The settlers went to the Chimneys to take some pickaxes, iron-tipped spears, string made of fibers, flint and steel; they then returned to the plateau, Top accompanying them.

On the way the sailor could not help saying to the engineer,—­

“Don’t you think, captain, that by means of that charming liquid you have made, one could blow up the whole of our island?”

“Without any doubt, the island, continents, and the world itself,” replied the engineer.  “It is only a question of quantity.”

“Then could you not use this nitro-glycerine for loading firearms?” asked the sailor.

“No, Pencroft; for it is too explosive a substance.  But it would be easy to make some guncotton, or even ordinary powder, as we have azotic acid, saltpeter, sulphur, and coal.  Unhappily, it is the guns which we have not got.

“Oh, captain,” replied the sailor, “with a little determination—­”

Pencroft had erased the word “impossible” from the dictionary of Lincoln Island.

The settlers, having arrived at Prospect Heights, went immediately towards that point of the lake near which was the old opening now uncovered.  This outlet had now become practicable, since the water no longer rushed through it, and it would doubtless be easy to explore the interior.

In a few minutes the settlers had reached the lower point of the lake, and a glance showed them that the object had been attained.

In fact, in the side of the lake, and now above the surface of the water, appeared the long-looked-for opening.  A narrow ridge, left bare by the retreat of the water, allowed them to approach it.  This orifice was nearly twenty feet in width, but scarcely two in height.  It was like the mouth of a drain at the edge of the pavement, and therefore did not offer an easy passage to the settlers; but Neb and Pencroft, taking their pickaxes, soon made it of a suitable height.

The engineer then approached, and found that the sides of the opening, in its upper part at least, had not a slope of more than from thirty to thirty-five degrees.  It was therefore practicable, and, provided that the declivity did not increase, it would be easy to descend even to the level of the sea.  If then, as was probable, some vast cavity existed in the interior of the granite, it might, perhaps, be of great use.

“Well, captain, what are we stopping for?” asked the sailor, impatient to enter the narrow passage.  You see Top has got before us!”

“Very well,” replied the engineer.  “But we must see our way.  Neb, go and cut some resinous branches.”

Neb and Herbert ran to the edge of the lake, shaded with pines and other green trees, and soon returned with some branches, which they made into torches.  The torches were lighted with flint and steel, and Cyrus Harding leading, the settlers ventured into the dark passage, which the overplus of the lake had formerly filled.