"Ay," answered Jack with a laugh; "I remember that ill-fated model. No, I never recovered her, but she nevertheless served her purpose; for her mishap gave me my first really useful idea in connection with the design of a submarine boat. And at last I have completed a working model which thus far has answered exceedingly well. She is only a small affair, you know, five feet in diameter by twenty-five feet long, but she is big enough to accommodate two men--or even three, at a pinch. I have been as deep as ten fathoms in her, and have no doubt she could descend to twice that depth; while she has an underwater speed of twenty knots, which she can maintain for five hours." "By Jove, that's splendid--very much better than anything that anyone else has done, thus far!" exclaimed Montijo admiringly. "You ought to make your fortune with a boat of that sort. And you are pining for an opportunity to subject her to a thoroughly practical test? Well, my scheme, which I will explain in full when we get into the Park, will enable you to do that." "Is that so?" commented Jack. "Then that alone would very strongly predispose me in favour of it. But why make such a secret of it, old chap? Is it of such a character that a passer-by, catching a few words of it, would be likely to hand us over to the nearest policeman as a couple of conspirators?" "Well, no; it is scarcely so bad as that," answered Montijo, laughing: "but it is of such a nature that I would prefer not to speak of it, if you don't mind, until we are somewhere in the Park where we can converse freely without the fear of being overheard. You see, the Pater and I are pretty well-known to--and not too well liked by--the Spanish authorities in Cuba, and it is by no means certain that they may not think it quite worth their while to have us watched over here; therefore--" "Yes, of course, I understand," returned Jack; "therefore for the present `mum's the word', eh?" Montijo nodded, and the two lads strode along, conversing upon various topics, until they reached Hyde Park Corner, and swung in through the Park gates, and so on to the grass. "Ah, now at last I can speak freely!" remarked Montijo with a sigh of relief. "First of all, Singleton," he continued, "you must understand that what I am about to say will be spoken in the strictest confidence; and, whether you should agree to my proposal or not, I must ask you to pledge your honour as a gentleman that you will not repeat a single word of what I say to anyone--anyone, mind you--without first obtaining my consent, or that of my Pater." "All right, Carlos, my boy," answered Singleton, cheerily; "I promise and vow all that you ask. There is nobody on the face of this earth of ours who can keep a secret better than I can, as you ought to know by this time."