They found that they were on the north-eastern extremity of the island of Baru, with the whole of the harbour of Cartagena before them, the roofs and spires of the town just showing waveringly, in a sort of mirage, over the low land which forms the easternmost extremity of the island of Tierra Bomba. It is this same island of Tierra Bomba, by the way, which converts what would otherwise be an open roadstead into a landlocked harbour, for it forms the western side of the harbour, and serves as a natural breakwater, sheltering the roadstead very effectually when the wind happens to blow from the westward. Also, being roughly triangular in shape, its eastern and western sides each measuring about four miles long, and its northern side about three miles, it divides the entire harbour into two parts, namely, the upper and the lower bay. The upper bay in its turn is divided into the inner and the outer harbour by two irregularly shaped spits of low land, the western spit jutting out south and east in a sort of elbow from the promontory on which the city is built, while the eastern spit is divided from the mainland by a narrow channel, and is called Manzanillo Island. The foregoing is a brief and rough description of Cartagena harbour, given for the information of the reader and to enable him the better to understand what follows; but comparatively few of the above details were apparent to the two Englishmen lurking among the mangroves on the north-eastern extremity of Baru, for the island of Tierra Bomba, the most prominent object in sight, shut out much of the upper bay. They obtained, however, a good view of the Boca Chica, or harbour entrance, and took careful note of the fact that it was effectually commanded, at its narrowest and most difficult point, by a battery built on the very beach itself, and a fort, or castle, crowning the crest of a hill immediately above. They both agreed that if this was the only entrance to the harbour, and if the garrisons of those forts maintained a proper lookout, it should be quite impossible for a ship to enter or leave Cartagena harbour, except with the full permission of the authorities. "Well," exclaimed Marshall at last, when they had both familiarised themselves with everything that there was to see from their viewpoint, "this is all very well, and we have already learned quite enough to repay us for all our trouble in taking this trip. But I have not yet seen nearly all that I want to see; therefore, by hook or by crook, I must get ashore upon that island yonder"--pointing to Tierra Bomba. "That hill at its north-eastern angle ought to command a view of the whole harbour and town, and I must get up there. Now, how is it best to be done?"