hreads had been cut in reverse, with a lefthand thread.

Throughout the galaxy there existed as many technical and cultural
differences as there were planets, yet one of the few things they all
had in common, inherited from their terrestrial ancestors, was a
uniformity of thread. Jason had never thought about it before, but
when he mentally ran through his experiences on different planets he
realized that they were all the same. Screws went into wood, bolts
went into threaded holes and nuts all went onto bolts when you turned
them with a clockwise motion. Counterclockwise removed them. In his
hand was the crude D'zertano nut and bolt, and when he tried it it
moved in the same manner. But the engine bolt did not work that
way--it had to be turned clockwise to _remove_ it.

* * * * *

Dropping the nut and bolt he placed the wrench on the massive engine
bolt and slowly applied pressure in what felt like the completely
wrong direction, as if he were tightening not loosening. It gave
slowly, first a quarter then a half turn. And bit by bit the
projection threads vanished until they were level with the surface of
the nut. It turned easily now and within a minute it fell into the
pit--he threw the wrench after it and scrambled out. Standing at the
edge he carefully sniffed the air, ready to run at the slightest smell
of gas. There was nothing.

The second nut came off as easily as the first and with no ill
effects. Jason pushed a sharp chisel between the upper case and the
baseplate where he had removed the solder, and when he leaned on it
the case shifted slightly, held down only by its own weight.

From the entrance to the enclosure he shouted to the group huddled in
the distance. "Come on back--this job is almost finished."

They all took turns at sliding into the pit and looking at the
projecting bolts and made appreciative sounds when Jason leaned on the
chisel and showed how the case was free.

"There is still the little matter of taking it off," he told them,
"and I'm sure that grabbing and heaving is the wrong way. That was my
first idea too, but the people who assembled that thing had some bad
trouble in store for anyone who tightened those nuts instead of
loosening them. Until we find out what that is we are going to tread
very lightly. Do you have any big blocks of ice around here, Edipon?
It is winter now, isn't it?"

"Ice? Winter?" Edipon mumbled, caught off guard by the cha