"Why should we run?" the voice of Sitsumi suddenly rang out in the control room. "Must we admit in the very beginning of our revolution that we are vulnerable? Must we confess the fears to which all humanity is heir? We had not thought ourselves liable to attack, but there still is a way to destroy these upstarts. To your places, everyone! We shall fight these winged upstarts and destroy them!"

The denizens of the space ship were at their stations. Jeter and Eyer could imagine the minions of Sitsumi and the Three, below the floor of the white globe, standing-to on platforms about the unseen engines which gave life and movability to this ship of the stratosphere. How many there were of them there was no way of knowing. They had guessed two hundred. There might have been a thousand. It scarcely mattered.

Sitsumi's face was set in a firm mask. He, of all the "lords of the stratosphere," seemed to possess endless courage. His example fired the three.

"What do you plan?" asked Wang Li.

Jeter and Eyer listened with all their ears.

"We have only one weapon in this unexpected emergency," said Sitsumi quietly. "We cannot direct the ray upward or laterally: it is not so constructed. But we can attack with the space ship itself! And remember that so long as our outer rind remains intact and hard we are invisible to attackers."

Jeter and Eyer exchanged glances.

"If only we could find the way to break or soften that outer rind," said Jeter.

"What can we do?" asked Eyer. "If it is impervious to the cold of these heights; if it is so strong that it is impervious to the tremendous pressure inside the globe—which must be kept at a certain degree to maintain human life—what can we do? We tried bullets. We might as well have used peas and pea-shooters. If our friends try bombs they will still be unsuccessful. If only we could somehow open up the outer rind or soften it, so that our friends could see the inner globe and reach it with their bombs!"

Jeter's face was now dead white. His eyes were aglow with excitement.

"Tema," he whispered, "Tema, that's their vulnerability! That's what they fear! They're scared that the outer rind may be broken—which would spell destruction to the space ship and everybody in it."

"Including us," replied Eyer, "but, anyway—well, what's the odds? We're only two—and with this thing destroyed the nightmare will end. Of course there should be some way to raid the Lake Baikal area and destroy any other ships in the making, besides ferreting out the secret of the invisible substance and the elements of the gravity inverter. If we somehow survive, and this ship is destroyed, that's the next thing to do."

Jeter nodded and signaled Eyer to cease whispering.

They devoted their attention now to the six planes. They were coming up in battle formation. They were in plain view and through the telescopes it could be seen that each was armed with bombs of some kind. Useless against the invisible space ship as matters now stood; but what would those bombs do to the inner globe?

It still lacked several hours of the time allowed in the ultimatum to Jeter and Eyer of Sitsumi and the Three, when the six planes leveled off within a couple of miles of the space ship. They knew about where the stratosphere had swallowed up Jeter and Eyer. Now they were casting about for a sign, like bloodhounds seeking the spoor of an enemy.

Jeter and Eyer held their breaths as they watched. Now and again they stole glances at Sitsumi and the Three, who were watching the six planes with the intensity of eagles preparing to dive.

Naka stepped up close to Jeter.

"When the time comes," he said menacingly, "and it appears that we may be in difficulties with the fools who think to thwart Sitsumi and the Three and rescue you, it shall give me great pleasure to destroy you with your own automatic."

"Pleasant fellow," said Eyer. "Shall I smash him, Lucian?"

Jeter shook his head.

"Our friends out there will look after that, Tema," he said in a natural tone of voice. "I'll bet you two to one they get this ship within an hour. Not that a bet will mean anything, as they'll get us, too!"

"Your friends," said Naka, "will be destroyed. They will not even be given the opportunity you were given. Sitsumi and the Three will waste but little time on them!"

"What," said Jeter calmly "is Sitsumi's hurry? Why is he scared?"

"Scared?" Naka seemed on the point of hitting Jeter for the blasphemy. "Scared? He fears nothing. We'll down your friends long before their motors—"

Sitsumi suddenly turned and looked at Naka. The look in Sitsumi's eyes was murderous, Naka went dead white.

"I think your master believes you talk too much, Naka," said Jeter, but Jeter's eyes were gleaming, too.

As soon as Sitsumi had turned back to his station Jeter's lips began to move.

"See?" he said. "It isn't their machine guns these people fear. It isn't their bombs—it's their motors! I wonder why...."

By now the six planes were flying abreast, in battle formation, almost above the space ship, at perhaps a thousand feet greater elevation. A strange humming sound was traveling through the space ship. The whole inner globe was vibrating, shaking—and vibration was a menace to glass or crystal!

"We've got the answer!" said Jeter. "The outer rind, while capable of being softened—in sections at least, with safety—for special reasons, such as happened when we were 'swallowed,' can be hardened to the point of disruption. It can be shattered, Tema, by vibration! That's why the space ship keeps far above the roar of cities! The humming of countless automobile engines might shatter the rind! God, I hope this is the answer!"

In his mind's eye Eyer could picture it—the outer rind "freezing" solid, and cracking with the thunderous report of snapping ice on a forest lake. No wonder Sitsumi and the Three must destroy the six planes.

"Now!" yelled Sitsumi. "Shift positions! The space ship will be hurled directly at the formation of planes! Wang Li, to the beam controls!"

Wang Li sprang to the table, pressed a button. The humming sound in the space ship grew to mighty proportions. The trembling increased.

Jeter and Eyer kept their eyes glued to the six planes above. Without tilting their noses the six planes seemed to plunge straight down toward the surface of the space ship. Thus the two knew that the space ship was in motion—itself being bodily hurled, as its only present weapon of offense, against the earthling attackers.

A split second—

One of the planes struck the surface solidly and crashed. Instantly its wheels and its motor were caught in the outer rind.

The other five ships scattered wildly, escaping the collision by some sixth sense, or through pure chance.

"Poor devil!" said Jeter. "But his buddies can see his plane and know that it marks the spot where they could conveniently drop their bombs."

Eyer was on the point of nodding when Sitsumi shouted.

"Quickly, Wang Li! Spin the outer shell before the enemy uses the wrecked plane as an aiming point!"

Awhirring sound. The plane whirled around as though it were twirled on the end of a string. To the five other pilots it must have seemed that the plane had struck some invisible obstruction, been smashed, and now was whirling away to destruction after a strange, incomprehensible hesitation in the heart of the stratosphere.

"Quickly, you fool!" shouted Sitsumi at Wang Li. "You're napping! You should have got all those planes! And you should have spun the outer globe instantly, before the remaining enemy had a chance to find out our location."

"I can move away a half mile," suggested Wang Li.

"We've got to silence those motors, fool!" yelled Sitsumi. "You know very well that we can't run. Charge them again, and take care this time that you crash into the middle of their formation."

"They're scattered over too great an area. I should wait for them to reform."

"Fool! Fool! Don't you think I know the weakness in my own invention? The proper vibration will destroy us! If the rind is softened we become visible. We dare not wait for them to reform! Attack each plane separately if necessary, and at top speed!"

Jeter began to speak rapidly out of the corner of his mouth. Even Naka's attention was fastened on the five planes and Wang Li's efforts to destroy them.

"Gag Naka!" said Jeter. "The keys! In some way we've got to get to our plane. It's barely possible. If we can start the motor.... Hurry! Now, while the whole outfit is watching our friends out there!"

Eyer rose and reached for Naka with his right hand.

He dared not miss his lunge. He did not. His huge hand fastened in the throat of their keeper. Nobody—neither Sitsumi nor the Three—turned as Naka gasped and struggled. Eyer pulled the man back over the table and, his neck thus within reach of both hands, snapped it as he would have broken the neck of a chicken.

Jeter was already searching the body for the keys. He found them.

Their leg irons were just falling free when Sitsumi turned. Eyer was feeling for the automatics in Naka's belt.

"We won't need them!" yelled Jeter. "There isn't time. Let's go!"

Jeter was away at top speed, almost pulling Eyer off his feet because their hands were still fastened together with the handcuffs.

They were outside on the floor level.

And through many doors denizens of the lower control room, hurried out by the commands of Sitsumi, were racing to head them off. But nothing could stop them. One man got in their way and Eyer's right fist caved in his face with one deadly, devastating blow. They had now reached the stairs.

The space ship was being hurled at the five remaining planes. Even as the two men reached the stairs and started up, another of the dauntless rescuers paid with his life for his courage. Several bombs exploded as his plane struck the space ship, but they caused no damage whatever. The hard outer rind seemed to be impervious to the explosions. Obviously no explosive could destroy the space ship.

"Quickly, Tema," said Jeter. "The rind can be shattered by vibration, and we've got to do it somehow."

"And after that?" panted Eyer.

"Our friends out there can then see the inner globe. They'll drop bombs. They'll smash in the globe and—"

"I know," said Eyer. "Its inhabitants, including us, will start off in all directions through the stratosphere, with great speed, and probably in many pieces."

Jeter laughed. Eyer laughed with him. They didn't fear death, for now they felt they were on the verge of destroying this monster of space.

Their pursuers were following them closely.

Jeter frantically tried to unfasten the handcuffs as they ran. He didn't manage it until the door was almost reached. He left one cuff dangling on his right wrist.

Then, they were through the door.

"Now, Tema," shouted Jeter, "if you believe in God—if you have faith—pray for strength to move this plane!"

"Where?"

"So that its wheels and nose go through this open door! Then it won't travel forward when we start the motor—and our pursuers won't be able to get through to stop us."

"You think of everything, don't you?" There was a grin on Eyer's face. But his eyes were stern. He wasn't belittling their deadly danger. And there was also a chance that Jeter's vibration idea was wrong.

"Those four planes," panted Jeter, as the two tried to get their plane in motion toward the door, "cause, from a distance, through thin air, a slight vibration, varying with their distance from the globe; our plane motor racing and actually in contact with the globe, can set up a tremendous vibration by its great motor speed. If we can vibrate the globe up to its shattering point there's a chance!"

"We can't pull her, Lucian," said Eyer. "I'll do a Horatius at the door. You get in, start the motor, taxi her until the wheels go through. I'll keep the crowd back."

"Right!"

Jeter went through the doors into the plane. In a few seconds the propeller kicked over, hesitated, kicked again. Then the motor coughed, coughed again, and broke into a steady roaring.