'Did you feel a sideways pull?' inquired Danielson.

'That's what bowled me over. I had one foot in the air when someone raised the floor and then gave me a shove.'

'I guess maybe I did,' answered Gantt, 'but I was sitting down, so that took some of the edge off.'

'You're right. The thing must have come up just outside the tent,' Runyan joined in. 'Must have been one of those tunes when it got jarred off course somehow. Actually, in spite of the low probability, it's lucky no one was hit. I was thinking. Pat may have had a good idea: it might be of some interest to find the hole it made coming out and the other falling back in. Apparently that occurred just a bit further to the east, near the edge of camp. I think we may have learned something important here, in addition — to having the wits scared out of us.'

'What's that?' asked Gantt.

'Well, there are three things that come to mind. First, we've confirmed the fact that it comes down near where it went up. That's significant.'

'I thought of that. It's the same as Dallas ,' said Danielson, her eyes shining. 'It must be moving with the same tangential velocity as the surface of the earth as it comes up.'

Gantt looked puzzled, and Danielson explained to him, 'Remember that, because it rotates, the surface of the earth is actually moving at about a thousand miles an hour. If this thing were literally moving on a line pointed at a fixed direction in space, then as it reached the surface we would move out from under it at just that speed. How long did you say it was up? About a half of a minute? Let's see, the earth's surface rotates about twenty miles in a minute or about ten in the time the thing was up.'

'Closer to seven,' said Runyan with unconscious pedanticism, 'but clearly the relative motion could have been much greater than it actually was.'

'I guess I still don't quite see,' began Gantt.

'The point is,' explained Runyan, 'that when it comes to the surface of the earth it's virtually at rest with respect to the local terrain. That can't be an accident. It must have begun that way. We can rule out the idea that it's a naturally occurring black hole. To have it moving at precisely the earth's orbital velocity so that it could be trapped was asking a lot. To insist that it also move in consonance with the rotation of the earth is out of the question. I could never put any store in the idea anyway, but now I think we can really lay it to rest.

'Let me put it another way,' he continued, 'if you were to imagine taking a black hole and holding it in your hand so that both you and it were moving along with the surface of the earth, and then you were to drop it, and let it orbit freely, the result would be just what we have seen. It would drop down, pass to the far side of the earth and return. It must return to precisely the same altitude as that from which it was dropped, and at its highest point, when it momentarily has no velocity towards or away from the earth's centre, it must have precisely the same sideways motion as when it was released. To someone moving with the same motion, that is, with the velocity of the earth's surface, it would seem to come momentarily to an exact standstill.'

'But it didn't stand still,' objected Gantt, 'that is, it continued on up.'

'That's my second point,' replied Runyan. 'One we kicked around in La Jolla. We know how far up it went. It took about fifteen seconds to go up and an equal amount to return. At one gee, that's a distance of about three thousand four hundred feet. What's the altitude here?'

'About twenty-three hundred feet,' said Gantt.

'Then apogee is about five thousand seven hundred feet above sea level. A bit over a mile. That must be the altitude from which it was originally dropped.'

Before either Danielson or Gantt could comment, Runyan was on his feet. 'Let me get something out of my luggage.' He tossed off the remaining bourbon in his cup, set the cup on the chair arm, and strode purposefully over to the mess tent where their luggage had been placed. The cup blew off, and Gantt rescued it from the ground. Runyan rummaged for a moment and then returned with a stack of computer output. He regained his seat and balanced the paper on his knees so he could easily riffle the accordion— folded sheets.

'Another little project of mine,' he explained. 'Pat, you said that in Dallas your agents thought about forty seconds elapsed from the time you first heard the noise to when it returned. That gives an estimate of the altitude to which it rises. I figured they could be off by ten per cent either way. The Seamount event gave a more accurate estimate. I narrowed down the maximum altitude to within three hundred feet. What I've got here is a list of every point on earth which falls along the locus of the orbit and within three bins in altitude, each spanning a hundred feet. With this new precise data of yours, Ellison, we can throw out two-thirds of the possibilities. There are surprisingly few left. Few enough that they can all be checked in a finite time. There are a couple in California , a few in Arizona , a small batch in New Mexico and that's for the continental United States.' He looked on down the list, 'There's a couple of places in Morocco, one in Algeria, some in Iran, Afghanistan, and Pakistan, none in Tibet, it's all too high, and finally, in the northern hemisphere, several places in China.' He flipped to another sheet. 'The southern hemisphere is even more sparse. A few places on either side of the Andes, in Chile and Argentina. That's about it. Everything else is lower, mostly ocean.'

Gantt's brows knitted in concentration.

'We're ahead of you there, Alex,' Danielson smiled slightly, her voice touched with pride. 'I made up a similar list of sites after we got back from La Jolla. Bob Isaacs had ordered up a photomontage along the trajectory several months ago. The problem was we didn't know what to look for, and there was too much area to cover. He just told me that we are collecting new satellite photos of the spots on my list; they'll be ready tomorrow morning.'

She craned her neck and looked down his list, flipping the pages back and forth.

'I think I've got everything you have here, and a few more. Here in Chile , for instance, north of Santiago. There's a shallow valley there and actually two points, not just one, a few kilometres apart.'

She looked up at Runyan, and he locked her eyes with a long, cool stare. Then he gave her a broad, friendly wink, and her heart jumped.

'You said you had three points?' Gantt prompted him to continue.

'This may be a bit more subtle, but just as important.' Runyan leaned forward and put his stack of computer printout on the ground. He retrieved his cup from Gantt rod poured himself a small bit of bourbon. Resting his upper forearms on both knees and rotating the cup between his palms, he looked up at Danielson from beneath his brows. 'Let me ask you, why is there such a small motion with respect to the surface?'

'But you just answered that!' objected Danielson. 'Its motion at its highest point is set by the initial conditions with which it's released. If it moved with the surface at first, it always will.'

'Always?'

Danielson stopped and stared at the bewhiskered scientist, her eyes shifting back and forth between his. Finally she said, 'You said earlier there must be perturbations, friction. The orbit can't be perfect, it must shift slowly with time.'

'Now I'm with you,' broke in Gantt. 'The orbit must shift slowly with time, but it hasn't shifted much.' He looked at both of them. 'So it hasn't had time.'

'That's just the sort of dung I've been trying to compute,' fold Runyan. 'My model isn't perfect yet, but I have some Feeling for the scale of dungs. I would have to say this thing couldn't have been around for more than ten years, and probably less.'