 Now, this model is even more Earth-like than the previous two. There's two big differences here. This model doesn't have a core in it at all. And also, you need to have a little bit of imagination here because what I'm hoping to have you think about is that you've got this gradation from dark to light that's smooth. I know you can see rings here, but pretend they're not there. So, what this model would show is if the Earth only had a mantle and it successfully got denser and denser and denser and denser and denser from the outside to the inside, therefore the waves would be transmitted faster and faster and faster. Now, the effect of this would be to make the rape heads curved because at each layer, it's boundary, the waves would still have to obey Snell's law and refract. But if each layer is infinitely thin, then mathematically that ends up being a curve. So, let me just draw some sample waves. Here is an earthquake right here, and so rape heads would curve like this or like that or like this, for example. Anything that went straight down would just head straight through the middle, but that's the only path that would be exactly the same as the analogous path in homogeneous Earth. Every other one curves. You might want to look back at the previous sketches to see the difference between these rape heads and the rape heads that went through homogeneous Earth or even one that just had a couple of layers at the top.