After another careful survey, the boys owned, somewhat regretfully, that their leader was right. The mirage--if it was a mirage--was stationary, and localised to one part only. "It is no deceptive phenomenon," said Ned, decidedly, as he replaced his glass in its case. "That is a real lake, and those are substantial buildings, and bona fide trees, or I am a Boer, which is about the last kind of beast I'd like to be." "Then, I suppose, that means you intend to risk the desert?" answered Clarence. "Yes; if we can get down to it. I reckon we can cross that strip of sand in two or three days at most, and we have water enough to carry us over it." "But, supposing it is a real lake, that doesn't say it is a fresh-water one," replied Fred. "The buildings and trees are vouchers for that. Now let us examine this present hill. Before we do so, however, we'll climb one of those towers, and find out what is to be seen from there." Ned crossed the court, where his followers were still standing in square, and entered the doorway of the centre tower on the other wall. The stairs were still standing, and firm underfoot; therefore they were soon on the upper platform or roof. Here they could see down the pass which they had come, also two other approaches or chasms in the mountain. They had likewise glimpses of high bare centre peaks that reached above the lower ridges. It was a sterile prospect. Not a tree broke the hard outlines of the ruddy rocks. They lay baking and bald under the bright, hot rays of the tropical sun. A large flat-roofed building filled out the centre of this fortification. It was smaller at the top than the bottom, for the sides sloped outwards as they descended; yet a broad parapet surmounted the top for several feet, projecting from its sides. The roof was completely covered over, while the walls were pierced with windows and loopholes. On the farther side of this building they were able to see other walls and towers stretching along to a considerable distance. "This has been a stronghold to keep invaders from the inhabitants of yonder lake-watered land; and as far as I can see, the necessity for keeping it garrisoned has passed away. That heap of bones we saw must be the remains of some demolished race of invaders. I think we may venture with all safety inside this building. There does not seem to be even a jackal about." Saying these words, Ned led the way down, and, marshalling his force in double line, he advanced with revolvers in his hands towards the portal. The walls were of immense thickness, and made quite a long passage, through which they passed silently. Before them opened an apartment, so spacious that the entire company, with their luggage, only occupied an isolated portion of it. It had accommodation space for at least five thousand people.