Brunelleschi, Dome of the Cathedral of Florence.

STEVEN ZUCKER: We're in Florence. And we're standing outside of the Duomo. BETH HARRIS: The cathedral of Florence, and we're looking up at Brunelleschi's dome. STEVEN ZUCKER: It's huge. Until St. Peter's, it was the highest dome that had ever been raised. And in its width, it was as wide as the Pantheon. BETH HARRIS: Almost. STEVEN ZUCKER: If you think about the Duomo itself had been planned in the 14th century. BETH HARRIS: The plan was to build a dome that had a span nearly equal to that of the Pantheon. STEVEN ZUCKER: And of course, the Pantheon had been built in the ancient world. And that technology had largely been lost. BETH HARRIS: So first and foremost, what Brunelleschi did was an amazing engineering achievement. The challenge was how to build a dome this wide without wooden centering. Generally, when you build an archway-- and the dome is really just an arch-- STEVEN ZUCKER: In the round. BETH HARRIS: --in the round-- you put up a wooden framework. STEVEN ZUCKER: So this is the wood to actually support the dome until it can be locked in place by the keystone. BETH HARRIS: Exactly. So you don't even really need mortar to hold it together because you've got the keystone. STEVEN ZUCKER: The problem is that this was so big, they couldn't actually get enough lumber, and lumber that was strong enough, to hold the thing up until they could lock it in place. BETH HARRIS: And so there was no way to do wooden scaffolding or centering to hold it up as it was being built. So how do you build this dome that inclines inward and not have it fall down? STEVEN ZUCKER: There's two problems. You've got that issue. And then you've got the problem of it wanting to splay outward. BETH HARRIS: A dome exerts pressure not only down, but down and out. And so one of the biggest challenges is how to raise the dome and deal with that downward and outward pressure, not cracking the walls underneath. STEVEN ZUCKER: Now, in the ancient world, for the Pantheon, for example, they had dealt with that by just creating sheer bulk. In other words, the walls got to be 10 feet thick. BETH HARRIS: I think, actually in the Pantheon, they're something like 20 feet thick of concrete. STEVEN ZUCKER: But Brunelleschi couldn't do that here. So what he's done instead is, first of all, he made the decision to make the dome as light as possible. And that means that it's basically hollow. It's a double shell. And within the shell is a staircase that snakes around that allows one to actually get to the top. And if you look, you can see people just below the lantern, up at the top of the dome, taking in the view of the city. BETH HARRIS: He also created ribs. STEVEN ZUCKER: Which are doing a lot of the weight bearing. BETH HARRIS: And then in between each of the major ribs, which are visible on the outside, there are two within that we can't see. STEVEN ZUCKER: And those are actually locked in place by a series of horizontals, as well. So there's this whole skeletal structure that's actually holding this piece together. I think, most importantly, he was able to develop a system where, as the dome was being raised up, as each course of stone and brick was added, it was actually locking itself in place. And so it was self sustaining. BETH HARRIS: Another way that Brunelleschi dealt with the downward and outward thrust was to create chains inside the dome made out of stone and wood, locked together with iron, like a girdle, to hold the dome in and to counter that downward and outward thrust. STEVEN ZUCKER: You might think of an old-fashioned wooden barrel that has a couple of iron rings around it to help keep the wood together. BETH HARRIS: Brunelleschi created cantilevered scaffolding. STEVEN ZUCKER: That could rise as the building went up. BETH HARRIS: And so the workmen had a place to work. Brunelleschi also built new kinds of pulleys and hoists to bring up his heavy, massive pieces of stone to the top of the dome. So he created this ox hoist, just these remarkable machines that no one had ever seen before. STEVEN ZUCKER: He actually even designed a special barge to go down the Arno to be able to bring the materials to the city itself. If you think about the sheer quantity of material that had to be imported, and had to be hoisted up, and had to be put in place, it is just this remarkable project. BETH HARRIS: Bricks that had to be created, stone that had to be quarried and brought here, platforms for the workmen to work on, machines to hoist everything. And I think it was Alberti who said something like, what Brunelleschi did, he did without-- STEVEN ZUCKER: --without a precedent. BETH HARRIS: Without having any example to lean on. STEVEN ZUCKER: Utter invention. Now, we think that Brunelleschi may have gone to Rome and may have studied ancient architecture as well as sculpture there. But there is no precedent in the ancient world, even, for what Brunelleschi accomplished here. BETH HARRIS: Now, it's important to say that the dome is not hemispherical, like the dome on the Pantheon. STEVEN ZUCKER: It's actually kind of tall. BETH HARRIS: Right. It's kind of pointed. In a way, it has more of a Gothic shape than a classical shape. But in that way, it matches the Gothic church itself. If you look closely, you can see these exedrae, or blind tribunes, that Brunelleschi added around the outside of the dome. They actually look very classical compared to the Gothic church there. In fact, look like Roman triumphal arches. So there's this curious classical moment here in an otherwise very Gothic church. STEVEN ZUCKER: And it's a church that is not only Gothic, but really referring back to the Tuscan Romanesque tradition. Especially in terms of the polychromy, the colored marbles, which Brunelleschi also carries up into the barrel just below the dome itself. But ultimately, you've got Brunelleschi, who, through his engineering genius, is solving a problem the Western tradition had never been capable of solving before. How does one span this enormous space? And in order to do it, he's surpassing the ancients that he's even here paying reverence to.