“I mean that we have only reached the level of the island, long vertical tube, which terminates at the mouth of the crater, has its lower end only at the level of the sea.”

“Are you sure of that?”

“Quite sure. Consult the barometer.”

In fact, the mercury, which had risen in the instrument as fast as we descended, had stopped at twenty-nine inches.

“You see,” said the Professor, “we have now only the pressure of our atmosphere, and I shall be glad when the aneroid takes the place of the barometer.”

And in truth this instrument would become useless as soon as the weight of the atmosphere should exceed the pressure ascertained at the level of the sea.

“But,” I said, “is there not reason to fear that this ever-increasing pressure will become at last very painful to bear?”