At last he finished the final equation, and, with a smile of triumph and relief, got to his feet again.

Back to the girl, who was cooking over an odorous fire of cedar, he made his way, rejoicing.

"I've got it!" he shouted gladly. "Making reasonable allowances for depth, I've got it!"

"Got what?"

"The probable width!"

"Oh!" And she stood gazing at him in admiration, beautiful and strong and graceful. "You mean to say--"

"I'm giving the chasm a hundred miles' depth. That's more than anybody could believe possible--twice as much. On that a.s.sumption, my tests show the distance to the other side--and there is another side, by the way!--can't be over--"

"Five hundred miles?"

"Nonsense! Not over one hundred to one-fifty. I'm going on a liberal allowance for error, too. It may not be over seventy-five. The--"

"But if that's as far as it is, why can't we see the other side?"

"With all that chemicalized vapor rising constantly? Who knows what elements may be in it? Or what polarization may be taking place?"

"Polarization?"

"I mean, what deflection and alteration of light? No wonder we can't see! But we can fly! And we're going to, what's more!"

"Going to make a try for Chicago, then?" she asked, her eyes lighting up joyfully at thought of the adventure.

"To-morrow morning, sure!"

"But the alcohol?"

"We've still got what we started with from Detroit, minus only what we've burned reaching this place. And we reckoned when we set out that it would far more than be enough. Oh, that part of it's all right!"

"Well, you know best," she answered. "I trust you in all things, Allan. But now just look at this roast partridge; come, dear, let to-morrow take care of itself. It's supper-time now!"

After the meal they went to the flat rock and sat for an hour while the sun went down beyond the void. Its disappearance seemed to substantiate the polarization theory. There was no sudden obliteration of the disk by a horizon. Rather the sun faded away, redder and duller; then slowly losing form and so becoming a mere blur of crimson, which in turn grew purple and so gradually died away to nothing.

For a long time they sat in the deepening gloom, their rifles close at hand, saying little, but thinking much. The coming of night had sobered them to a sense of what now inevitably lay ahead. The solemn purple pall that adumbrated the world and the huge nothingness before them, so silent, so immutable and pregnant with terrible mysteries, brought them close together.

The vague, untrodden forest behind them, where the night-sounds of the wild dimly reechoed now and then, filled them with indefinable emotions. And that night sleep was slow in coming.

Each realized that, despite all calculations and all skill, the morrow might be their last day of life. But the morning light, golden and clear above the eastern sky-line of tall conifers, dispelled all brooding fears. They were both up early and astir, in preparation for the crucial flight. Stern went over the edge of the chasm, while Beatrice prepared breakfast, and made some final observations of wind, air currents and atmosphere density.

An eagle which he saw soaring over the abyss, more than half a mile from its edge, convinced him a strong upward current existed to-day, as on the day when they had made their short flight over the void. The bird soared and circled and finally shot away to northward, without a wing-flap, almost in the manner of a vulture. Stern knew an eagle could not imitate the feat without some aid in the way of an up-draft.

"And if that draft is steady and constant all the way across," thought he, "it will result in a big saving of fuel. Given a sufficient rising current, we could volplane all the way across with a very slight expenditure of alcohol. It looks now as though everything were coming on first-rate. Couldn't be better. And what a day for an excursion!"

By nine o'clock all was ready. Along the land a mild south wind was blowing. Though the day was probably the 5th of October or thereabout, no signs of autumn yet were blazoned in the forest. The morning was perfect, and the travelers' spirits rose in unison with the abounding beauty of the day.

Stern had given the Pauillac another final going over, tightening the stays and laterals, s.c.r.e.w.i.n.g up here a loosened nut, there a bolt, making certain all was in perfect order.

At nine-fifteen, after he had had a comforting pipe, they made a clean getaway, rising along the edge of the chasm, then soaring in huge spirals.

"I want all the alt.i.tude I can get," Stern shouted at the girl as they climbed steadily higher. "We may need it to coast on. And from a mile or two up maybe we can get a glimpse of the other side."

But though they ascended till the aneroid showed eight thousand five hundred feet, nothing met their gaze but the same pearly blue vapor which veiled the mystery before them. And Stern, satisfied now that nothing could be gained by any further ascent, turned the machine due west, and sent her skimming like a swallow out over the tremendous nothingness below.

As the earth faded behind them they began to feel distinctly a warm and pungent wind that rose beneath--a steady current, as from some huge chimney that lazily was pouring out its monstrous volume of hot vapors.

Away and away behind them slid the lip of this gigantic gash across the world; and now already with the swift rush of the plane the solid earth had begun to fade and to grow dim.

Stern only cast a glance at the sun and at his compa.s.s, hung there in gimbals before him, and with firm hand steadied the machine for the long problematical flight to westward. Behind them the sun kept even with their swift pace; and very far below and ahead, at times they thought to see the fleeing shadow of the biplane cast now and then on ma.s.ses of formless vapor that rose from the unsounded deeps.

Definitely committed now to this tremendous venture, both Stern and the girl settled themselves more firmly in their seats. No time to feel alarm, no time for introspection, or for thoughts of what might lie below, what fate theirs must be if the old Pauillac failed them now!

No time save for confidence in the stout mechanism and in the skill of hand and brain that was driving the great planes, with a roaring rush like a gigantic gull, a swooping rise and fall in long arcs over the hills of air, across the vast enigma of that s.p.a.ce!

Stern's whole attention was fixed on driving, just on the manipulation of the swift machine. Exhaust and interplay, the rhythm of each whirling cam and shaft, the chatter of the cylinders, the droning diapason of the blades, all blent into one intricate yet perfect harmony of mechanism; and as a leader knows each instrument in the great orchestra and follows each, even as his eye reads the score, so Stern's keen ear a.n.a.lyzed each sound and action and reaction and knew all were in perfect tune and resonance.

The machine--no early and experimental model, such as were used in the first days of flying, from 1900 to 1915, but one of the perfected and self-balancing types developed about 1920, the year when the Great Death had struck the world--responded n.o.bly to his skill and care.

From her landing-skids to the farthest tip of her ailerons she seemed alive, instinct with conscious and eager intelligence.

Stern blessed her mentally with special pride and confidence in her mercury equalizing balances. Proud of his machine and of his skill, superb like Phaeton whirling the sun-chariot across the heavens, he gave her more and still more speed.

Below nothing, nothing save vapors, with here and there an open s.p.a.ce where showed the strange dull purple of the abyss. Above, to right, to left, nothing--absolute vacant s.p.a.ce.

Gone now was all sight of the land that they had left. Unlike balloonists who always see dense clouds or else the earth, they now saw nothing. All alone with the sun that rushed behind them in their skimming flight, they fled like wraiths across the emptiness of the great void.

Stern glanced at the barometer, and grunted with surprise.

"H'm! Twelve thousand four hundred and fifty feet--and I've been jockeying to come down at least five hundred feet already!" thought he. "How the devil can _that_ be?"

The explanation came to him. But it surprised him almost as much as the noted fact.

"Must be one devil of a wind blowing up out of that place," he pondered, "to carry us up nearly four thousand feet, when I've been trying to descend. Well, it's all right, anyhow--it all helps."

He looked at the spinning anemometer. It registered a speed of ninety-seven miles an hour. Yet now that they were out of sight of any land, only the rush of the wind and the enormous vibration of the plane conveyed an idea of motion. They might as well have been hung in mid-s.p.a.ce, like Mohammed's tomb, as have been rushing forward; there was no visible means of judging what their motion really might be.

"Unique experience in the history of mankind!" shouted Stern to the girl. "The world's invisible to us."

She nodded and smiled back at him, her white teeth gleaming in the strange, bluish light that now enveloped them.

Stern, keenly attentive to the engine, advanced the spark another notch, and now the needle crept to 102 1/2.

"We'll be across before we know it," thought he. "At this rate, I shouldn't be surprised to sight land any minute now."

A quarter-hour more the Pauillac swooped along, cradling in her swift flight to westward.