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Language: en-GB

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This is the Falkirk Wheel, and it's one of
the greatest bits of modern industrial design

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in the world.

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On the top of that hill is the Union Canal,
and down here is the Forth and Clyde Canal.

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And the traditional way to get boats between
them was a flight of locks, which'd take about

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a day to get through, and besides, they were
dismantled here in the 1930s. The solution:

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the world's only rotating boat lift.

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Now boat lifts aren't a new thing. They've
been around for centuries. Most of them, like

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this, have two caissons, which is the technical
term for the tubs that the boats sit in, although

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usually they're next to each other rather
than rotating round a central point. And the

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reason for two is this: to lift 500 tonnes
of water 24 metres up, you need a minimum

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of 32 kilowatt-hours of power. That's about
what an average British house uses in three

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days, or an average American house in one
day. And that's before friction, that's just

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the potential energy you need to put into
the system to raise it up that far. But: if

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you're also having the same weight of water
descend at the same rate, at the same time,

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then you're not adding any potential energy
at all. On average, nothing is going up or

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down. You're just getting a really heavy thing
moving, and then stopping it again. Because

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of that, this wheel only needs one and a half
kilowatt-hours for one lift: less than a twentieth

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of what it'd need otherwise.

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There are even some boat lifts in the world
powered entirely by gravity, where local conditions

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and the architecture permit it: you just fill
the top one with a bit more water, let it

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go, and then apply the brakes when you need
to. Bit tricky with something that's as fancy

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as this though. It has to be massively reinforced,
there is an incredible amount of momentum

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there.

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And here's the really clever bit: remember
Archimedes' principle, the Greek bloke with

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the bathtub. A floating object displaces an
amount of water equal to its weight. An object

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that sinks, sure, that displaces equal to
volume, but a floating object like a boat

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displaces water equal to its weight. When
they're full of water, those caissons weigh

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about 500 tonnes. You add ten tons of floating
boat into one of them, then ten tons of water

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flows out into the canal.

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Which means this is balanced no matter how
much is actually floating in it.

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