the northwest, and we followed their trail for about forty rods and then lost it. We weren’t able to pick it up again.”
“Thanks, Bill,” replied the doctor. “Well, Carnes, that seems to add more weight to the theory that the spot of cold was made and didn’t just happen. If a prospecting party had just happened along they would either have left the wreck alone or would have made some attempt to inter the bodies. That cold belt must have been produced artificially by men who planned to rob this plane after bringing it down and who were near at hand to get their plunder. Is there any chance of following that trail?”
“I doubt it, Doc. Walter and I scouted around quite a little, but we couldn’t pick it up again.”
“Is there any power line passing within twenty miles of here?”
“None that Walter and I know of, Doc.”
“Funny! Such a device as must have been used would need power and lots of it for operation. Well, I’ll try my luck. Carnes, help me unpack and set up the rest of my apparatus.”
With the aid of the operative, Dr. Bird unpacked two of the burros and extracted from cases where they were carefully packed and padded some elaborate electrical and optical apparatus. The first was a short telescope of large diameter which he mounted on a base in such a manner that it could be elevated or depressed and rotated in any direction. At the focal point of the telescope was fastened a small knot of wire from which one lead ran to the main piece of apparatus, which he sat on a flat rock. The other lead from the wire knot ran into a sealed container surrounded by a water bath under which a spirit lamp burned. From the container another lead led to the main apparatus. This main piece consisted of a series of wire coils mounted on a frame and attached to the two leads. The doctor took from a padded case a tiny magnet suspended on a piece of wire of exceedingly small diameter which he fastened in place inside the coils. Cemented to the magnet was a tiny mirror.
“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
