"I can't see a thing, Carnesy," he said. "We'll have to move over to the next crest and make a new set-up. Plant a rod on the hill so that we can get an azimuth bearing and get the airline distance with a range finder."

On the hilltop which Dr. Bird had pointed out the apparatus was again set up. For several minutes Carnes swept the hills before an exclamation from the doctor told him to pause. He read the new azimuth, and the doctor laid off the two readings on a sheet of paper with a protractor and made a few calculations.

"I don't know," he said reflectively when he had finished his computations. "This darned instrument is still so sensitive that you may have merely focused on a deep shadow or a cold spring or something of that sort, but the magnet kicked clear around and it may mean that we have located what we are looking for. It should be about two miles away and almost due west of here."

"There is no spring that I know of, Doc, and I think I know of every water hole in this country," remarked Bill.

"There could hardly be a spring at this elevation, anyway," replied the doctor. "Maybe it is what we are seeking. We'll start out in that direction, anyway. Bill, you had better take the lead, for you know the country. Spread out a little so that we won't be too bunched if anything happens."

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 s.p.a.ce before it was a ma.s.sive 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!" e.j.a.c.u.l.a.t.ed 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."

Pistols in hand, Carnes and the doctor crawled over the crest and joined the advance. Again and again the rifle spoke from the hollow and was answered by the vicious barks of the rifles in the hands of the guides, Carnes and the doctor resting their pistols on rocks and sending an occasional bullet toward the loophole. The conditions of light and the moving target were not conducive to good marksmanship on the part of the besieged man, and none of the attackers were hit. Presently Walter succeeded in sending a bullet through the loophole. The rifle barrel suddenly disappeared. With a shout the four men rose from their cover and advanced toward the building at a run.

As they did so an ominous whirring sound came from the apparatus in front of the house and a sudden chill filled the air.

"Back!" shouted Dr. Bird. "Back below the hill if you value your lives!"

He turned and raced at full speed toward the sheltering crest of the hill, the others following him closely. The whirring sound continued, and the concave reflector turned with a grating sound on its gears. As the path of its rays struck the ground the rocks became white with frost and one rock split with a sharp report, one fragment rolling down the slope, carrying others in its trail.

With panic-stricken faces the four men raced toward the sheltering crest, but remorselessly the reflector swung around in their direction.

The intense cold numbed the racing men, cutting off their breath and impeding their efforts for speed.

"Stop!" cried the doctor suddenly. "Fire at that reflector! It's our only chance!"

He set the example by turning and emptying his pistol futilely at the turning mirror. Bill, Walter and Carnes followed his example. Nearer and nearer to them came the deadly ray. Bill was the nearest to its path, and he suddenly stiffened and fell forward, his useless gun still grasped in his hands. As his body struck the ground it rolled down hill for a few feet, the deadly ray following it. His head struck a rock, and Carnes gave a cry of horror as it broke into fragments.

Walter threw his rifle to his shoulder and fired again and again at the rotating disc. The cold had became intense and he could not control the actions of his muscles and his rifle wavered about. He threw himself flat on the ground, and, with an almost superhuman effort, steadied himself for a moment and fired. His aim was true, and with a terrific crash the reflector split into a thousand fragments. Dr. Bird staggered to his feet.

"It's out of order for a moment!" he cried. "To the house while we can!"

As swiftly as his numbed feet would allow him, he stumbled toward the house. The muzzle of the rifle again projected from the loophole and with its crack the doctor staggered for a moment and then fell. Walter's rifle spoke again and the rifle disappeared through the loophole with a spasmodic jerk. Carnes stumbled over the doctor.

"Are you hit badly?" he gasped through chattering teeth.

"I'm not hit at all," muttered the doctor. "I stumbled and fell just as he fired. Look out! He's going to shoot again!"

The rifle barrel came slowly into view through the loophole. Walter fired, but his bullet went wild. Carnes threw himself behind a rock for protection.

The rifle swung in Walter's direction and paused. As it did so, from the house came a strangled cry and a sound as of a blow. The rifle barrel disappeared, and the sounds of a struggle came from the building.

"Come on!" cried Carnes as he rose to his feet, and made his stumbling way forward, the others following at the best speed which their numbed limbs would allow.

As they reached the door they were aware of a struggle which was going on inside. With an oath the doctor threw his ma.s.sive frame against the door. It creaked, but the solid oak of which it was composed was proof against the attack, and he drew back for another onslaught. From the house came a pistol shot, followed by a despairing cry and a guttural shout. Reinforced by Carnes, the doctor threw his weight against the door again. With a rending crash it gave, and they fell sprawling into the cabin. The doctor was the first one on his feet.

"Who are you?" asked a voice from one corner. The doctor whirled like a flash and covered the speaker with his pistol.

"Put them up!" he said tersely.

"I am unarmed," the voice replied. "Who are you?"

"We're from the United States Secret Service," replied Carnes who had gained his feet. "The game is up for you, and you'd better realize it."

"Secret Service! Thank G.o.d!" cried the voice. "Get Koskoff--he has the plans. He has gone out through the tunnel!"

"Where is it?" demanded Carnes.

"The entrance is that iron plate on the floor."

Carnes and the doctor jumped at the plate and tried to lift it, without result. There was no handle or projection on which they could take hold.

"Not that way," cried the voice. "That cover is fastened on the inside.

Go outside the building; he'll come out about two hundred yards north.

Shoot him as he appears or he'll get away."

The three men nearly tumbled over each other to get through the doorway into the bitter cold outside. As they emerged from the cabin the gaze of the guide swept the surrounding hills.

"There he goes!" he cried.

"Get him!" said Carnes sharply.