We glided on down the line to 1330 to look in on the Church of Our Savior in Chora. The tourists had already seen it down the line in Istanbul under its Turkish name, Kariye Camii; now they saw it in its pre-mosquified condition, with all its stunning mosaics intact and new. “See, there,” I said. “There’s the Mary who married the Mongol. She’s still there down the line. And this — the early life and miracles of Christ — that one’s gone from our time, but you can see how superb it was here.”

The Sicilian shrink holographed the whole church; he was carrying a palm camera that the Time Service regards as permissible, since nobody up the line is likely to notice it or comprehend its function. His bowlegged tempie waddled around oohing at everything. The Ohio people looked bored, as I knew they would. No matter. I’d give them culture if I had to shove it up them.

“When do we see the Turks?” the Ohioans asked restlessly.

We skipped lithely over the Black Death years of 1347 and 1348. “I can’t take you there,” I said, when the protests came. “You’ve got to sign up for a special plague tour if you want to see any of the great epidemics.”

Mr. Ohio’s son-in-law grumbled, “We’ve had all our vaccinations.”

“But five billion people down the line in now-time are unprotected,” I explained. “You might pick up some contamination and bring it back with you, and start a world-wide epidemic. And then we’d have to edit your whole time-trip out of the flow of history to keep the disaster from happening. You wouldn’t want that, would you?”

Bafflement.

“Look, I’d take you there if I could,” I said. “But I can’t. It’s the law. Nobody can enter a plague era except under special supervision, which I’m not licensed to give.”

I brought them down in 1385 and showed them the withering of Constantinople, a shrunken population within the great walls, whole districts deserted, churches falling into ruins. The Turks were devouring the surrounding countryside. I took my people up on the walls back of the Blachernae quarter and showed them the horsemen of the Turkish sultan prowling in the countryside beyond the city limits. My Ohio friend shook his fist at them. “Barbarian bastards!” he cried. “Scum of the earth!”

Down the line to 1398 we came. I showed them Anadolu Hisari, Sultan Beyazit’s fortress on the Asiatic side of the Bosphorus. A summer haze made it a trifle hard to see, so we shunted a few months into autumn and looked again. Surreptitiously we passed around a little pair of field glasses. Two elderly Byzantine monks appeared, saw the field glasses before I could get hold of them and hide them, and wanted to know what we were looking through.

“It helps the eyes,” I said, and we got out of there fast.

In the summer of 1422 we watched Sultan Murat II’s army bashing at the city walls. About 20,000 Turks had burned the villages and fields around Constantinople, massacred the inhabitants, uprooted the vines and olive trees, and now we saw them trying to get into the city. They moved siege machines up to the walls, went to work with battering rams, giant catapults, all the heavy artillery of the time. I got my people right up close to the battle line to see the fun.

The standard technique for doing this is to masquerade as holy pilgrims. Pilgrims can go anywhere, even into the front lines. I distributed crosses and icons, taught everybody how to look devout, and led them forward, chanting and intoning. There was no hope of getting them to chant genuine Byzantine hymns, of course, and so I told them to chant anything they liked, just making sure it sounded somber and pious. The Ohio people did The Star Spangled Banner over and over, and the shrink and his friend sang arias from Verdi and Puccini. The Byzantine defenders paused in their work to wave to us. We waved back and made the sign of the Cross.

“What if we get killed?” asked the son-in-law.

“No chance of it. Not permanently, anyway. If a stray shot gets you, I’ll summon the Time Patrol, and they’ll pull you out of here five minutes ago.”

The son-in-law looked puzzled.

“Celeste Aida, forma divina—”

“…so proudly we hailed—”

The Byzantines fought like hell to keep the Turks out. They dumped Greek fire and boiling oil on them, hacked off every head that peered over the wall, withstood the fury of the artillery. Nevertheless it seemed certain that the city would fall by sunset. The evening shadows gathered.

“Watch this,” I said.

Flames burst forth at several points along the wall. The Turks were burning their own siege machines and pulling back!

“Why?” I was asked. “Another hour and they’ll have the city.”

“Byzantine historians,” I said, “later wrote that a miracle had taken place. The Virgin Mary had appeared, clad in a violet mantle, dazzlingly bright, and had walked along the walls. The Turks, in terror, withdrew.”

“Where?” the son-in-law demanded. “I didn’t see any miracle! I didn’t see any Virgin Mary!”

“Maybe we ought to go back half an hour and look again,” said his wife vaguely.

I explained that the Virgin Mary had not in fact been seen on the battlements; rather, messengers had brought word to Sultan Murad of an uprising against him in Asia Minor, and, fearing he might be cut off and besieged in Constantinople if he succeeded in taking it, the sultan had halted operations at once to deal with the rebels in the east. The Ohioans looked disappointed. I think they genuinely had wanted to see the Virgin Mary. “We saw her on last year’s trip,” the son-in-law muttered.

“That was different,” said his wife. “That was the real one, not a miracle!”

I adjusted timers and we shunted down the line.

Dawn, April 5, 1453. We waited for sunrise on the ram-part of Byzantium. “The city is isolated now,” I said. “Sultan Mehmet the Conqueror has built the fortress of Rumeli Hisari up along the European side of the Bosphorus. The Turks are moving in. Come, look, listen to this.”

Sunrise broke. We peered over the top of the wall. A deafening shout went up. “Across the Golden Horn are the tents of the Turks — 200,000 of them. In the Bosphorus are 493 Turkish ships. There are 8,000 Byzantine defenders, 15 ships. No help has come from Christian Europe for Christian Byzantium, except 700 Genoese soldiers and sailors under the command of Giovanni Giustiniani.” I lingered on the name of Byzantium’s last bulwark, stressing the rich echoes of the past: “Giustiniani… Justinian—” No one noticed. “Byzantium is to be thrown to the wolves,” I went on. “Listen to the Turks roar!”

The famous Byzantine chain-boom was stretched across the Golden Horn and anchored at each bank: great rounded logs joined by iron hooks, designed to close the harbor to invaders. It had failed once before, in 1204; now it was stronger.

We jumped down the line to April 9, and watched the Turks creep closer to the walls. We skipped to April 12, and saw the great Turkish cannon, the Royal One, go into action. A turncoat Christian named Urban of Hungary had built it for the Turks; 100 pair of oxen had dragged it to the city; its barrel, three feet across, fired 1,500-pound granite projectiles. We saw a burst of flame, a puff of smoke, and then a monstrous ball of stone rise sleepily, slowly, and slam with earthshaking force against the wall, sending up a cloud of dust. The thud jarred the whole city; the explosion lingered in our ears. “They can fire the Royal One only seven times a day,” I said. “It takes a while to load it. And now see this.” We shunted forward by a week. The invaders were clustered about the giant cannon, readying it to fire. They touched it off; it exploded with a frightful blare of flame, sending huge chunks of its barrel slewing through the Turks. Bodies lay everywhere. The Byzantines cheered from the walls. “Among the dead,” I said, “is Urban of Hungary. But soon the Turks will build a new cannon.”