"Why, you were lying with your back toward the direction it came from," explained Pete. "An interesting optical delusion," declared the professor. "I must make a note of it, and----" "Wow! There it goes ag'in." "Where? Where?" chorused the boys. "Right off there! Look! Look!" The lanky cow-puncher, fairly dancing about with excitement, pointed out beyond the shadow of the solitary mesa. Sure enough, there were three or four enormous, black, shadowy shapes, traveling across the sands at a seemingly great speed. "Get your rifles, boys!" yelled Jack. The weapons lay handy, and in a jiffy four beads had been drawn on the immense, vague shapes. But even as their fingers pressed the triggers, and the four reports rang out as one, the indefinite forms vanished as mysteriously as they had appeared. CHAPTER IV. SOME QUEER TRACKS. The hour, the surroundings, and the utter mystery of the whole affair combined to make the sudden appearance and vanishment of the great shadowy shapes the more inexplicable, not to say alarming. Small wonder was it that the inquiring faces that turned toward each other were a trifle whiter than usual. "What do you make of it, Pete?" asked Jack. "Stumped, by the big horn spoon!" was the expressive response. "No doubt, some natural phenomena, with a simple explanation," came from the professor. It was noted, though, that his angular form seemed to be somewhat shivery as he spoke, and that his teeth chattered like dice rattling in a box. "Natural phe-nothings!" burst out Pete. "The things, whatever they was, were as solid as you or me." "How was it they didn't make any noise, then?" inquired Ralph, practically. "Waal, son, you jes' take a run on the bunchgrass, and you'll see that you won't make no racket, nuther." Ralph did as he was directed, and it was really wonderful how silently he sped over the springy vegetation. "Maybe it was somebody putting up a scare on us," suggested Walt, rather lamely. "They couldn't rig up anything as big as that," said Jack decisively, "besides, there's another thing--I didn't tell you because I thought I might have been mistaken, but I saw those same things this afternoon." "What?" went up in a perfect roar of incredulity. "Say, is this some kind of a josh?" asked Coyote Pete suspiciously. "Never more serious in my life," Jack assured him, and then went on to relate the strange experience that had befallen him when he had poked his head out from under the saddle in the sand-storm. "If they weren't so enormous, I should say they was horses," said Pete; "but the biggest horses that ever growed never even approached them critters--spooks, er whatever they are." "There are giants among men," suggested Walt, "why shouldn't there be giants among spooks, too?"