Extending out from the brow of the precipice, and resting upon the tops of the conduits where they plunged downward, was a narrow bridge of iron lattice-work which connected all six of the pipes and gave access to the bolts which tightened the steel elbows. Through holes in this grating, iron ladders fastened between the pipes and the granite cliff back of them descended clear to the bottom of the precipice.
A slight rail only three feet high protected the outer edge of this grid—a little hand-hold for the workmen in case of a misstep. From this dizzy balcony it would be possible to drop a stone almost upon the roof of the powerhouse.
After a quick look around, Ensign Hallock chose a spot a little back from the cliff to set up the mortars that were to throw explosives upon the building. He also prepared to place mines under the conduits. But first the machine guns were planted to command the surrounding timber, in case of an attack.
There still was no indication that the sorcerers suspected our presence in their vicinity; so, inasmuch as Hallock said his preparations would take some little time, Dr. Gresham determined to employ the interval in getting a closer look at the power plant.
One of the ladders down the precipice, he had noticed, was in such a position behind its water main that it could not be seen from the building; and he decided to attempt the approach by this means. To my delight, he made no objection to my accompanying him.
As we slipped through an opening in the iron bridge and started our dizzy descent of the ladder—which seemed to sway beneath our weight—I felt a thrill of exultation, in spite of our peril, at the thought that at last we were to solve the mystery of the Seuen-H’sin’s terrible power over our planet!
The trip was slow and risky, but finally we came abreast of a window in the rear wall of the building, and by stretching around the side of the thick water main we could see into the place.
The workshop of the sorcerers was a long, low, narrow structure directly beside the river. Like the houses back in the Chinese village, it was a mere shell of corrugated iron, its steel framework so bolted together that it could sway with the earth tremors.
In a row down the centre of the structure were six huge turbines, operating electric generators.
Along one side of the room was the largest switchboard I had ever seen, while the whole of the other lengthwise wall was flanked with a series of massive induction coils, elaborately insulated from each other and from the ground. Although I knew little about electricity, I was certain that if the combined electrical output of those dynamos were directed through that maze of coils, the resulting voltage could only be measured in the millions—perhaps hundreds of millions!
From one large, enclosed object, supported on steel uprights over the row of induction coils, two electric cables, more than two inches in diameter, ran off through the north end of the building. One of these ended in a tiny structure about eighty yards from the powerhouse. The other ran on up the valley.
But, most curious of all, in the center of the switchboards was an apparatus surmounted by a large clock, before which a Chinese attendant sat constantly. Precisely every eleven minutes and six seconds a bell on this clock clanged sharply, and there was a bright flash in a long glass tube, followed by an earth shock.
For some time we clung there in the shadows, while Dr. Gresham studied every detail of the amazing workshop. Then, calling my attention to the fact that the place outside the powerhouse, where one of the cables ended, was hidden from view of the attendants inside by a thick clump of trees, the astronomer said he wanted a closer look at this place.
Creeping through the timber, we reached the tiny structure over the cable’s end. Not the slightest watch seemed to be kept anywhere about the plant. The door to the house was not fastened, so we entered and looked hurriedly about.
The room was absolutely empty except for the heavy cable, which came to the center of the floor and there connected with a copper post about four inches in diameter that ran straight down into the ground.
Without lingering further, we crawled back to the ladder and commenced our long climb up the cliff.
Upon reaching the top again, we found the ensign and his men still busy with their preparations for the bombardment. Withdrawing far enough to be out of their hearing, the astronomer turned to me and remarked:
“Well, what do you think of the scientific achievements of the sorcerers now?”
“I don’t know what to think!” I replied. “It’s utterly beyond my comprehension!”
The doctor chuckled at my dismay.
“Forgive me,” he said, “for having kept you so long in the dark. Until today I could never prove my theories—certain as I was of their correctness—and I did not wish to attempt any explanations until I was sure of my ground. But now you have seen enough to understand the solution of the puzzle.”
To my delight, the scientist was dropping into one of his most communicative moods. After a moment he went on:
“To comprehend, even in a general way, what the Seuen-H’sin has done, you must understand the principle of resonance.
“Let us start with the swinging pendulum of a clock. What keeps it in motion? Nothing but a slight push, delivered at exactly the right time. Any swinging object can be kept swinging, even though it weigh many tons, if it is given a touch by the finger of a baby at _just the right moment_. By the same principle, the amount of swing can be increased enormously if the successive pushes are correctly timed.
“But we need not limit our illustration to swinging objects. Everything in the word has a natural period of vibration, whether it be a violin string, or a battleship, or a forty-story skyscraper.
“Fifty men can capsize a twenty-thousand-ton battleship merely by running back and forth from one side of the deck to the other and carefully timing their trips to the vessel’s rolling. A child with a tack hammer can shake down a forty-story skyscraper if he can discover the natural period of the building’s vibration and then tap persistently upon the steel framework at the correct interval.
“Even the earth itself has its natural period of vibration.
“If you exploded a ton of dynamite on top of the ground it would blow quite a hole and jar the earth for several miles around it; and that would be all. But if you set off another ton of dynamite, and then another and another, and kept it up continuously—always timing the explosions to the period of the earth’s vibration—eventually the jar would be felt clear through the globe. And if you still persisted, in time you would wreck the world.
“Such is the accumulative power of many little blows correctly timed. The principle of timing small impulses to produce large effects is the principle of resonance.
“But there are other forces in nature which can produce vibration—electricity, for instance, Nikola Tesla demonstrated a number of years ago that the globe is resonant to electric waves.
“Now, suppose some person constructed an apparatus that could suddenly turn a tremendous flood of electric waves into the earth. That energy would go clear through the globe, imparting a tiny impulse to every atom of matter of which the sphere is composed—like a push upon the pendulum of a clock.
“And suppose that person knew the exact period of the earth’s vibration, and sent another bolt, and another and another, into the globe—all exactly timed to impart a fresh impulse at the correct moment—to give the pendulum another push, so to speak. Then let him pile electric impulse upon electric impulse, each at just the right second, until the accumulation of them all represented millions of horsepower in electric oscillations. In time, _the world would be shaken to pieces_!
“And—impossible as it sounds—that is the very principle the Seuen-H’sin is using there beneath your eyes! The dynamos furnish the power, and that great battery of induction coils magnifies it to an almost inconceivable voltage. By those cables attached to copper plugs, the impulses are conveyed to the earth.
“Every blow of that tremendous electric hammer is heavier than the preceding one because it has the accumulated power of all the others behind it. With every blow the earth grows weaker—less able to stand the shock. Continued, the planet’s doom would be inevitable—if it is not already so!”
I had been listening to this recital with amazement too profound to admit of interruption. When Dr. Gresham finished I sat silent, turning it all over in my mind, and reflecting how simple the explanation seemed. Finally—
“Was it those electric waves being discharged into the ground,” I asked, “that Professor Howard Whiteman in Washington mistook for wireless signals from Mars?”
“Precisely!” was the answer.
“And how,” I inquired, “was it possible for the sorcerers to discover the exact period of the earth’s vibration? That seems little short of superhuman.”
“Doubtless you remember the newspaper accounts published that night when we returned from Labrador,” replied the doctor. “They told how the electric whispers, when first noticed, occurred exactly two minutes apart; then the interval increased one minute each night until the signals were separated by more than thirty minutes; afterward the lulls altered erratically for some time, until they became fixed at eleven minutes and six seconds.”