"Oh! nothing but a narrow passage through which only one man at a time could squeeze, a hole easy enough to block, and the outside end of which would be hidden among the rocks." "Of what use could it be to us, Serko?" "I have often thought about the utility of having some other way of getting out besides the submarine tunnel. We never know what the future may have in store for us." "But the walls are so thick and hard," objected Ker Karraje. "Oh, with a few grains of Roch's explosive I undertake to reduce the rock to such fine powder that we shall be able to blow it away with our breath," Serko replied. It can easily be imagined with what interest and eagerness I listened to this. Here was a ray of hope. It. was proposed to open up communication with the outside by a tunnel in the wall, and this held out the possibility of escape. As this thought flashed through my mind, Ker Karraje said: "Very well, Serko, and if it becomes necessary some day to defend Back Cup and prevent any ship from approaching it----. It is true," he went on, without finishing the reflection, "our retreat would have to have been discovered by accident--or by denunciation." "We have nothing to fear either from accident or denunciation," affirmed Serko. "By one of our band, no, of course not, but by Simon Hart, perhaps." "Hart!" exclaimed Serko. "He would have to escape first and no one can escape from Back Cup. I am, by the bye, interested in this Hart. He is a colleague, after all, and I have always suspected that he knows more about Roch's invention than he pretends. I will get round him so that we shall soon be discussing physics, mechanics, and matters ballistic like a couple of friends." "No matter," replied the generous and sensible Count d'Artigas, "when we are in full possession of the secret we had better get rid of the fellow." "We have plenty of time to do that, Ker Karraje." "If God permits you to, you wretches," I muttered to myself, while my heart thumped against my ribs. And yet, without the intervention of Providence, what hope is there for me? The conversation then took another direction. "Now that we know the composition of the explosive, Serko," said Ker Karraje, "we must, at all cost, get that of the deflagrator from Thomas Roch." "Yes," replied Engineer Serko, "that is what I am trying to do. Unfortunately, however, Roch positively refuses to discuss it. Still he has already made a few drops of it with which those experiments were made, and he will furnish as with some more to blow a hole through the wall." "But what about our expeditions at sea?" queried Ker Karraje. "Patience! We shall end by getting Roch's thunderbolts entirely in our own hand, and then----" "Are you sure, Serko?" "Quite sure,--by paying the price, Ker Karraje."