Like the three men across the hog-back, and the two whom he had so swiftly slain, the Hun cable-patrol evidently fought shy of the Boche uniform here on the edge of the Forbidden Forest.

Two of the cable-guard lay smashed to a pulp thousands of feet below. Where was the remainder of the patrol? Were the men with the shotguns part of it?

McKay stood alone in the silent hut, still breathless from his struggle, striving to think what was now best to do.

And, as he stood there, through the front window of the hut he saw an aviator and another man come down from the crest of Thusis to the chasm's edge, jump into the car which swung under the cable, and begin to pull themselves across toward the hut where he was standing.

The hut screened his retreat to the wood's edge. From there he saw the aviator and his companion land on the platform; heard them shouting for the dead who never would answer from their Alpine deeps; saw the airman at last go away toward the plateau where he had left his machine; heard the clanking of machinery in the hut; saw the steel cable begin to sag into the canyon; AND REALISED THAT THE AVIATOR WAS GOING BACK OVER FRANCE TO THE BOCHE TRENCHES FROM WHENCE HE HAD ARRIVED.

In a flash it came to McKay what he should try to do--what he MUST do for his country, for the life of the young girl, his comrade, for his own life: The watchers at the hog-back must never signal to that airman news of his presence in the Forbidden Forest!

The clanking of the cog-wheels made his steps inaudible to the man who was manipulating the machinery in the hut as he entered and shot him dead. It was rather sickening, for the fellow pitched forward into the machinery and one arm became entangled there.

But McKay, white of cheek and lip and fighting off a deathly nausea, checked the machinery and kicked the carrion clear. Then he set the drum and threw on the lever which reversed the cog-wheels. Slowly the sagging cable began to tighten up once more.

He had been standing there for half an hour or more in an agony of suspense, listening for any shot from the forest behind him, straining eyes and ears for any sign of the airplane.

And suddenly he heard it coming--a resonant rumour through the canyon, nearer, louder, swelling to a roar as the monoplane dashed into view and struck the cable with a terrific crash.

For a second, like a giant wasp suddenly entangled in a spider's strand, it whirled around the cable with a deafening roar of propellers; then a sheet of fire enveloped it; both wings broke off and fell; other fragments dropped blazing; and then the thing itself let go and shot headlong into awful depths!

Above it the taut cable vibrated and sang weirdly in the silence of the chasm.

The girl was still lying flat under the walnut-tree when McKay came back.

Without speaking he knelt, levelled his pistol and fired across at the man beyond the hog-back.

Instantly her pistol flashed, too; one of the men fell and tried to get up in a blind sort of way, and his comrades caught him by the arms and dragged him back behind the ledge.

"All right!" shouted one of the men from his cover, "we've plently of time to deal with you Yankee swine! Stay there and rot!"

"That was Skelton's voice," whispered Miss Erith with an involuntary shudder.

"They'll never attempt that hog-back under our pistols now," said McKay coolly. "Come, Yellow-hair; we're going forward."

"How?" she asked, bewildered.

"By cable, little comrade," he said, with a shaky gaiety that betrayed the tension of his nerves. "So pack up and route-step once more!"

He turned and looked at her and his face twitched:

"You wonderful girl," he said, "you beautiful, wonderful girl! We'll live to fly our pigeons yet, Yellow-hair, under the very snout of the whole Hun empire!"

CHAPTER VIII

THE LATE SIR W. BLINT

That two spies, a man and a woman, had penetrated the forest of Les Errues was known in Berlin on the 13th. Within an hour the entire machinery of the German Empire had been set in motion to entrap and annihilate these two people.

The formula distributed to all operators in the Intelligence Department throughout Hundom, and wherever Boche spies had filtered into civilised lands, was this:

"Two enemy secret agents have succeeded in penetrating the forest of Les Errues. One is a man, the other a woman.

"Both are Americans. The man is that civilian prisoner, Kay McKay, who escaped from Holzminden, and of whom an exact description is available.

"The woman is Evelyn Erith. Exact information concerning her is also available.

"The situation is one of extremest delicacy and peril. Exposure of the secret understanding with a certain neutral Power which permits us certain temporary rights within an integral portion of its territory would be disastrous, and would undoubtedly result in an immediate invasion of this neutral (sic) country by the enemy as well as by our own forces.

"This must not happen. Yet it is vitally imperative that these two enemy agents should be discovered, seized, and destroyed.

"Their presence in the forest of Les Errues is the most serious menace to the Fatherland that has yet confronted it.

"Upon the apprehension and destruction of these two spies depends the safety of Germany and her allies.

"The war can not be won, a victorious German peace can not be imposed upon our enemies, unless these two enemy agents are found and their bodies absolutely destroyed upon the spot along with every particle of personal property discovered upon their persons.

"More than that: the war will be lost, and with it the Fatherland, unless these two spies are seized and destroyed.

"The Great Secret of Germany is in danger.

"To possess themselves of it--for already they suspect its nature--and to expose it not only to the United States Government but to the entire world, is the mission of these two enemy agents.

"If they succeed it would mean the end of the German Empire.

"If our understanding with a certain neutral Power be made public, that also would spell disaster for Germany.

"The situation hangs by a hair, the fate of the world is suspended above the forest of Les Errues."

On the 14th the process of infiltration began. But the Hun invasion of Les Errues was not to be conducted in force, there must be no commotion there, no stirring, no sound, only a silent, stealthy, death-hunt in that shadowy forest--a methodical, patient, thorough preparation to do murder; a swift, noiseless execution.

Also, on the 14th, the northern sky beyond the Swiss wire swarmed with Hun airplanes patrolling the border.

Not that the Great Secret could be discovered from the air; that danger had been foreseen fifty years ago, and half a century's camouflage screened the results of steady, calculating relentless diligence.

But French or British planes might learn of the presence of these enemy agents in the dark forest of Les Errues, and might hang like hawks above it exchanging signals with them.

Therefore the northern sky swarmed with Boche aircraft--cautiously patrolling beyond the Swiss border, and only prepared to risk its violation if Allied planes first set them an example.