As the white-clad maiden made her way along the well-trodden route, Cloelia grabbed Helena and me by the hand and towed us along after her.

PAST THE DUST and commotion of the huge building site for the Flavian Amphitheatre and then beyond the massive plinth for the Temple of Claudius, which Vespasian was also at last completing out of gratitude to his political patron, lay the Caelian Hill. This quiet, wooded haven looks south over the Capena Gate and the Circus Maximus. It is one of the most ancient, unspoiled parts of the city, the rocky hillside rich with springs. They were originally the province of water goddesses called the Camenae, but the nymph Egeria, saucy lass, rather usurped their dominance. Here is the famous grove where King Numa Pompilius consulted (his word for it) the darling nymph night after night while she (he alleged) dictated political edicts to him; here too is the spring named after his lovely, helpful muse, to which the Vestals daily traipse.

Egeria’s Spring must have been extremely handy for the Palace of King Numa. He would not have had too long a stroll in his search for inspiration. (One more example, Helena explains to me, of a dumb but well-intentioned man in power being brought to greater glory than he ever deserved by a much more intelligent lady friend.) Egeria kept old Numa going strong to over eighty, anyway.

Constantia approached the ancient watering hole with the stately gait that her sisterhood cultivates. Carrying a water vessel on the head is supposed to improve the posture; it certainly draws attention to a full womanly figure in a way that is not supposed to happen with the damsels in white. Having a girdle tied in a Hercules knot right under a well-rounded bust is bound to draw attention to the bust. Generations of Vestals have probably been well aware of this. Constantia no doubt viewed such thoughts with disdain. She looked to be in her early twenties; she must have completed the first ten years of learning her duties and was now equipped to carry them out in a reverential-though slightly distracting-style.

While Constantia was filling the pitcher, Helena Justina took Cloelia by the hand and-with gestures to me to wait behind-they walked sedately forwards. Helena addressed the Virgin by name. The lictor immediately told Helena to get lost. Offered the threatening points of his ceremonial rods, she backed away.

Constantia, perhaps long practiced, had ignored the small flurry as her petitioners were discouraged. Now the pitcher was full it was much heavier; she needed to concentrate. She swung it up on to her head, straight-backed and superior. I began to appreciate that the complex arrangements of braids worn by the Virgins might actually make a coiled mat to support their water jars and save them bruised heads. Eyes straight ahead like a tightrope walker, the Vestal moved to retrace her steps back to the Forum. She held her free arm very slightly apart from her body for balance, but mainly swayed gently as women in far-off provinces do as they visit wells outside their mudhut villages, appearing to enjoy their carrying skills.

The stones around Egeria’s shrine were green with slimy algae. Constantia seemed to be prepared for trouble. When her foot slipped, she regained her balance with commendable aplomb. Only a little water slopped out of her jar. It probably happened every day-and every day, Constantia probably looked just as annoyed when her ankle turned.

Helena was still standing nearer than I was. I think what she muttered to me afterwards, keeping it quiet from Cloelia with a genuinely shocked air, must have been a mistake. She surely misheard what Constantia had gasped as she skidded.

“Well, you believe what you like, Marcus. You are so innocent, I expect you would have thought Numa Pompilius was just a man who liked to work with a female secretary. Egeria proved to be efficient, and of course he never laid a finger on the nymph… But I could swear that when the venerable Virgin nearly turned her ankle, she winced and cursed.”

Little Cloelia looked up scornfully. “Of course she did, Helena. She said ‘Balls!’ ”

WE TRAILED Constantia all the way back to the House of the Vestals, keeping at a safe distance in case the lictor got frisky with his rods. Helena, who could be sensationally persistent, went straight back to the door porter and asked if her request for an interview had been considered yet. Far too soon for an answer. Ladies who lead lives of traditional simplicity observe the traditional rules for correspondence too: they do not follow up messages until the feast has gone cold.

Constantia herself had an excuse: ferrying water from the shrine. But do not imagine the Virgins are so geared to simplicity they read letters from the public personally. They have a large staff, and it certainly includes secretaries.

No, of course I don’t think they employ the secretaries to write their love letters. Saying that would be blasphemy.

We made our second attempt at going home. Leaving the enclosure on the Sacred Way side this time, we emerged onto the small Street of the Vestals opposite the Regia-once the grand Etruscan Palace of Numa Pompilius, aforementioned aficionado of nymphs. I shrugged off the swathes of my toga and slung that hot, hated garment over my shoulder casually.

The Regia had long ago ceased to be occupied domestically, and few traces now remained of whatever ancient buildings had once occupied the site. It was a sacred area, used for centuries by the College of Pontiffs. They know how to earmark good accommodation. Some consul had rebuilt everything in sight using his spoils of war, a plunder so magnificent he had been able to floor and wall the new edifice with solid white and gray marble. As a result, this strongly constructed area had survived the Great Fire when all the huge patrician houses farther along the Sacred Way had been swept to destruction. Facing us now were the Temple of Mars, containing the spears that generals shook before departing for battle; an integral vestibule; and the Temple of Ops, the old-fashioned goddess of plenty, which only the Vestals and Pontifex Maximus were allowed to enter. To our right, at the far end of the complex, was a small porch, under whose columns we saw a disturbance.

A litter with an eagle on top and purple curtains was being lifted by bearers, who set off at a smart pace. Noisily tramping ahead went a phalanx of plumed helmets: Praetorian Guards. As they spread across the road, looking for more scope to knock passersby aside, we knew we were witnessing the departure of the Emperor. Presumably, he had been there in his capacity as Pontifex, pootling around the priestly college on some religious business.

I would have thought nothing of it. But a crowd of hangers-on had been waiting for Vespasian to leave. As they now scattered, one man broke free of the rest; he was going at a fast lick. He saw me. A relieved expression lit his face. He slowed up.

“Falco! What a coincidence-I was sent out to find you. I thought it would take me half the day.”

I recognized him. I last saw him in Lepcis Magna, just a few weeks back. A calm, sensible slave, he attended the Emperor’s envoy, Rutilius Gallicus. At present the last thing I wanted was a social invitation from the man who gave the order to send my brother-in-law to the lions. But nobody issues their dinner invitations from the Regia. This was about something else. As I suspected, the message for me was to see Rutilius urgently-on official business. There had to be a religious connection. However, I did not suppose it would involve geese or chickens.