“What is that apparatus?” asked Carnes as the doctor finished his set-up and surveyed it with satisfaction.
“Merely a thermocouple attached to a D’Arsonval galvanometer,” replied the doctor. “This large, squat telescope catches and concentrates on the thermocouple and the galvanometer registers the temperature.”
“You’re out of my depth. What is a thermocouple?”
“A juncture of two wires made of dissimilar metals, in this case of platinum and of platinum-iridium alloy. There is another similar junction in this case, which is kept at a constant temperature by the water bath. When the temperatures of the two junctions are the same, the system is in equilibrium. When they are at different temperatures, an electrical potential is set up, which causes a current to flow from one to the other through the galvanometer. The galvanometer consists of a magnet set up inside coils through which the current I spoke of flows. This current causes the magnet to rotate and by watching the mirror, the rotation can be detected and measured.
“This device is one of the most sensitive ever made, and is used to measure the radiation from distant stars. Currents as small as .000000000000000000000000001 ampere have been detected and measured. This particular instrument is not that sensitive to begin with, and has its sensitivity further reduced by having a high resistance in one of the leads.”
“What are you going to use it for?”
“I am going to try to locate somewhere in these hills a patch of local cold. It may not work, but I have hopes. If you will manipulate the telescope so as to search the hills around here, I will watch the galvanometer.”
For several minutes Carnes swung the telescope around. Twice Dr. Bird stopped him and decreased the sensitiveness of his instrument by introducing more resistance in the lines in order to keep the magnet from twisting clear around, due to the fluctuations in the heats received on account of the varying conditions of reflection. As Carnes swung the telescope again the magnet swung around sharply, nearly to a right angle to its former position.
“Stop!” cried the doctor. “Read your azimuth.”
Carnes read the compass bearing on the protractor attached to the frame which supported the telescope. Dr. Bird took a pair of binoculars and looked long and earnestly in the indicated direction. With a sigh he laid down the glasses.
“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 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.”
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!”