"That right? Geometry is an amazing science! And I thought this was just any old length of twine. May I have a private word, Magnus? And bring your instruments, please."

Magnus came to my office without a quibble. He realised his setting-out string was what had strangled Pomponius. Now I had to decide, did he know that before I produced it or did he simply work out why the knotted twine was in my possession today?

We walked the short distance to my office. Gaius the clerk prepared to leave, but I signalled him to remain as a witness. He sank back on his seat, undecided whether this was to be a routine interview or something more serious.

"You've declared your movements last night, Magnus." For a second the surveyor looked at Gaius. There was no doubt about it. The glance, involuntary and cut short, was enough to make me wonder if my clerk was his pretty boy. Did everyone on this site have unmanly Greek tastes? "One of my team is working on the witness statements, so I've not seen them yet. Remind me, please."

"What team, Falco?"

"Never mind what bloody team!" I snarled. "Answer the question, Magnus."

"I was in my quarters."

"Anyone vouch for that?"

"Afraid not."

"Always the clever witness answer," I told him. "Avoids what sounds like easy collusion, after the event. Genuinely innocent men quite often lack alibis- that's because they had no idea they needed to fix one." It would not clear Magnus but it would actually not condemn him either.

I took the satchel from him and flapped it open on a table. In silence we both studied the neatly ranged equipment, all secured under stitched leather loops. Spare pegs and a small mallet. A pocket sundial. Rulers, including a fine, well-worn folding one marked with both Roman and Greek measurements. Stylus and wax tablets. And a hinged metal pair of mapping compasses.

"Used these today?"

"No."

I carefully released the compasses from their restraining strip of leather, using only my fingertips. I teased them open. Barely visible along one pointed prong was a faint brown stain. But under the leather band into which the instrument had been pushed more staining was obvious.

"Blood," I decided. It certainly was not cartography ink.

Magnus was watching me. He was intelligent, forthright and highly respected on this site. He also hated Pomponius, and had probably clashed with him as many times as anyone except Cyprianus -who seemed a close ally to Magnus. I thought two people had combined to murder the project manager. Those two, perhaps.

I spoke quietly. We were both subdued. "You've worked it out, Magnus. Your five-four-three was unravelled from around the dead architect's neck. That and your set of compasses are the murder weapons. If Pomponius had been impaled on the bath-house floor with yourrowd, you couldn't be in more trouble."

Magnus said nothing.

"Did you kill him, Magnus?"

"No!"

"Short and sharp."

"I did not kill him."

"You're too shrewd?"

"There were other ways to get rid of him from the project. You were here to do that, Falco."

"But I'm working with the system, Magnus. How long would it have taken me? Incompetence is a persistent weed."

Magnus sat quietly. He had chosen an X-shaped stool, one that must once have folded, though I knew it had seized up. Grey-haired and controlled, he had a still core that would not be easily broken into. His grim expression and tone of voice almost suggested it was him testing me, not the other way round.

I put my palms on the edge of the table and pushed back, as if distancing myself from the whole situation. "You don't say much, for a prime suspect."

"You do enough talking!"

"I shall act too, Magnus, if I have to. You always knew that."

"I thought you capable," Magnus agreed. "You had assessed the situation. You would have tackled Pomponius and not necessarily by removing him. You have the ear of high authority, Falco; you even summon up a kind of tact sometimes. You could have imposed workable controls, when you were ready."

I gazed at him. This speech of his was a compliment, yet sounded like a condemnation.

"Well, that's what I thought until this morning, when you came up with the damned idea of bringing Marcellmus back on site Magnus added. He now spoke with pent-up fury.

"He's the King's darling," I replied curtly. Magnus had just told me why the project plotters were against me. They had loathed Pomponius, sure enough- but they did not want him replaced by another disaster. A worse one, maybe. "This morning we had Verovolcus listening in, Magnus. The King, his master, is the client. But don't suppose the client will be allowed to impose a no-hoper on this scheme. If I have to thwart him, believe me I'll do it but I'll do it with sensitivity if possible. If you don't know my views on Marcellinus, Magnus, that's because you never asked."

We glared at one another in silence.

"So if I believed you could handle Pomponius," Magnus muttered at last, 'why would / take the personal risk of killing him?"

I let the Marcellinus issue go, though clearly it needed sorting, and fast.

The surveyor was right. I could just about believe a scenario where he came upon Pomponius at the wrong moment and then snapped suddenly but premeditated killing, when there were other solutions, contradicted this man's natural restraint. Still, self-control would not impress a court as evidence, whereas the murder weapons his possessions- could.

"Risk is not your style," I agreed. "You're too fastidious. But you don't tolerate bungling either. You are vocal and you're active. You are a suspect for this murder precisely because you don't stand back."

"What does that mean?"

"You have strict standards, Magnus. That could make you lose your temper. Yesterday we had all endured a long, irritating day. Suppose you went to bathe, very late, to relax and forget the Mandumerus fiasco. Just when you were calming down, you came to the last hot caldarium. That fool Pomponius was there. You flared up. Pomponius ended up dead on the floor."

"I do not take my five-four-three string inside the baths, Falco."

"Somebody did," I answered him.

"I use a strigil, not a damn set of compasses."

"What's your tool for excavating eyeballs?"

Magnus breathed hard and did not reply.

"Did you see Cyprianus yesterday evening?" I demanded.

"No." Magnus looked at me sharply. "Does he say I did?"

I gave no answer. "There are some half-baked workmen at the baths this morning. Are you part of that?"

"No. I gave Togidubnus an estimate, way back. Anything after that is his affair."

"Is much work needed?"

"Needed- none at all," Magnus opined acidly. "Possible as much as a rich client, urged on by a shameless contractor, wants to waste his money on."

"So you say you are not connected with the wastrels on site today?"

"No."

"Let's get to the main point. Did you go to the bath house last night, Magnus?"

Magnus held back his answer. I waited stubbornly. He continued to maintain his silence, trying to force me to break in, to take back the initiative. He was desperate to know whether I had any firm information.

After an age, he decided what to say. "I did not go to the baths."

Overcome by the tension, the clerk, Gaius, let out a gasp. Magnus kept his eyes on me.

"You're lying, Magnus." My arm gave a wild sweep. I dashed the satchel of instruments right off the table. I then yelled out at full pitch, "Oh shit in Hades, Magnus! Just tell me the truth, will you?"

"Steady, Falco!" Gaius squeaked in great alarm. He spoke for the first time since we came in. His eyes flickered, blinking too rapidly.

I really let my temper rip. "He was at the baths!" I roared at the clerk. "I have a witness who says so, Gaius!" I would not look at Magnus. "If you want to know why I'm raving about it, I thought he was a man of superior quality. I thought I could trust him- I did not want the killer to be him!"