"Um--ah--well," began Pete, somewhat stumped by this last, "I don't see what that's got to do with it." "But I do. Mexicans, my friend, are, as you should know, a cunning race. Moreover, those of them who dwell along it know the border far better than any white could ever hope to. By the admission of our own secret agents, it has hitherto been impossible to find how the arms, which the Chihuahua rebels are receiving, can reach them. It is obvious, however, that there must be some way in which they do, hence----" "Waal, perfusser, hev it your own way," grunted Pete, rather red and angry. The professor's logic did indeed seem unassailable. The rebels of Northern Chihuahua were getting arms--but how? The cow-puncher and the boys recalled now a visit made to Mr. Merrill's ranch some weeks before by a party of United States secret agents. The men were puzzled and angry over their failure to locate the "leak." Somehow arms were being shipped across the border into Chihuahua from American soil, but just how had hitherto baffled all the efforts of their ingenuity to discover. "There, there, don't be so easily offended," counseled the professor, perceiving Pete's palpable irritation. "After all, the matter has nothing to do with us. We are here to measure the mesa for scientific purposes, not to get into arguments over how a band of insurrectos are getting their arms. Come, boys, to work. Let us begin at the top, by measuring the altar. Suppose, Jack, you lay the tape on it, while I make a rough field sketch of the structure." The boys, now over their first repulsion to having anything to do with the altar, about which such grisly memories clustered, eagerly began to carry out these orders, while Coyote Pete seated himself on the side of the summit overlooking the travelers' camp below, and amused himself by throwing small bits of detached rock down at the unoffending One Spot, Two Spot and Three Spot. The base of the altar being duly measured and recorded, Jack, tape in hand, followed by the others, clambered up its rough sides, which afforded an easy foothold, for the purpose of ascertaining the dimensions of the top. To the lad's astonishment, however, there was no summit. That is to say, the altar was hollow. The professor exhibited considerable scientific excitement on hearing this. The man of science had been greatly puzzled over the total absence of any traces of the human sacrifices he knew must have taken place there. He now hailed Jack eagerly. "Are there not some bones or traces of sacrifices inside it, my boy?" he inquired excitedly. "Nary a bone," shouted Walter cheerfully. "Hold on, though," cried Jack. "There are some queer-looking things down in one corner." Lowering himself inside the altar, he made for one corner of the erection, in which he had spied a heap of fragile-looking bones of some kind.