 I mentioned in the video with Maureen that tuft is used in the dome of the pantheon. And so I want to give you a little bit more explanation of that, but in a form that's a little closer to one of the case studies that you'll get in this course. So here we're looking at a couple of 3D computer reconstructions of the pantheon in Rome. And hopefully you're aware that this is a monumental round temple based on a typical Roman rectangular temple design. And in fact the viewer coming to see the pantheon would have approached it from this atrium area with the triumphal arch in the foreground. And so their view would have sort of been cropped by the colonnades on either side, and they might enter it initially thinking that this would be like any other Roman temple rather than an enormous domed interior once they get inside. This is a temple that was built by Hadrian to celebrate all of the gods, and there are a series of altars inside, including one to an unknown god. And the altars have an additional role in that they put decoration in front of areas that are actually supporting and holding up the dome, so they help to sort of disguise the structure in a way. And of course what we're most interested in is the construction of the dome itself, which was made of seven distinct layers of Roman concrete. And Roman concrete was something that was really thick and chunky and full of stuff. They used it for a lot of really rapid construction, and then they would put marble or travertine or other type of stone on the exterior in a veneer in order to make it look like it had been constructed way more expensively and with a lot more time involved. And at the Pantheon, they formulated the individual layers of the dome so that they would be varying densities of fill material inside these layers, and you wanted to have, of course, the heaviest layers with the densest fill in the areas closest to the bottom, to the base of the dome, because you want the thrust of the dome to be concentrated downward rather than outward. If the thrust were to be concentrated too far outward, then you have problems with cracking and instability and your dome collapses. And then, of course, in the upper layers of the dome, it becomes thinner, as you can see in the cutaway view, and it also is much lighter. And that's where we have tuft and other lighter materials being used as the fill. Now we're standing inside the Pantheon looking up into the dome. That oculus, or great opening in the center, oculus means eye, is 27 feet, or slightly over 8 meters in diameter. And then you can see, coming outward from it, we have five concentric rings of coffering, and these are the inset squares that grow increasingly larger as we get to closer and closer to the drum of this particular dome. Those coffers serve to lighten the dome itself, so they act with the concrete, and they're also molded out of concrete, to lighten the dome and to make it more self-supporting. The dome itself is 142 feet in diameter, and it's, there's almost a complete, a perfect sphere of space inside the Pantheon. So that diameter that I tell you is from side to side of the drum, but also from the center of the floor to the oculus as well. Here's a wonderful photo taken from a rooftop close to the dome of the Pantheon, and here you can see the exterior of the dome and the great stepped rings that help to further direct the forces of the dome downward rather than outward, and thus make it more stable. And of course, for the interest of this class, we have those seven distinct layers in the concrete of the dome, each one using a different combination of infill materials, and these materials varied from heavy to late, starting with travertine, which is that shelly limestone that polishes up to a shining white like marble. That's what's on the exterior of the Colosseum and so many other buildings in Rome itself. Tuff, which is sort of a compressed volcanic ash and a really favored building material throughout the Roman Empire because it was relatively easy to cut and work, and so it was used to do sort of rapid wall construction and so on. The Romans used a lot of bricks, and those were slightly less dense and heavy than Tuff, and then finally pumice, which is essentially the foam from a volcanic eruption with lots and lots of trapped gases in it that actually floats. This was incredibly important in the uppermost layers of the Pantheon dome. And so when engineers have studied this amazing dome, which has stood perfectly well without any structural problems for nearly 2,000 years, so I'm sorry, they discovered these varying layers of stone, and that's one of the things that you could consider in this course. You could write about the properties of a particular stone like travertine or tuff or pumice and the way that it would be used in things like Roman construction.