down on the concrete to rest, leaning their backs against the safe.

How time counted in hyperspace, neither Phil nor Ione could tell; Phil knew that his watch was running. He knew that it was ages and ages that he sat with his back against the safe, reviewing all the events of his put life, and thinking of this ignominious end to a lively career! He swore half aloud; then suddenly looked at Ione, ready to apologize. He found her weeping silently.

"I should never have let you come into the building with me," he stammered in confusion at her tears.

"Oh, what do I care what becomes of me!" she exclaimed angrily. "But who will take care of poor daddy? He doesn't even know when it's time to eat." And she burst into a fresh fit of weeping.

Phil bent his head in the dumbness of profound despair.

PART V

The Reversible Equation

Despair, however, is a luxury. Necessity is a stimulus. With the parchings of thirst and the gnawings of hunger, the two young people ceased swearing and weeping. Phil got up and paced about and sat down again. Ione's tears stopped and dried, and she sat and thought.

In the back of her mind there had been forming a vague sort of an idea, which had signalled ahead of itself that there was hope. She sat there and desperately drove her reason to its utmost efforts, to find that idea and bring it to the surface of consciousness. Hand to hand fights with wild animals, battles between ships of the line, vicious duels between ace-aviators in the clouds are tense fights; but they cannot compare in anxious difficulty with the struggle to bring up an unformed idea out of the subconscious mind—especially when one knows that the idea is there, and that it must be found to save one's life.

"Ione!" exclaimed Phil. It was the first time he had used the name. "What is the matter? You are as tense as a—"

"Ah!" cried Ione, springing up. "Tense! Tensors! I have it!"

Phil gazed at her in alarm. She laughed; at first it was a strained laugh, but gradually it melted into her sunny one.

"No, I'm not crazy. I knew there was a way out, and I've been trying to reason it out. How simple. You remember the little jolts when you pulled at the vines and when you kicked the funny animal? Tensors. Matter and space are so closely interrelated that you can't move matter in or out of space without causing disturbance, recoils, and tremors in space. Those bits of matter were small, and produced only a slight disturbance. It takes about a hundred pounds of lead to swing this segment—"

"Oho! Got you!" exclaimed Phil. "Not so dumb! The safe!"

"Yes. The safe!" Ione cried.

"Throw it off and watch us swing, eh? What would happen?"

"I might calculate it if I knew the weight of the safe."

"No calculating when I'm around," Phil said. "It couldn't make things any worse. Try it first and calculate afterwards."

They got behind the safe and pushed, and their combined strength against it was about as effective as it would have been in moving the Peoples' Gas Building. They sat down again in despair.

"Suppose we could budge it," Ione said. "All we could do would be to push it around, this piece of matter we are on. That wouldn't help. We've got to get it out of space. We can't push it hard enough to do that. It's got to be shot out suddenly—"

"And we haven't got a gun handy," Phil remarked droopingly.

"Not exactly a gun. A sort of sling—"

Phil leaped to his feet.

"A sling. Why! To be sure! The vines!"

Without another word, both of them got up and ran. They hastened in a direction opposite to the one they had at first taken on their trip of exploration, and this brought them first past the "space" of the Tinkertoy-like animals. As they went by, several of these beasts darted at them, one of them snapping at Ione's heels. She uttered a scream, causing Phil to turn about and kick right and left among them. He drove them back and escaped from them, rejoining Ione.

"Wait," he said, when they reached the vines. "Remember those wooden balls. If I could get a few to throw at those critters—"

In a moment they were off, and finally arrived at the point from which they first saw the balls. Odd it seemed, how they hung suspended in space, thousands of them, all sizes. Phil reached out and grasped one about the size of a baseball and drew it toward himself. He felt a dizzy lurch and heard Ione scream.

"Let go!" she screamed again.

When he suddenly realized what was going on, he found himself prostrate on the ground, with Ione across him, her arms about his knees.

"Do you realize," she panted, disentangling herself, "that you were pulling yourself out of this space into that one?"

"Thanks!" said Phil. "Never say die. More careful this time, and a smaller one."

He reached out and grasped a ball smaller than a golf-ball, and pulled carefully, keeping an eye upon Ione. There was resistance to his pull, but gradually the ball came. It seemed heavy. There was a crack as of breaking wood, and he fell backward, with a wave of nausea sweeping strongly over him. He gazed in amazement at a heavy wooden stick that he held in his hands. The only thing about it that suggested the ball for which he had reached was its diameter.

"Can't understand it, but appreciate it just the same," he said. He broke the stick in two, and had two excellent clubs.

"Simple," Ione replied. "The balls are cross-sections of these trees or sticks which grow in a 'space' at right angles to our own; and we only see their three-dimensional cross-sections."

"Yes," said Phil. "Cabbages and kings. I'm for you and the party."

A short walk brought them to the "space" of the vines. After testing the matter out carefully, they found that they could each pull two of them at a time. The vines stretched amazingly when they found those whose far ends were fixed firmly in the tangle, permitting them to carry their own ends along with them toward the safe. Phil wound his vines around his left arm and stuck one club through his belt. The other he got ready for the wooden animals.

He needed it. The size of the pack was doubled, and he rapped them till his hand was numb before he and Ione got by. Their vines drew out thin, but held until they were firmly tied about the safe. They went back after four more.

"I should judge," said Phil, "that by the time we get thirty or forty, the elastic pull will be strong enough to drag the safe back with them as they snap back home."

Trip after trip they made, fighting the wooden animals with their clubs each time. Their clothes were torn, and their legs bleeding; their throats were dry and lips cracked. The hard animals seemed to have a persistent, mechanical ferocity that was undismayed by hammering with the clubs and by repeated repulses. Phil could not seem to hurt them; he merely knocked them away. Finally, on the ninth trip, Ione collapsed when she reached the safe. As she fell, the elasticity of the vines began slowly to drag her back with them. Phil was forced to sit across her knees while he tied his own vines about the safe. Then he released her and added her vines to the great cable about the safe.

An overbold hard animal rattled up and snapped at her. Goaded to fury, Phil swung at it with his club and hurled it through the air. He could feel the lurch as it left his space and entered another. Then he pushed with his mightiest effort against the safe. It budged, and slid a few inches. He used his stick as a lever. It moved again, a little faster. Ione struggled to her feet and tried to help, but her efforts were ineffectual.

With one arm about her, Phil pried again under the safe, knowing that another trip after vines was out of question. Another animal snapped at their heels. For a while, it was