My last day in Stirling I had a choice between the castle, which is a fine one, and the Falkirk Wheel. I've seen enough castles, having visited Donegal and Crac de Chevaliers. So I decided to see this remarkable piece of modern engineering, the Falkirk Wheel.

British Waterways have done a good job trying to establish some kind of Disneyland-type theme park around their engineering, and thus the site entails a large cafe, a children's playground, a trampolining area, a picnic area with 22 tables, a steak bar, a dairy ice-cream hut etc. Indeed they've really gone to town on it. There is a small exhibition about it, and the model in it wasn't operating.

So what's it all about, this Falkirk Wheel thing? It is a device for carrying barges between two canals, and these two canals differ in elevation by 34m. In the old days there was a series of eleven locks joining the canals, but these fell into disrepair in the 1930's and so the canals became separated. This was considered a bit of a shame because the route used to join the east of the country to the west. Oh dear. Somehow British Waterways managed to raise enough money for the project, partly through National Lottery aid, and it was opened by the Queen in 2002.

The Falkirk Wheel

So how technical do you want to be about this wheel then? If you don't want to be technical, you can just say wow, it looks really nice and everything and that's the end of your trip. You can admire the funnily shaped curvey bits on the end of the wheel and say oh yes, that's real Art that is. However, if you did you'd be missing the point, and I expect that most people who visit it do exactly that. I found this written at the exhibition which I thought was great:

If you recall, at the Wallace Monument I discovered that the Scot's claim to grandeur is that James Watt's name is written on nearly every lightbulb in the world. Here they may have decided that they couldn't expect the average Scot to know what a Watt is, and hence they wrote "eight electric kettles" instead of 16kW. How ironic! perhaps the Scottish are not as smart as they give themselves credit for.

My big issue with the wheel is why it should require any electricity to turn it at all. Why don't you just fill the top caisson a bit more full than the bottom one, and let the top-heaviness of the wheel turn it? I had a technical conversation with Alison Lennox, 38, a member of staff who knew quite a lot more about it than the exhibition displays. I was trying to discover why you can't just assume a bit of water-flow out of the Union Canal. "Is that the high or the low end of the Union Canal then?" I asked.

"Neither, the Union Canal is a contour canal and it follows a 240ft contour," she said

"Oh, I see. How long does it go on for then?"

"thirty-one miles."

"Wow, not bad. So how much water comes down from the Union Canal into the Forth and Clyde Canal then?"

"None, in fact we backpump water up into the Union Canal."

"Why do you do that?, isn't there a general flow down?"

"No, the Union canal has a feeder problem so we need to backpump water to it, especially after the boats come down through the locks to get onto the aqueduct."

Well I think that's clear. To people with degrees in engineering perhaps. I'll explain it later. I asked her more:

"So if the whole point of the wheel is to avoid a series of locks, why do boats need to descend through locks to get onto the aqueduct, why don't you just make the wheel a bit bigger to cover more elevation?"

"They wouldn't let us. There is the Antonine wall and a railway line in the way, so we had to build a tunnel underneath them."

That's all crystal clear now. Let me explain: the place to start is with the fact that the two gondolas (aka caissons) that carry the boats are equal in weight; the two arms of the wheel counterbalance each other reasonably exactly. This is actually achieved after the boats have entered the caisson, and the gates have closed to the canal: at this point water is pumped in / out of the caisson until its weight is 360 tonnes. Thus you can expect that the net flow of water between the canals is zero. This assumes that the weight of shipping going up is the same as going down.

The canal at the top, the Union Canal, doesn't have enough water in it, so they are always pumping water up to it, indeed keeping it filled is probably a separate issue, and best not confused with the Falkirk Wheel.

Bingo! The next big question to ask is why didn't they build one of these in the nineteenth century? The Falkirk wheel is not rocket-science. They had suspension-bridges, steam engines and railways then, so why didn't they build one of these instead of having tedious sequences of locks, like the series of 16 locks at Caen hill which lift the canal by 72 metres. I don't know the answer to that. Even if the technology wasn't available then, I would certainly expect someone smart like Brunel to conceive the notion of such a device.

The other thing you can visit at this site is the Antonine wall, which was built 142-154ad, on the orders of Roman emperor Antonius Pius. There isn't a lot left of it, but I found a big ditch running east-west which I guess is the structure, so I climbed onto it. As we saw earlier, Stirling was the gateway to the North of Scotland in the middle-ages, so you'd think that the obvious place to put a wall would be near Stirling; well here it is.

There are forts every two miles along the wall, giving a total of nineteen forts along its length. The wall was abandoned soon after building, around 162ad when the legions withdrew to another wall, 100 miles further south. Although that was not the end of the story, and the Romans did not leave Britain until 400ad, it seems that the area between the two walls was considered something of a buffer-state, and the Romans paid the tribes there in silver to keep the picts of the north in the north. You can see a hoard of that silver in the National Museum.

The Antonine wall