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 intervals.
"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 age 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."
"Yes," I assented.
"Well," continued the scientist, "those variations simply denoted the experiment of the Seuen-H'sin to ascertain the period of the globe's vibration. If, after continuing their discharges all one night, their seismographs showed no response from the earth, they knew their bolts were wrongly timed, and they experimented with another period.
"Eventually they found that their impulses penetrated the earth with a speed of approximately 709 miles a minute—in other words, in precisely eleven minutes and six seconds the waves passed clear through the plant. This, then, was demonstrated to be the length of time that must elapse before the pendulum—figuratively speaking—could be given another electrical push. You saw just now, on the switchboard down there, the clockwork apparatus which times those bolts."
After a moment's consideration I remarked:
"Your own electrical equipment on board the Albatross—those big induction coils and the rest of it—what did you plan to do with that?"
"I had meant to fight the Seuen-H'sin with its own methods," the doctor replied. "I was going to throw a high-power electric current into the earth at intervals between those of the sorcerers'—say five minutes apart. That would have interfered with the acceleration of the vibrations—like setting a second group of men to run across the ship's deck between the trips of the first group. One set of vibrations would have neutralized the other.
"But," Dr. Gresham added, "the time for such methods is past. We must end the whole thing immediately—at one stroke!"
Receiving a signal from Ensign Hallock that he was ready, we started to rejoin the ship's party. But before we had gone a dozen steps we were rooted to the spot by a new terror!
Off in the east, where the snow-covered: peaks lifted into the sky, suddenly burst forth an awful crashing sound, as of a colossal cannonade—a ponderous and unbroken thunder-roll, terrible as the enormous tumult of the day of doom. As our gaze followed the nightmare sounds to the edge of the world we beheld the lofty mountains oscillate, crack, disjoint, and crumble into seething ruin.
The noise that accompanied this destruction came roaring and booming across the intervening miles—a stupendous and unearthly commotion, shattering the very atmosphere to fragments.
For a minute Dr. Gresham stood petrified. But as the enormity of the cataclysm became evident, an unconscious cry, almost a groan, escaped him:
"Too late! Too late! The beginning of the end!"
Suddenly he wheeled—almost livid with excitement—to the naval officer and screamed at the top of his voice:
"Fire! For God's sake destroy that power plant! Fire! FIRE!"
CHAPTER XIII
PLAYING OUR FINAL CARD
IN THEIR ASTONISHMENT at the terrible upheaval, Ensign Hallock and his men had left their posts und crowded toward the end of the promontory, a few feet away from the mortars. At Dr. Gresham's command to fire, most of them leaped to obey the order.
Instantly the woods behind us sprang
