If you wanted to sort out a lot of the world’s problems, you’d take George Bush and Mahmoud Ahmadinejad and send them on a canal holiday.

As a recipe for world peace, you could do a lot worse. There’s something about narrowboats that makes you incapable of doing an unkind thing or thinking a negative thought. In the first two hours on the Llangollen Canal, I was wished “good morning” 23 times.

As most people went on to comment, it was indeed a very nice day, unseasonably mild, with intense splashes of sun between the scudding clouds. But that wasn’t the point: I got the feeling I’d have received the same treatment in the January sleet, or if a nuclear strike had just taken out Wrexham and the mushroom cloud was looming.

All this bonhomie was an unexpected bonus, because my brother-in-law and I hadn’t come up to Chirk Marina, on the north Welsh borders, to have our faith in humanity restored. We’d come to see one of the wonders of the world.

It isn’t officially recognised as such yet, but it soon might be: the Pontcysyllte aqueduct on the Llangollen Canal is going to be Britain’s nomination for Unesco World Heritage status next year — which would put it in the same league as the Taj Mahal, Machu Picchu and the Great Wall of China.

Ambitious stuff for what is, after all, just a bridge with water in it. But in its day — it was completed in 1805, having been 10 years in the building — Pontcysyllte was the toast of Britain. It was then, and remains now, the longest and highest navigable aqueduct in the world, a technical marvel that nobody believed the engineers would pull off.

It’s 1,007ft long and 127ft high, and the ribbon of water it hoists miraculously above the flood plain of the meandering River Dee is contained in a vast cast-iron trough, produced in sections sealed together by Welsh flannel boiled in sugar, bedded into mortar made of lime and ox blood.

It all sounds fantastically unlikely, but, 200 years on, it’s still doing exactly what it was designed for: carrying a canal through midair. We were both schoolboyishly keen to try it out, but it was no foregone conclusion that we’d get there. First we had to get under the bridges.

LEAVING CHIRK Marina with Pete the boatman’s baffling volley of operating instructions still ringing in our ears (“It’s imperative that you grease your nipples morning and night,” he told us, a ritual we observed religiously), we started getting to grips with the steering.

A narrowboat is, in essence, a 12-ton bathtub with a motor at the back, and its handling characteristics are everything you’d expect: it doesn’t have any.

Pootling up and down the Llangollen Canal takes you through some lovely countryside — rolling moorland, looming Welsh hills, that sort of thing — and most of the time you can just relax, watch the kingfishers darting and chat to the sheep.

But every 20 minutes, you’ll meet a cutesy humpback bridge, where the waterway narrows sharply and there’s only a couple of inches clearance on either side of the boat.

It’s tricky. Approaching at the speed of a gouty stick insect, you squint down the length of the boat to judge you’re a smidgen too far to the left, and adjust the tiller accordingly. You wait for a bit and nothing happens.

The bridge is quite close now, so you wrench the tiller savagely, hoping for some effect, and a second later the thing spins irreversibly clockwise and, with a gentle 12-ton crump, the prow dislodges a piece of crucial masonry, bringing the entire bridge down on top of you.

Well, we didn’t actually demolish any bridges, but we came pretty close. So it was something of a relief when we rounded the corner to approach the aqueduct.

In front of us stretched a ruler-straight strip of water. To one side, strollers clanked along a metal walkway, holding tight to the guard rail. And to the other? Nothing. Air. Standing on the gunwale, you’re just a step and a few seconds’ free fall from the tumbling, spuming River Dee.

We crossed slowly, taking turns to hang off the side over the sheer drop, like the stupid teenagers we’d suddenly become. Embarrassing, but worth it: the view down the valley was stunning; the sense of space and air overwhelming. No wonder that, at the time, the Pontcysyllte aqueduct was touted as the closest mankind had got to flying.

On the other side, I had a chat with Peter Birch, the local heritage and environment manager for British Waterways. “This was the first structure engineers had built that really caught the public imagination,” he said. “People came from all over the country to marvel at it, society artists travelled up from London to paint it.

It was so audacious, it gave British engineers tremendous confidence: they thought, ‘Well, if we can do this, we can do anything.’ ” It’s that marriage of efficiency and aesthetics that really makes the aqueduct so thrilling. The 18 tapered stone piers that stride across the valley dovetail perfectly with the 19 cast-iron arches, gleaming in their black paint, that carry the canal.

The elegance of form and line is born of slide rules and equations, rather than some wild artistic vision, but it’s no less lovely for that. This is engineering as sculpture.

THERE’S a final irony to the tale of Pontcysyllte: really, it should never have been built at all. Buoyed up by canal-mania, overoptimistic investors pumped the vast sum of £45,000 into a bridge that, in the words of the architecture critic of The Sunday Times, Hugh Pearman (another Pontcysyllte enthusiast), “went nowhere much and then stopped”. It was a financial disaster.

Until now. Ten-thousand boats and 25,000 pedestrians crossed the bridge last year, not as part of the commercial traffic it was designed for, but simply to experience the wonder of the thing. In the process, they brought £66m to the local economy. The Pontcysyllte aqueduct is finally earning its keep, not because it does anything useful, but because it looks so damn splendid while it doesn’t do it.

It was time to turn back to Chirk, and proudly display our well-greased nipples to Pete the boatman. Confident I’d now mastered the steering, I slung the stern of the barge out and went for a three-point turn in the tiny marina at the north end of the aqueduct.

With the benefit of hindsight, the result was predictable. In the half-hour it took to get the thing pointing the right way, we caused a narrowboat jam that stretched back for 300 yards. Finally, a head thrust out from the window of the boat behind us, and I steeled myself for the tirade of abuse.

“Not to worry,” it said. “I’ll put the kettle on.”

Politeness and tolerance. Narrowboats and tea. That’s what George and Mahmoud need. Forget the UN, the key to averting the third world war is a nice weekend in Pontcysyllte. If it is declared a world wonder, maybe they could hold a summit here. Only one problem: they’d never be able to pronounce it.

Need to know

Drifters (0845 762 6252, www.drifters.co.uk ) has three-night short breaks in a four-berth narrowboat, moored at Chirk Marina on the Llangollen Canal, from £369 in low season. The aqueduct is off the A5, nine miles north of Oswestry. If you get lost, ask a local for directions: it’s pronounced Pont-ker-sill-ter

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