One idea overpowered me almost, fear lest the rock should give way from which I was hanging. This cord seemed a fragile thing for three persons to be suspended from. I made as little use of it as possible, performing wonderful feats of equilibrium upon the lava projections which my foot seemed to catch hold of like a hand.

When one of these slippery steps shook under the heavier form of Hans, he said in his tranquil voice:

"_Gif akt!_ "

"Attention!" repeated my uncle.

In half an hour we were standing upon the surface of a rock jammed in across the chimney from one side to the other.

Hans pulled the rope by one of its ends, the other rose in the air; after passing the higher rock it came down again, bringing with it a rather dangerous shower of bits of stone and lava.

Leaning over the edge of our narrow standing ground, I observed that the bottom of the hole was still invisible.

The same manœuvre was repeated with the cord, and half an hour after we had descended another two hundred feet.

I don't suppose the maddest geologist under such circumstances would have studied the nature of the rocks that we were passing. I am sure I did trouble my head about them. Pliocene, miocene, eocene, cretaceous, jurassic, triassic, permian, carboniferous, devonian, silurian, or primitive was all one to me. But the Professor, no doubt, was pursuing his observations or taking notes, for in one of our halts he said to me:

"The farther I go the more confidence I feel. The order of these volcanic formations affords the strongest confirmation to the theories of Davy. We are now among the primitive rocks, upon which the chemical operations took place which are produced by the contact of elementary bases of metals with water. I repudiate the notion of central heat altogether. We shall see further proof of that very soon."

No variation, always the same conclusion. Of course, I was not inclined to argue. My silence was taken for consent and the descent went on.

Another three hours, and I saw no bottom to the chimney yet. When I lifted my head I perceived the gradual contraction of its aperture. Its walls, by a gentle incline, were drawing closer to each other, and it was beginning to grow darker.

Still we kept descending. It seemed to me that the falling stones were meeting with an earlier resistance, and that the concussion gave a more abrupt and deadened sound.

As I had taken care to keep an exact account of our manœuvres with the rope, which I knew that we had repeated fourteen times, each descent occupying half an hour, the conclusion was easy that we had been seven hours, plus fourteen quarters of rest, making ten hours and a half. We had started at one, it must therefore now be eleven o'clock; and the depth to which we had descended was fourteen times 200 feet, or 2,800 feet.

At this moment I heard the voice of Hans.

"Halt!" he cried.

I stopped short just as I was going to place my feet upon my uncle's head.

"We are there," he cried.

"Where?" said I, stepping near to him.

"At the bottom of the perpendicular chimney," he answered.

"Is there no way farther?"

"Yes; there is a sort of passage which inclines to the right. We will see about that to-morrow. Let us have our supper, and go to sleep."

The darkness was not yet complete. The provision case was opened; we refreshed ourselves, and went to sleep as well as we could upon a bed of stones and lava fragments.

When lying on my back, I opened my eyes and saw a bright sparkling point of light at the extremity of the gigantic tube 3,000 feet long, now a vast telescope.

It was a star which, seen from this depth, had lost all scintillation, and which by my computation should be 46; _Ursa minor._ Then I fell fast asleep.

CHAPTER XVIII.

THE WONDERS OF TERRESTRIAL DEPTHS

At eight in the morning a ray of daylight came to wake us up. The thousand shining surfaces of lava on the walls received it on its passage, and scattered it like a shower of sparks.

There was light enough to distinguish surrounding objects.