The clock itself is built out of these very long-lived materials, like titanium, 316 stainless steel and ceramic and quartz. So for instance, we just finished grinding, I think, probably the world's largest quartz lens. It's about this big in diameter. And what that does is that when the sun goes overhead at the Summer Solstice, it shines down the shaft and this lens focuses the light onto a box that warms it up and it knows that the sun is directly overhead, and that allows it to synchronise and move, adjust the clock so that it's perfectly on time. And so as long as it gets one of those sunny days once every 20 or 30 years, then it will always stay adjusted. So it's just been a great thing. I get to learn about materials, I get to learn about history of the Mayan calendar system, I get to learn about bells. I mean, one of the neat things in the clock is when you wind it, if it's fully wound, when noon comes, it will play the bells. And it has ten bells. And Brian Eno noticed that the number of permutations of 10 bells is almost exactly equal to the number of days in 10,000 years. So every day for 10,000 years it plays a different permutation of bells. So there's a lot of machinery to do that. But even the bells themselves turned out to be fun. So I invented a new kind of bell which, if I had invented this 400 years ago, would have been a very important invention, but...

Normally, if you want to go down an octave in a bell, you have to make it twice as high, twice as wide, twice as thick. So it's eight times as heavy, so it costs you ten times as much. So to shift an octave down it costs you ten times as much. So that's why you only see a few bells that are like the size of the Liberty Bell or Big Ben or something like that. Mostly bells are this size. Those are the ones that are down an octave and they're very difficult to cast. And so one of the things I realised is that there's a psychoacoustic effect, that the pitch that you hear isn't necessarily the actual pitch that is generated. So these days, with the computer, we can tune bells to have different harmonics. So a normal church bell, for instance, has a minor third in it, which is why it sounds kind of whiney and sad. And people have developed, with computer models, they've actually made bells that have major thirds, which sound a little bit more cheerful. And I realised the same technique could be used to tune the harmonic structure so that you had, for instance, a fifth. And if you built the right harmonic structure, then your ear would hear the pitch to be the least common multiple of all the things in the harmonic structure. So let's say that I want a bell that plays 220A, but I can only afford a bell that plays 440A. I don't want to pay ten times as much for the 220. Well, I can make a bell that's tuned to 440 and it's also tuned to 660, it's also tuned to 880, so every multiple of 220. And so when you hear that bell, the only way your ear can make sense of it is to imagine that it's hearing a 220 bell, and these are just the higher harmonics of it. So your ear will actually hear the pitch as lower than any of the things you're actually hearing. So I explained this to a bunch of bell makers and they all told me it wouldn't work and none of them would make it. But I finally found this bell maker in Australia who knew how to tune these bells and he was willing to try it. And I agreed to pay for the bell. And he made it and it really does work. I can play you the bell sound, but you hear these higher harmonics, but your ear hears it as a lower bell. So I now know how to make a bell at a tenth the cost of a normal bell. And so all the bells that are in the clock are made like that. The last one just got cast.

So the great thing about the clock is it has all these wonderful little details like that, that it's just been such a fantastic way of learning. And also such a fantastic way of engaging with people, because, you know, I get to talk to Brian Eno about music or bell makers about bell-casting, or archaeologists about the Mayan calendar system, or astronomers about the slowing rate of the earth, or... so for me, it's just been a wonderful way into  knowledge about everything. And also, it has accomplished the thing I originally set out to be, which is it has given me a longer-term view of human history. I really do feel a part of something that goes forward 10,000 years as well as back 10,000 years.

So of all the projects I've done, I think it's the thing that is truest to my heart.