not leave without his mules and stores. Jules and Scott tookup the watch at the inn, while Yvette shadowed the suspect. It wasthought best that Simmons should not appear. It soon became evidentthat the man had no associates in Christiansand. All he did was tovisit shops, paying cash for all his purchases and having them sent tothe inn where his mules were stabled.
The next day, with his mules heavily loaded, he set out fromChristiansand, taking the road to Trygstand and Ostersluis.
Yvette, Jules, and Scott decided to follow him on foot. To have takenhorses would have told him he was being followed as soon as he left theroad, as they were pretty sure he would, sooner or later. Luckily allthree were splendid walkers and felt they would have no trouble inkeeping up with the heavily-laden mules. Cramming a few necessitiesinto rucksacks they were soon on the track of their quarry.
Man and mules made steady progress. They were soon through Trygstandand, shortly after, caught sight of the Mohawk high above them andevidently following the road on the watch for them.
With a handkerchief tied to a stick Yvette swiftly signalled to Dick thebrief facts, and the Mohawk passed on towards Christiansand. When thesailor and the mules were hidden in a dip in the road Dick landed, andall four held a brief consultation as to their future plans.
As a result Scott put on his best speed and soon passed the sailor whohad stopped for a rest. The man was now between two parties on theground and under observation from Dick from the air. He certainly couldnot escape.
A few miles beyond Trygstand he suddenly left the high road, and turnedwestward and north across the open country. Evidently he was not boundfor Ostersluis. But where could he be going? For miles there was noteven a house in the deserted track of country into which he had plunged.
But it was evident he knew his bearings thoroughly. Hour after hour hejogged along, and soon the pursuers realised that they had been wise notto bring horses. No horse could have crossed the country over which thesure-footed mules went swiftly without a stagger.
At nightfall the man camped. Apparently he paid no attention to thepassing of the aeroplane, for he barely glanced at it. Building a smallfire under the shelter of a rock, the three pursuers spent a comfortlessnight. Dick had flown to his camp, intending to pick the party up againat dawn.
Early next morning the man was afoot and continued his journey. He wasnow in the wild country well to the west of Ostersluis, and travellingdue north. Yvette, Jules, and Scott were a mile behind, following withthe utmost care not to reveal their presence and so rouse the man'ssuspicions.
They had gone but a few miles when the man paused on the flat top of ahigh hill, which on the side away from them sloped steeply into a deepgorge at the foot of which ran a small stream. They watched himnarrowly.
With great care he got the four mules together, standing side by side.He himself took up a position directly in front of them and almosttouching the animals' heads.
A moment later man and mules sank together, apparently into the earthand disappeared!
They could hardly believe their eyes! Surely the man must have gonedown the reverse slope of the hill. But they were confident that he hadnot moved.
They hurried to the spot. Not a sign of any living thing was to beseen! The mystery was profound.
While they stood gazing at one another in speechless amazement, theMohawk, which they had not perceived above them, dropped verticallydownwards and landed a few yards away. Dick sprang out.
"Did you see?" he gasped. "The man and mules went down into some sortof pit. But where was it?"
The flat top of the hill was broken into a series of narrow cracks;apparently the rock of which it was composed was of volcanic origin.They examined it closely, but they could discover nothing which offereda solution of the mystery.
Dick described closely what he had seen from the sky. It agreed withwhat the others had observed. The man had got the mules together, andall had sunk slowly downward. Dick had seen the black mouth of the pitfor a few moments and a blaze of light. Then the pit had disappeared,and the ground resumed its normal appearance.
"We shall have to camp here to-night," said Dick. "We must get to thebottom of this. We shall have to take turns to watch. In the meantimewe had better have a look round."
Having closely examined the top of the hill, they turned to the deepgorge and descended to the bottom. The stream, they found, issued fromthe hill itself, flowing out from a low tunnel high enough to admit thepassage of a man. From it also issued a cloud of mist which spread overthe bottom of the little valley in a thick blanket which completelyconcealed the surface of the ground from anyone at the top of the hill.
But still more remarkable was that the bed of the little stream wasdeeply covered with what appeared to be recently melted lava. In manyplaces it was still hot, and the water, they found, was nearly boiling.The first traces of this were found at the mouth of the tunnel fromwhich the stream emerged, and for hundreds of yards the molten rockcould be traced, as though it had poured from the tunnel and flowed downthe bed of the brook.
Wood and water were available in abundance, and soon they had pitchedtheir camp, near enough to the top of the mysterious hill to enable themto watch it closely and yet well concealed so that if the man reappearedthey would have no difficulty in escaping observation.
The first watch fell to Yvette, and with a revolver ready for instantuse, she prepared to spend a couple of lonely hours on the edge of thehill. The camp was but a quarter of a mile away so that a shot wouldbring her speedy help at any time.
A brilliant moon lit up the country for miles.
There was no trace of any living thing. Everything was still andsilent.
Yvette had been on watch about an hour when she became aware that theair was full of a dull murmur of sound. She listened intently. Therewas no mistake about it. A dull throbbing noise was distinctlydiscernible.
She walked round the flat top of the hill, looking keenly in everydirection and trying to locate the position from which the mysterioussound was coming. But it was in vain.
Glancing into the gorge, she saw a strange and terrible phenomenon. Thecourse of the little brook was traced in a dull fiery glow. Clouds ofsteam were rising thickly into the night air; she could plainly hear thesharp hiss of water on something hot.
She ran swiftly down the hill. At the bottom she paused on the edge ofthe stream. The water had disappeared and in its place ran a river ofmolten rock! Through her boots she felt the heat of the ground.
Returning to the top of the hill she waited for Dick, who was now almostdue to relieve her. In a few moments he appeared and listened inamazement as she gasped out her story.
The dull, throbbing noise was still audible.
"Machinery," said Dick laconically, "but where?"
Suddenly he flung himself on his face, and pressed his ear close to theground.
"Listen," he said.
Yvette followed his example. There could be no mistake; the mysterioussound was coming from the ground beneath their feet! The earth was fullof muffled thunder.
Dick took from his pocket a hammer and struck a sharp blow on the flatrock beneath their feet. It rang hollow! Unmistakably they werestanding on the roof of a cavern.
Walking to the camp they roused the others and told them what they hadseen and heard.
"We have got to catch that sailor if we wait here a month," said Scott."He must come out again some time. But how about food?"
"We have enough tinned stuff in the Mohawk for a week," said Dick, "sowe shall be all right for a few days. In the meantime we must watch theplace closely."
Next day passed without incident until evening was drawing on. ThenYvette, who was watching the top of the hill while the others rested, atsix o'clock gave a low whistle. She was lying on the ground keepingobservation between a couple of rocks which hid her completely. In amoment the others had crawled to her side.
"Look!" she said.
On the
top of the hill, three hundred yards away, stood the sailor andthe four mules, clearly silhouetted against the evening glow. He hadappeared suddenly, Yvette told them, just on the spot where he haddisappeared on the previous day.
"We must get him," said Dick.
The man with the mules started to return along the way he had come.They saw at once that the path he was taking would bring him close tothem.
With the mules unloaded the man evidently had no intention of walking.He mounted one of the animals and rode towards them at a fast trot.
He was within twenty yards when Dick aimed his revolver and fired. Themule the man was riding bolted, throwing its rider heavily. Before hecould recover himself he was bound and helpless. The other three mulesstampeded wildly and were soon out of sight.
Carried to the camp the man soon recovered. But he resolutely refusedto say a word.
"Well," said Dick. "We must try to get into the cave. Perhaps thetunnel out of which the brook runs will lead us to it."
They were soon at the mouth of the strange tunnel. There was no sign ofthe molten matter of the previous night. The stream, thick with mud,flowed sluggishly, but the water was cool, and the ground, which thenight before had been too hot to walk upon, was now not more thanuncomfortably warm.
With Dick leading, Scott and Yvette next in order, and Jules bringing upthe rear they entered the mouth of the tunnel. There was, they found,just room for them to pass, stooping low and walking knee deep in thelittle stream. They were, of course, in total darkness, for Dick wasafraid to show a light for fear of betraying their presence.
For a hundred yards Dick groped his way onward. Then his outstretchedhands struck something soft. It was a kind of curtain hung across thestream, thick and heavy.
Cautiously he slightly raised one corner and peered through. The sightthat struck his eyes filled them with amazement.
They were at the entrance to an enormous chamber, a hundred and fiftyyards across, dimly lighted by a single big electric lamp, the only onealight out of dozens which hung from the roof. The floor sloped steeplyupwards at the far end where they could make out a kind of platform,reaching nearly to the roof and with steps leading downward into thegreat hall. All round the side were a series of openings, apparentlysmall chambers cut into the solid rock. From one of these the streamthey had followed seemed to issue, crossing the floor of the great cavein a narrow deep channel.
But what fascinated Dick's attention was a great table, apparently ofiron, which occupied the centre of the cave. It was heavily constructedand seemed to be based on massive legs which went down into the rock.Upon it stood a strange machine unlike anything he had ever seen beforeand of the use of which he could not form the smallest idea. Surmountedby two huge governor balls, it was a complicated mass of polishedwheels, of some metal which Dick could not identify, and which gleamedwith a strange radiance in the light of the huge electric lamp overhead.From the machine a bewildering mass of wires led to a series of pointsat the face of the rock.
So much Dick could make out in the dim light. He was keenly anxious tolearn more. But how was it to be done? No sign of any human being wasto be seen, but he could not imagine that what lay before their eyes wasthe work of the solitary sailor who now lay bound in their camp.
At any rate they could not remain where they were. Dick decided to tryto gain entrance to one of the wall chambers where they could shelterwith a better chance of seeing what would happen in this undergroundhome of mystery. But which should they choose?
Some of the chambers were half-way to the roof and were reached by stepscut in the solid rock. Dick decided on one of these not far from wherethey were standing. They crept cautiously from their hiding-place andstole along to the bottom of the cave. A moment later they were at thefoot of the steps. These they hastily climbed, and soon foundthemselves in a fair-sized cave, fifteen or sixteen feet above the floorof the main cavern and commanding a good view of the entire area. Itwas dry and warm and formed an ideal post of observation, provided theirpresence remained undiscovered.
Suddenly a blaze of light struck their eyes. Some one had turned on thewhole of the electric lamps which hung in clusters from the roof.
Peering cautiously out they saw, to their amazement, half a dozen menissue from different chambers near the floor of the cave. All wore biground spectacles of deep blue glass and were clothed in close-fittinggarments of rubber, with heavy gauntletted gloves of the same material.Apparently they could not see well, for the spectacles must have beenalmost impervious to ordinary light.
One of the men, fixing his spectacles on more firmly and, drawing hisrubber overall more closely around him, approached the strange machinewhich stood on the table. The others proceeded to the points at whichthe wires from the machine reached the side of the cave. Here they tookup some kind of tool which looked like a gigantic blowpipe and stoodready as if awaiting a signal.
A low whistle sounded from the man at the table, as he grasped a smallwheel and gave it a quick turn.
An instant later an appalling blaze of light burst from the strangemachine, and the cave was filled with a roar of sound, a terrible deepdrone of such frightful intensity that the hidden watchers shuddered asif with actual physical agony. Dick felt the sweat start suddenly fromhis forehead and pour down his face. Anxiously he glanced towardsYvette. She lay with her face buried in her arms, her body tremblingconvulsively. Scott and Jules, their faces white as chalk, were gazingat the unearthly light which streamed from the whirling machine, shadingtheir eyes with their hands to shelter them from its blinding radiance.They could not look at it for more than a few seconds; it was liketrying to gaze at the sun at midday.
Taking a letter from his pocket, Dick bored a tiny hole in it with hisscarf pin. Through this hole he found he could see in comparativecomfort. He signed to the others to do the same, and soon all four--forYvette quickly recovered her self-possession--were eagerly watching thestrange scene before them. Speech, in the deafening noise by which theywere surrounded, was, of course, out of the question.
The man at the great table in the centre of the cavern evidently had atask of great difficulty to control the movements of the strangemachine, which he seemed to do by means of a large wheel something likethe steering wheel of a steamer. Long streamers of flame shot from itin all directions, and as its mass of wheels revolved at terrific speedit shook and trembled as if it would actually leap from the table.
In the meantime the men at the rock face were hard at work with bigblowpipes, from the muzzles of which shot streams of fire of suchintensity that the solid rock seemed to melt away like butter. Themolten matter was led by ducts in the ground through a grid of somemetal, evidently highly refractory to heat, for it appeared to do nomore than glow white-hot even in the terrific temperature of the meltedrock. After passing through this grid the molten matter was led to thebed of the stream, from which the water had in some manner been cut off,and flowed out the way Dick and his companions had entered.
What was the object of the work?
Dick could not guess, but every now and again one of the men would walkto the grid and with a long implement shaped like a hoe would scrape offsomething adhering to the bars, which he deposited in a big tank ofwater. Dick determined that, sooner or later, he would obtain aspecimen.
But in the meantime their position was decidedly precarious. If theywere observed there was no possible way of escape, for the tunnel bywhich they had entered was barred by the stream of molten matter. Theycould only lie still and hope that no one would enter the gallery inwhich they lay concealed.
After two hours of work, the man at the table stopped the machine, andall the men straightened out for a rest. Evidently they were very muchexhausted. The lights were extinguished, except for the single onewhich was burning when they entered, and the men returned to theirquarters, evidently almost falling with weariness. Dick came to theconclusion that they could only carry on the work on which they wereengag
ed for a short time and that after that sleep and rest wereimperative. The flow of molten metal had stopped and the water wasagain allowed to flow along its ordinary channel, from whence it sent uphuge clouds of dense steam.
This gave Dick his chance.
Sending the others to the mouth of the exit, he cautiously crept towardsthe tank in which were deposited the scrapings from the grid whichfiltered the molten rock. He reached it safely, and plunging in his armup to the shoulder, abstracted a couple of handfuls of what seemed likeheavy shot. These he placed at once in his pocket.
He was about to return to the others when his attention was caught bythe queer platform at the one end of the cave. Looking at thiscarefully he found that it was really a huge lift, and at once themysterious disappearance of the sailor and the mules was explained. Itwas evident that the top of the lift was really the thin covering ofrock which had sounded hollow when tapped and that this had been so cutthat when the lift forced it into position only traces of raggedcrevices were left on the surface. Dick could not but admire theingenuity with which this approach to the subterranean retreat had beendevised.
Presently he heard a heavy knocking above his head and, guessing thecause, shrank back for shelter into the mouth of a small cave adjoining.A moment later a man emerged from one of the other chambers andapproached the lift. Dick was curious to see how it worked. There was,as he could see, a small electric motor fitted to it, but where couldthe necessary power come from?
The new-comer carried in his hand a tiny machine which was in everyrespect a duplicate in miniature of the big one on the central table.But it was so small that the man carried it easily in one hand. From itran a pair of electric cables which the man proceeded to connect withthe terminals of the motor.
Placing the machine on the ground he gave the wheel a sharp turn.Immediately the tiny machine began to revolve, throwing out flashes andflames exactly like the larger one but on a miniature scale.
Clearly, however, there was considerable power in it, for the lift atonce