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"How do you do?" said Calhoun evenly. "I gather we are to work together—again, Doctor Kelo."

Doctor Kelo's mouth opened, and shut. His face went gray. He made an inarticulate sound. He stared at Calhoun in absolute stupefaction. Murgatroyd squirmed past Calhoun's body to look into the communicator screen. He saw a man, and to Murgatroyd that meant that shortly he would be aground among people who admired him adoringly and would therefore stuff him with all the things he liked to eat and drink.

"Chee!" said Murgatroyd cordially. "Chee-chee!"

The stark incredulity of the bearded face changed to shock. That expression became purest desperation. One of Doctor Kelo's beautifully manicured hands disappeared. It appeared again. There was a tiny snapping sound and the grid operator became suddenly boneless. He seemed to bend limply in all his joints and almost to pour downward to the floor.

Doctor Kelo turned swiftly to the dials of the landing-grid control-board. He surveyed them, panting suddenly. Of course, a landing grid can do its work in many different fashions. It can use the processes of normal space commerce to make space war impossible. Because it can be deadly.

Doctor Kelo reached out. Calhoun could not see exactly what he did, but he could guess its purpose. Immediately he felt a surging of the Med Ship which told him exactly what had been done. It was an increased downward velocity of the ship, which had to be brought down rapidly for most of its descent, or otherwise the grid would swing around to the night side of this world where, with a planet's bulk between, it could not do anything with the Med Ship at all. However, high acceleration toward the ground could be used to a certain point only. Below a critical distance the ship couldn't be stopped. It would be bound to crash to flaming destruction against the world it had meant to land on.

The ship surged again. It plunged planetward with doubled acceleration. In the grip of the landing grid's force fields, it built up to a velocity far beyond any at which it could be slowed for a safe landing. It was building up toward the speed of shooting stars, which consume themselves when they touch atmosphere. It was still thousands of miles out in emptiness, still speeding crazily to inevitable destruction.

V

Calhoun said coldly, "I've got to learn how a murderer thinks, Murgatroyd. While I'm thinking there's a situation they have to meet, these characters work out a way to kill me, as if that was bound to settle everything. I can't anticipate the ideas they get automatically!"

He placed his hands on the control-board where he could act in an infinitesimal fraction of a second. He waited. The Med Ship was in the grip of an immaterial field of force which was capable of handling a merchant ship of space, whereas the Aesclipus Twenty was as small as a ship could be and still perform a Med Ship's functions.

The fact that a field of force is not a solid object has its consequences. A solid object can exert a thrust in three dimensions. If it is rigid, it can resist or impose thrusts in any direction, up or down, right or left, and away from or toward itself. However, a field of force can only act in one: toward or away from, or up or down, or left or right. It cannot push in one direction while resisting a thrust from another. So a grid field could pull a ship downward with terrific force, but it could not pull the ship sidewise at the same moment, and that happened to be what was necessary.

There is a certain principle known as the conservation of angular momentum. A ship approaching a planet has always some velocity relative to the planet's surface. Within a wide range of speeds, that angular velocity will make a ship take up an orbit at a distance appropriate to its speed. The greater the speed, the lower the orbit. It is like a weight on a string, twirled around one's finger. As the string winds up, the weight spins faster. It is like a figure skater spinning in one spot on the ice with arms outstretched, who spins more and more rapidly as he brings his arms closer to his body. The Aesclipus Twenty had such orbital, angular momentum. It could not descend vertically without losing its velocity. If it was to land safely, it would have to lose its velocity and at the moment it touched ground it must have exactly the motion of the ground it touched, for exactly the same reason that one stops a ground-car before stepping out of it.

But a grid field could only push or pull in one direction at a time. To land a ship it must cease to pull planetward from time to time, and push the ship sidewise to match its speed to that of the ground. If it didn't, the ship would go on beyond the horizon—or seem to.

So Calhoun waited. Grimly. The ship, plunging vertically, still retained its lateral speed. That speed drove it toward the horizon. It was necessary to pull it back to pull it down. So the bearded man, cursing as the ship swung away from the vertical, fumbled to pull it back.

An extremely skilled operator might well have done so, even against Calhoun's resistance. The shift of directional pull—or thrust—could have been made so swiftly that the ship would be actually free of all fields for less than the hundredth of a second. However, such fine work required practice.

Calhoun felt the ship shiver for the fraction of an instant. For that minute portion of a heartbeat, the downward pull had to be cut off so the sidewise push could be applied. But in that instant Calhoun jammed down the emergency rockets' control to maximum possible thrust. He was flung back into the pilot's chair. The weight of his chest forced air explosively out of his lungs. Murgatroyd went skittering across the floor. He caught an anchored chair leg with a wide sweep of his spidery arms and clung there desperately, gasping.

Three. Four. Five seconds. Calhoun swung the ship's nose and went on. Seven. Eight. Nine. Ten.

He cut the rocket-blast at the last instant before he would have blacked out. He panted. Murgatroyd said indignantly, "Chee! Chee! Chee!"

Calhoun said with some difficulty, "Right! I did you a dirty trick, but it had to be done! Now if we can keep him from getting his field locked on us—again . . ."

He sat alertly in the pilot's chair, recovering from the strain of such violent acceleration for even so brief a period. A long time later there was a faint, fumbling sensation as if a force field, groping, touched the ship. He blasted off at an angle at high acceleration again.

Then the ship was clear. It reached a spot where the landing grid, on the curved surface of Kryder II, was below the horizon. The Med Ship had orbital velocity. Calhoun made certain of it when he looked at the nearest-object indicator. He was then very close to atmosphere but the planet now below him curved downward and away from his line of flight. The ship was actually rising from the planetary surface. Calhoun had escaped a collision with Kryder II by speeding up across its face. One can sometimes avoid a collision in traffic by speeding up, but it is not the safest thing for either ground-cars or spaceships to do.

Murgatroyd made querulous noises to himself. Calhoun got out the data on the planet Kryder II. There were continents and highways and mountain ranges and cities. He studied the maps and a view of the actual surface beneath him. The communicator screen was blank, and had been since the horizon rose between the grid and the fleeing ship. He flipped it off. At the sunset line there was a city. He located himself.

Murgatroyd said "Chee!" in an apprehensive tone as the emergency rockets roared again.

"No," said Calhoun. "No more full-force rocketeering, Murgatroyd. And I'm not going to take the chance of being outwitted again. I've been fooled twice by not knowing how a murderer's mind works. I'm going to operate out of contact with such characters for a while. I'm going to land and do a burglary and get back out to space again."