Bolanus frowned. `It's horrible to imagine someone going into a conduit with a human hand or foot in his pocket-'

`Foot?'

`One turned up here once.' I wondered how many other grim discoveries we were going to hear about. `Then he would have to wait until he was certain none of his work mates was looking when he threw it in.'

`Stupidity. Would it be worth the risk?'

`Taking the risk might be part of the thrill,' Bolanus suggested. -

I wondered whether he was revealing too much understanding of the killer's mind. After all, he worked on the aqueducts himself and as an engineer's assistant he could make inspections alone if he wanted to. He would also be well placed to hear about any enquiry, and attach himself to it so he could check what was going on.

Unlikely. Yes, he was, a loner, because of his specialist knowledge. But this was a man who made things work, not one who destroyed and hacked up women out of some dark inhuman motive. Bolanus was one of the skilled world movers who built the Empire and kept it in trim. Still, the killer too, with years of undetected crime behind him, must have his own efficiency. If we ever identified him, I knew clues to his madness would be there – and yet he would be somebody who had lived in society without arousing qualms in those he met. The real terror in such men is how closely they resemble the rest of us.

`You may be right,' I said, deciding to test Bolanus anyway. I didn't want to end up as the dumb informer who let himself be led all round the problem by some helpful volunteer, only to find after weeks of frustration that the volunteer was the real quarry. It's been done often enough. Too often. `His main thrill will be in assuming power over his victims. When we find him, he'll be somebody who hates women.'

`The odd man out in the crowd!' Bolanus jeered.

`He finds them awkward to approach; when he tries it they probably laugh at him. The more he resents them for their rejection, the more they sense trouble and shrink from him.'

`Sounds like every boy's nightmare.!

'But it's out of all proportion, Bolanus. And unlike most of us, he never learns to take a chance. He's more than just an awkward character. He has an inbuilt flaw so he doesn't want to win anybody over, and they know it. This man is locked in his refusal to communicate properly, whereas the rest of us make a lot of mistakes along the way but if we're lucky we do manage a few winning throws too.'

Suddenly Bolanus grinned, looking nostalgic. `And when we do, it's magic!'

That seemed all right.

Of course addictive killers are usually also cunning liars who can act well. This man could be one of those, a manipulating fraud who knew just what I wanted to hear. So pervertedly clever he could counterfeit normality and outwit me at every move.

'It could be me or you,' suggested Bolanus, as if he knew what I was thinking. He was still munching his snack. `He's not going to stand out like some mad-eyed monster, or he would have been apprehended years ago,.'

I nodded. `Oh yes, he probably looks very ordinary.'

Again, he gave me a narrow look, as if he read my mind.

We went back to discussing how the killer was disposing of the bodies.

`You know the water boatmen find torsos in the river as well?'

'Makes sense, Falco. He might have found a way to float the hands down the aqueducts, but the torsos are too., large. They would stick. The killer is presumably trying to disperse the pieces over a wide area to avoid being traced, so he certainly doesn't want a regular blockage half a mile from where he lives.'

`Right.'

Bolanus offered me his picnic again, but I had gone off the idea. `How long have you known about the finds in the aqueducts, Bolanus?'

`It goes back beyond my time.'

`How long's your career?'

`Fifteen years. I learned my stuff originally abroad in the legions, got invalided out, then came home just at the right time to work on the dams Nero built at his big villa at Sublaqueum. That's on the River Anio, you know – which is also the source of the four Sabine aqueducts.'

'Is this relevant?'

`I think it might be. As far as I know, the body parts only turn up in certain places in our system. I'm starting to have a little theory about this,' I perked up. A theory from Bolanus might be one to respect. `I became something of a specialist in all the aqueducts that come from the Anio.'

`These are the long ones built by Caligula and Claudius?'

`And the old monster, the Anio Vetus.'

`I've seen them marching across the Campagna, of course.'

`A grand sight. That's when you know why Rome rules the world. They pick up good cold water from the river and the springs in the Sabine Hills, take a detour around the gracious homes at Tibur, and travel for miles to get here. It's a staggering engineering feat. But let me tell this; my own way -'

`Sorry.' His theories might be sound, but I felt a sudden terror of his rhetoric. I had talked to engineers before. For hours, and hours. `Do go on, friend.'

`Let's jump back a bit. You had a spat this morning with Statius about the Aqua Alsietina.'

`He wanted us to ignore it. Have there been any grisly finds there?'

No. In my opinion it can safely be ignored. It comes from Etruria west of us – and I don't reckon the killer goes anywhere near it. Nor the Aqua Virgo either.'

`Isn't that the one Agrippa built specially for his baths near the Saepta Julia?' I knew the Saepta well. Apart from being a traditional haunt of informers, which I had to avoid to ensure I never encountered my low-class colleagues, the Saepta was full of antique dealers and jewellers – including my father, who had an office there.' I liked to avoid Pa too.

`Yes. The Virgo is drawn from a marsh near the Via Collatina, and it's almost entirely underground. I'd also rule out the Aqua Julia and the Tepula.'

'Why them?' I asked.

`I've never heard of anything that relates to these killings being discovered in either. The Julia has its source in a reservoir only seven miles outside Rome on the Via Latina. The Tepula isn't far from it.'

`Near the Alban Lake?'

`Yes. The Julia and Tepula come into Rome carried on the same arcades as the old Aqua Marcia – and that's where my theory might creak a bit, because the Marcia has had finds in

it.,

'Where does the Marcia come from?'

Bolanus opened his hand in a triumphant gesture. `It's one of the big four from the Sabine Hills!'

I tried to look as if I understood the significance. `Are all these various conduits linked at all? Can water be transferred between them?'

`They are indeed!' Bolanus seemed to think he was teaching me logic. `There are places throughout the network where water from one aqueduct can be diverted into another if we need extra supplies, or if we want to close part of the system to work on it. The only constraint is that you have to divert downwards from a high aqueduct to a lower one. You can't lift water up. Anyway, once they get here the Claudia, Julia and Tepula share one reservoir. That might be of interest. What could also be relevant is that the Marcia has a major link with the Claudia. The Claudia arrives in Rome with the Anio Novus; they are both carried on arcades which join on one set of arches near the city.'

`In one channel?'

`No, two. The Claudia was built first. It's coupled, underneath.' He paused. `Look, I don't want to confuse you with technicalities.'

Now you're sounding like bloody Statius.' He was right though; I had had enough, of this.

`All I mean to say is that I wouldn't be surprised if the human hands that turn up in Rome had been put into the water well outside the city.'

`You're saying they enter the system way back – before the channels are covered or go underground?'