Besides, who could say what might not be going on up there a little further, in those holes in the wall where, ages ago, the singular people whom Professor Oswald loved to study about, had their homes, and lived on from year to year? Old Hank, when he once more started upward, seemed to have become much more cautious. Frank could easily guess the reason. There was a strong possibility that the three schemers might have learned of their presence in the vicinity ere now. And of course Eugene knew full well why Frank and Bob had come to the Grand Canyon from their ranch home. Suspecting that sooner or later the two boys might discover the way up to the cliff house, they would be apt to lay a trap of some sort, thinking to catch them napping when they ascended. Old Hank could not be taken unawares any easier than might the wary weasel that has never been seen asleep by mortal eyes. Frank, keeping well up by the heels of the little cowboy's boots, was ready to draw himself upward at the first sign of trouble. He knew when Hank had reached the top of the singular stairway fashioned by Nature for the benefit of those who built their habitations near the top of the cliff, far beyond the reach of enemies in the valley below. A few seconds of suspense followed, while Chesty was following the veteran into the first hollowed-out apartment. Nothing followed where Frank had been expecting all manner of evil things. "Perhaps they're asleep," was the new thought that flashed through his brain. He did not know what manner of man Uncle Felix was. Now they were all gathered there in that outer chamber that might be called an ante-room of the various apartments running along the face of the cliff for some distance. Even Charley Moi was there, full of curiosity, and willing to lend a hand after a fashion. Bob looked around; just as his chum had done as soon as he entered. He saw that some one had certainly been there recently. There were plenty of evidences to that effect. Old Hank raised his hand with the forefinger elevated. It was recognized as a signal for absolute silence by all the others. Even Bob restrained his desire to ask questions; and every one listened, as if expecting to catch sounds. Was that a human voice? Frank started a trifle as the idea came to him. Still, it might only have been an additionally strong movement of the breeze; turning some angle that caused it to give forth a sound. He turned to see if any of the others had heard, and judged from the way old Hank had his head raised that he, too, had caught the sound; also that it appealed to him as full of significance. Again the veteran waved his hand. This time it meant not only caution, but an invitation to advance. Hank was about to pass into the next apartment, and wished the others to keep close at his heels.