of Ruhmkorff’s lamps and I the other. I was examining the beds of granite.