“Not quite invisible,” the professor said calmly. “What do you think I was trying to do with screen wire set up on the two sides of a bit of buzzard bait? I was trying to see what kept it from reeking to high heaven! Didn’t you ever hold a match six inches from your nose, and look at the world through the hot gases above the flame? Things wobble and waver when you do. How do you think I made up my mind there were gaseous dynamic systems around here? When you look through one of them, things waver and wobble! The things you’re talking about are just as invisible as the column of hot air above a match, which means they’re not easy to see—you have to know what to look for—but they can be seen!” “Then what tried to kill me?”

“Certainly a dynamic system,” the professor insisted. “It had to be. A dynamic system is a parcel of matter using energy in a patterned way. A whirlwind’s a dynamic system. So’s a gasoline engine. Or a rabbit, or a man. Whatever attacked you and the Monster had to be a dynamic system because it used energy in a patterned fashion. Look here! Blow a smoke ring.”

Lane blinked. The professor gestured impatiently. He blew a smoke ring. It went slowly across the stifling hot interior of the trailer, expanding as it went.

“That,” said the professor, “is a very simple dynamic system. It’s a quantity of air which happens to have a toroidal motion. It isn’t alive. It’s only a vortex ring. You can see it because the air of which it’s composed happens to contain smoke. But a vortex ring can exist in plain air just as—”

“Aunt Ann! Look at the smoke ring!” It was Carol, her voice strained.

The professor blinked. Then she looked at the thin, drifting ring of smoke. It was deformed. It was bent on one side exactly as if it had struck something solid.