des these sounds, the presence of chemical combinations was soon
betrayed by their powerful odor, and the engineer and his companion were
almost suffocated by sulphurous vapors.

"This is what Captain Nemo feared," murmured Cyrus Harding, changing
countenance. "We must go to the end, notwithstanding."

"Forward!" replied Ayrton, bending to his oars and directing the boat
towards the head of the cavern.

Twenty-five minutes after entering the mouth of the grotto the boat
reached the extreme end.

Cyrus Harding then, standing up, cast the light of the lamp upon the
walls of the cavern which separated it from the central shaft of the
volcano. What was the thickness of this wall? It might be ten feet or a
hundred feet--it was impossible to say. But the subterranean sounds were
too perceptible to allow of the supposition that it was of any great
thickness.

The engineer, after having explored the wall at a certain height
horizontally, fastened the lamp to the end of an oar, and again surveyed
the basaltic wall at a greater elevation.

There, through scarcely visible clefts and joinings, escaped a pungent
vapor, which infected the atmosphere of the cavern. The wall was broken
by large cracks, some of which extended to within two or three feet of
the water's edge.

Cyrus Harding thought for a brief space. Then he said in a low voice,--

"Yes! the captain was right! The danger lies there, and a terrible
danger!"

Ayrton said not a word, but, upon a sign from Cyrus Harding, resumed the
oars, and half an hour later the engineer and he reached the entrance of
Dakkar Grotto.

Chapter 19

The next day, the 8th day of January, after a day and night passed
at the corral, where they left all in order, Cyrus Harding and Ayrton
arrived at Granite House.

The engineer immediately called his companions together, and informed
them of the imminent danger which threatened Lincoln Island, and from
which no human power could deliver them.

"My friends," he said, and his voice betrayed the depth of his emotion,
"our island is not among those which will endure while this earth
endures. It is doomed to more or less speedy destruction, the cause of
which it bears within itself, and from which nothing can save it."

The colonists looked at each other, then at the engineer. They did not
clearly comprehend him.

"Explain yourself, Cyrus!" said Gideon Spilett.

"I will do so," replied Cyrus Harding, "or rather I will simply
af