al."

"You mean they're supposed to run that way? Overlapping so that for
five minutes we have Earth or Earth-and-a-half gravity and then none?"

"It's not _supposed_ to be that way," said Vogel. "But nobody ever
built a setup like this that worked any better." He added defensively:
"Of course, if you want, you can check with the company that makes
these units."

"I'm not trying to challenge your knowledge, and I'm not anxious to
make myself look silly. I have a sound reason for asking these
questions. There is a possibility of sabotage."

The engineer's grin was wider than the remark seemed to require.

"All right," said Cameron tiredly. "Suppose you tell me why sabotage
is so unlikely."

"Well," explained the gravital engineer, "it would have to be someone
living here, and he wouldn't like it if he suddenly got double or
triple gravity or maybe none at all. But there's another reason. Now
take a gravital unit. Any gravital unit. Most people think of it as
just that--a unit. It isn't really that at all. It has three parts.

"One part is a power source that can be anything as long as it's big
enough. Our power source is a nuclear pile, buried deep in the
asteroid. You'd have to take Handicap Haven apart to get to it. Part
two is the gravital coil, which actually produces the gravity and is
simple and just about indestructible. Part three is the gravital
control. It calculates the relationship between the amount of power
flowing through the gravital coil and the strength of the created
gravity field in any one microsecond. It uses the computed
relationship to alter the power flowing through in the next
microsecond to get the same gravity. No change of power, no gravity. I
guess you could call the control unit a computer, as good a one as is
made for any purpose."

The engineer rubbed his chin. "Fatigue," he continued. "The gravital
control is an intricate computer that's subject to fatigue. That's why
it has to rest an hour and a half to do forty-five minutes of work.
Naturally they don't want anyone tinkering with it. It's
non-repairable. Crack the case open and it won't work. But first you
have to open it. Mind you, that can be done. But I wouldn't want to
try it without a high-powered lab setup."

If it didn't seem completely foolproof, neither did it seem a likely
source of trouble. "Then we can forget about the gravital units," said
Cameron, arising. "But what about hand weapons? Are there any
available?