For three-quarters of an hour the little group of men made their way through the wilderness in the direction indicated by the doctor. Presently Bill, who was in the lead, held up his hand with a warning gesture. The other three closed up as rapidly as cautious progress would allow. "What is it, Bill?" asked the doctor in an undertone. "Slip up ahead and look over that crest." The doctor obeyed instructions. As he glanced over he gave vent to a low whistle of surprise and motioned for Carnes to join him. The operative crawled up and glanced over the crest. In a hollow before them was a crude one-storied house, and erected on an open space before it was a massive piece of apparatus. It consisted of a number of huge metallic cylinders, from which lines ran to a silvery concave mirror mounted on an elaborate frame which would allow it to be rotated so as to point in any direction. "What is it?" whispered Carnes. "Some kind of a projector," muttered the doctor. "I never saw one quite like it, but it is meant to project something. I can't make out the curve of that mirror. It isn't a parabola and it isn't an ellipse. It must be a high degree subcatenary or else built on a transcendental function." He raised himself to get a clearer view, and as he did so a puff of smoke came from the house, to be followed in a moment by a sharp crack as a bullet flattened itself a few inches from his head. The doctor tumbled back over the crest out of sight of the house. Bill and Walter hurried forward, their rifles held ready for action. "Get out on the flanks, men," directed the doctor. "The man we want is in a house in that hollow. He's armed, and he means business." * * * * * Bill and Walter crawled under the shelter of the rocks to a short distance away and then, rifles ready, advanced to the attack. A report came from the hollow and a bullet whined over Bill's head. Almost instantly a crack came from Walter's rifle and splinters flew from the building in the hollow a few inches from a loophole, through which projected the barrel of a rifle. The rifle barrel swung rapidly in a circle and barked in Walter's direction; but as it did so, Bill's gun spoke and again splinters flew from the building. "Good work!" ejaculated Dr. Bird as he watched the slow advance of the two guides. "If we just had rifles we could join in the party, but it's a little far for effective pistol work. Let's go ahead, and we may get close enough to do a little shooting."