He also took a bag of peanuts and, after some thought, a large ball of string. He bit off a short length of the string and used it-to tie the badge around his neck, like a talis­man. Then he tied one end of the ball to the desk and, after a moment's contemplation, knuckled off between the bookshelves, paying out the string behind him.

Knowledge equals power. . .

The string was important. After a while the Librarian stopped. He concentrated all his powers of librarianship.

Power equals energy . . .

People were stupid, sometimes. They thought the Li­brary was a dangerous place because of all the magical books, which was true enough, but what made it really one of the most dangerous places there could ever be was the simple fact that it was a library.

Energy equals matter. . . .

He swung into an avenue of shelving that was appar­ently a few feet long and walked along it briskly for half an hour.

Matter equals mass.

And mass distorts space. It distorts it into poly-fractal L-space.

So, while the Dewey system has its fine points, when you're setting out to look something up in the multidi­mensional folds of L-space what you really need is a ball of string.

Now the rain was trying hard. It glistened off the flag­stones in the Plaza of Broken Moons, littered here and there with torn bunting, flags, broken bottles and the oc­casional regurgitated supper. There was still plenty of thunder about, and a green, fresh smell in the air. A few shreds of mist from the Ankh hovered over the stones. It would be dawn soon.

Vimes's footsteps echoed wetly from the surrounding buildings as he picked his way across the plaza. The boy had stood here.

He peered through the mist shreds at the surrounding buildings, getting his bearings. So the dragon had been hovering-he paced forward-here.

"And," said Vimes, "this is where it was killed."

He fumbled in his pockets. There were all sorts of things in there - keys, bits of string, corks. His finger closed on a stub end of chalk.

He knelt down. Errol jumped off his shoulder and wad­dled away to inspect the detritus of the celebration. He always sniffed everything before he ate it, Vimes noticed. It was a bit of a puzzle why he bothered, because he always ate it anyway.

Its head had been about, let's see, here.

He walked backwards, dragging the chalk over the stones, progressing slowly over the damp, empty square like an ancient worshipper treading a maze. Here a wing, curving away towards a tail which stretched out to here, change hands, now head for the other wing . . .

When he finished he walked to the centre of the outline and ran his hands over the stones. He realized he was half-expecting them to be warm.

Surely there should be something. Some, oh, he didn't know, some grease or something, some crispy fried dragon lumps. Errol started eating a broken bottle with every sign of enjoyment. "You know what I think?" said Vimes. "I think it went somewhere."

Thunder rolled again.

"All right, all right," muttered Vimes. "It was just a thought. It wasn't that dramatic."

Errol stopped in mid-crunch.

Very slowly, as though it was mounted on very smooth, well-oiled bearings, the dragon's head turned to face up­wards.

What it was staring at intently was a patch of empty air. There wasn't much else you could say about it.

Vimes shivered under his cape. This was daft.

"Look, don't muck about," he said, "there's nothing there."

Errol started to tremble.

"It's just the rain," said Vimes. "Go on, finish your bottle. Nice bottle."

A thin, worried keening noise broke from the drag­on's mouth.

"I'll show you," said Vimes. He cast around and spotted one of Throat's sausages, cast aside by a hungry reveller who had decided he was never going to be that hungry. He picked it up.

"Look," he said, and threw it upwards.

He felt sure, watching its trajectory, that it ought to have fallen back to the ground. It shouldn't have fallen away, as if he'd dropped it neatly into a tunnel in the sky. And the tunnel shouldn't have been looking back at him.

Vivid purple lightning lashed from the empty air and struck the houses on the near side of the plaza, skittering across the walls for several yards before sinking out with a suddenness that almost denied that it had ever happened at all.

Then it erupted again, this time hitting the rim ward wall. The light broke where it hit into a network of search­ing tendrils spreading across the stones.

The third attempt went upwards, forming an actinic column that eventually rose fifty or sixty feet in the air, appeared to stabilize, and started to spin slowly.

Vimes felt that a comment was called for. He said: "Arrgh."

As the light revolved it sent out thin zigzag streamers that jittered away across the rooftops, sometimes dipping, sometimes doubling back. Searching.

Errol ran up Vimes's back in a flurry of claws and fas­tened himself firmly on his shoulder. The excruciating agony recalled to Vimes that there was something he should be doing. Was it time to scream again? He tried another "Arrgh". No, probably not.

The air started to smell like burning tin.

Lady Ramkin's coach rattled into the plaza making a noise like a roulette wheel and pounded straight for Vimes, stopping in a skid that sent it juddering around hi a semi­circle and forced the horses either to face the other way or plait their legs. A furious vision in padded leather, gauntlets, tiara and thirty yards of damp pink tulle leaned down towards him and screamed: "Come on, you bloody idiot!"

One glove caught him under his unresisting shoulder and hauled him bodily on to the box.

"And stop screaming!'' the phantom ordered, focusing generations of natural authority into four syllables. An­other shout spurred the horses from a bewildered standing start to a full gallop.

The coach bounced away over the flagstones. An ex­ploratory tendril of flickering light brushed the reins for a moment and then lost interest.

"I suppose you haven't got any idea what's happen­ing?" shouted Vimes, against the crackling of the spin­ning fire.

"Not the foggiest!"

The crawling lines spread like a web over the city, growing fainter with distance. Vimes imagined them creeping through windows and sneaking under doors.

"It looks as though it's searching for something!" he shouted.