ne of the women went back to the van and drove it into the turnaround. The others walked up, talking among themselves as the guard closed the gate and locked it again.

Sweat was pouring down my face; it felt like grease. My heart was triphammering.

They were out of my field of vision in the rear-view mirror. I took a chance and looked around.

I saw the back doors of the van swing open.

One of them carried a neat stack of sheets; another had towels; another had a pair of vacuum cleaners.

They trooped up to the door and the guard let them inside.

I drove away, shaking so badly I could hardly steer the car.

They were opening the house. He was coming.

Dolan did not trade in his Cadillac every year, or even every two the gray Sedan DeVille he was driving as that June neared its end was three years old. I knew its dimensions exactly. I had written the GM company for them, pretending to be a research writer. They had sent me an operator,s manual and spec sheet for that year,s model. They even returned the stamped, selfaddressed envelope I had enclosed. Big companies apparently maintain their courtesy even when they,re running in the red.

I had then taken three figures the Cadillac,s width at its widest point, height at its tallest, and length at its longest to a friend of mine who teaches mathematics at Las Vegas High School. I have told you, I think, that I had prepared for this, and not all my preparation was physical. Most assuredly not.

I presented my problem as a purely hypothetical one. I was trying to write a science fiction story, I said, and I wanted to have my figures exactly right. I even made up a few plausible plot fragments my own inventiveness rather I astonished me.

My friend wanted to know how fast this alien scout vehicle of mine would be going. It was a question I had not expected, and I asked him if it mattered.

"Of course it matters," he said. "It matters a lot. If you want the scout vehicle in your story to fall directly into your trap, the trap has to be exactly the right size. Now this figure you,ve given me is seventeen feet by five feet."

I opened my mouth to say that wasn,t exactly right, but he was already holding up his hand.

"Just an approximation," he said. "Makes it easier to figure the arc."

"The what?"

"The arc of descent," he repeated, and I cooled off. That was a phrase with which a man bent on revenge could fall in love. It had a dark, smoothly portentous sound. The arc of descent.

I,d taken it for granted that if I dug the grave so that the Cadillac could fit, it would fit. It took this friend of mine to make me see that before it could serve its purpose as a grave, it had to work as a trap.

The shape itself was important, he said. The sort of slit-trench I had been envisioning might not work in fact, the odds of its not working were greater than the odds that it would. "If the vehicle doesn,t hit the start of the trench dead-on," he said, "it may not go all the way in at all. It would just slide along on an angle for awhile and when it stopped all the aliens would climb out the passenger door and zap your heroes." The answer, he said, was to widen the entrance end, giving the whole excavation a funnel-shape.

Then there was this problem of speed.

If Dolan,s Cadillac was going too fast and the hole was too short, it would fly across, sinking a bit as it went, and either the frame or the tires would strike the lip of the hole on the far side. It would flip over on its roof but without falling in the hole at all. On the other hand, if the Cadillac was going too slowly and the hole was too long, it might land at the bottom on its nose instead of its wheels, and that would never do. You couldn,t bury a Cadillac with the last two feet of its trunk and its rear bumper sticking out of the ground any more than you could bury a man with his legs sticking up.

"So how fast will your scout vehicle be going?"

I calculated quickly. On the open highway, Dolan,s driver kept it pegged between sixty and sixty-five. He would probably be driving a little slower than that where I planned to make my try. I could take away the detour signs, but I couldn,t hide the road machinery or erase all the signs of construction.

"About twenty rull," I said.

He smiled. "Translation, please?"

"Say fifty earth-miles an hour."

"Ah-hah." He set to work at once with his slip-stick while I sat beside him, bright-eyed and smiling, thinking about that wonderful phrase: arc of descent.

He looked up almost at once. "You know," he said, "you might want to think about changing the dimensions of the vehicle, buddy."

"Oh? Why do you say that?"