That rules out travelling in time."

* * * * *

He jerked at his fishing-rod. He did not hook his fish.

"I don't think you understand me," he observed.

"No," said Fran matter of factly.

"It doesn't matter," Soames told him. "I'm saying that you can't put a
gallon of water in a full keg of wine. And you can't, unless you draw
off wine as fast as you add water. Unless you exchange. So you can't
shift an object from time-frame A to time-frame B without shifting a
corresponding amount of matter and energy from time-frame B to
time-frame A. Unless you keep the amount of matter and energy unchanged
in each. Unless you exchange. So you came to here and now from there and
then--your home time-frame, let's say--by a process of swapping. By
transposition. By replacement. Transposition's the best word. The effect
was time-travel but the process wasn't, like a telephone has the effect
of talking at a distance but the method is distinctly something else."

Fran jerked his fishing-rod. A nine-inch lake-trout flapped in the
boat's bottom.

"I'm supposed to be teaching you how to fish!" said Soames.

He watched as Fran rather gingerly extracted the hook and rebaited as
he'd seen Soames do.

Soames continued, "Your ship was transposed from your time into mine.
Simultaneously, gram molecular weight for gram molecular weight,
something had to be transposed into yours. Since you were to come into
my time twenty thousand feet high and there was nothing else handy to be
transposed into your time--why--air had to leave here and turn up there.
To make up the mass and energy of your ship and you and the other
children."

As if to indicate that he listened, Fran said:

"Zani, Mal and Hod."

"Right!" Soames jerked his rod and brought up a fingerling which he
silently unhooked and threw back overboard. "Considering the thinness of
the air where you came out, maybe half a cubic mile of it had to
transpose into your time to let your ship come into this."

He dropped the line overboard again.

"Which means that there was an implosion of anywhere from a quarter to
half a cubic mile of vacuum. It made an earth-shock and a concussion
wave, and it battered your ship until it went out of control. It would
seem to make sense that the tumult and the shouting would appear here,
where plain force was operating without much guidance, but not in your
time where the machinery and the controls were operating. Yo