“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.