The effect of the explosion, which was superior to any previously obtained by new explosives, was instantaneous. Even at the distance they were from the spot where it occurred, the four remaining ships felt the shock severely.
Two things were to be deduced from this sudden catastrophe:
1.--The pirate Ker Karraje was in possession of Roch's fulgurator.
2.--The new engine possessed the destructive power attributed to it by its inventor.
After the disappearance of the unfortunate cruiser, the other vessels lowered boats to pick up a few survivors who were clinging to the floating wreckage.
Then it was that the signals were exchanged and the warships started towards the island.
The swiftest of them, the _Tonnant_, a French cruiser, forged ahead while the others forced their draught in an effort to catch up with her.
The _Tonnant_, at the risk of being blown to pieces in turn, penetrated the danger zone half a mile, and then ran up her flag while manoeuvring to bring her heavy guns into action.
From the bridge the officers could see Ker Karraje's band scattered on the rocks of the island.
The occasion was an excellent one for getting a shot at them before the bombardment of their retreat was begun, and fire was opened with the result that the pirates made a rush to get into the cavern.
A few minutes later there was a shock terrific enough to shake the sky down.
Where the mountain had been, naught but a heap of smoking, crumbling rocks was to be seen. Back Cup had become a group of jagged reefs against which the sea, that had been thrown back like a gigantic tidal wave, was beating and frothing.
Had it been voluntarily caused by the pirates when they realized that escape was impossible?
The _Tonnant_ had not been seriously damaged by the flying rocks. Her boats were lowered and made towards all that was left of Back Cup.
The landing parties explored the ruins, and found a few horribly mangled corpses. Not a vestige of the cavern was to be seen.
One body, and one only, was found intact. It was lying on the northeast side of the reefs. In one hand, tightly clasped, was a note-book, the last line of which was incomplete.
A close examination showed that the man was still breathing. He was conveyed on board the _Tonnant_, where it was learned from the note-book that he was Simon Hart.