At the end of my PhD viva, just after they’d told me that I’d passed, the external examiner smiled across the table and asked what I was going to do with the rest of the afternoon. He was clearly expecting parties and pubs and inebriation to be high on the agenda. He was not expecting me to say that I was about to go cycling out into the Cambridgeshire countryside to see whether I could find a farmer who might lend me an old tractor tire or two. I explained that I was making a contraption to throw boots with, that I had to make it out of scrap material, and that I had to finish it within the next week. The examiner’s brow wrinkled, his eyebrows waggled uncertainly, and then he smoothly pretended he hadn’t heard and asked me what plans I had for a job. But it was true. I had agreed to be part of a rare all-female team taking part in a Scrap-heap Challenge road show, and the challenge was to build something that could compete at throwing wellington boots, to be put through its paces at the Dorset Steam Fair. There were three of us, we had no money and very little time, and as far as I could see the only option available was to use an ancient and very effective technology: the trebuchet.
A trebuchet is an extremely ingenious device that was developed over many centuries with the input of several civilizations: the early Chinese, the Byzantine and Islamic empires, and finally western Europe. When it came of age in the eleventh and twelfth centuries, it proved itself a monstrous lumbering brute, capable of demolishing castles that had previously been thought impregnable. A trebuchet could hurl 200-pound rocks over hundreds of yards. Siege engines like this contributed to the disappearance of motte-and-bailey castles (strategically useful but made only of wood and earth). Solid stone was the only defense, and so stone fortresses became the norm.
The benefits of the trebuchet were the same for me and my team as for the medieval warmongers: It’s mechanically simple and extremely effective. We borrowed scaffolding poles from a local building site, dug around in the college skip for stuff to make a sling from, persuaded the technicians in the Cavendish Laboratory to let me have a 16-foot—long metal beam, gathered all this stuff together at the top of the college playing fields, and set to work. Churchill College in Cambridge had been my home for nearly eight years by then, and the college staff were used both to me and to the sudden appearance of novel contraptions. Looking back, I’m still astonished at (and extremely grateful for) the cheerful acceptance that met any of the students whenever we had a new daft idea. At the opposite end of the playing field that week, someone else was testing a stratospheric balloon to send a teddy bear into space.
The basic structure of a trebuchet is very simple. You build a frame that gives you a pivot point which is perhaps 6 to 9 feet off the ground. Then you attach one long beam to that, like a giant seesaw, but you position the pivot point so that there’s much more of the beam on one side than the other. Now you’ve got an A-frame that looks as though it’s got a long stick laid across the top. The long end is the one that starts touching the ground. You attach a sling to the long end, and lay the sling on the ground underneath the frame. The first time we assembled it all, it was a beautiful sunny day, perfect for launching anything.
Then we hit a problem. The beautiful thing about a trebuchet (unless you’re the one who’s about to get a rock thrown at you) is that it uses gravity to spin the seesaw and the sling. You attach a heavy weight to the short end of the seesaw, and then as you drop the weight, it pulls the seesaw on your side down very quickly. The whole beam spins around the pivot, tracing out a vertical circle, and the sling also spins around the other end of the beam. So you’ve got lots of very fast rotation, and the projectile in the sling is spinning around the pivot because it’s being pulled inward by the sling. So far, so good. The first task was to get to this point, but we couldn’t find a weight that was heavy enough to move everything. I offered to swing from the beam myself as a human weight, but even I wasn’t quite heavy enough. We were stumped. That night, I spent a while pouring out my frustration to another set of friends, batting away their suggestions that I should just eat more cake. Then one of them offered me his scuba-diving weights. So the next day, I rigged myself up with a belt carrying 22 pounds of diving weights, and we tried again. This time, it worked perfectly. I swung under the pivot, the seesaw swung over the top, and the sling swung over the top of that. Everything was spinning. Now it was time for the next step.
The sling is only held in place by a small loop, and the trick to it all is that when the sling is almost at its highest point, the loop slips off. The sling is effectively broken. That means that the force that was pulling the projectile inward and keeping it on the circle has vanished. Now the situation has changed. At this moment, the projectile in the sling is traveling forward and upward very fast. As soon as it’s free of the inward force, it just keeps going in a straight line. Since it was traveling forward and upward before, it keeps going forward and upward. But it doesn’t go directly outward from the center of the spin. It carries on going sideways, as if following a line that sat on top of the spin circle. That was the theory. We put a shoe in the sling and lined everything up. I faced away from the playing field and swung down on the seesaw. The other end of the seesaw swung up, dragging the sling up, around, and over the pivot. At exactly the right moment (first time!) the sling released, and the shoe went flying over the top of my head, out on to the playing field. I wouldn’t ever want to do that with a rock, but the shoe proved the point perfectly. Our contraption could at least throw a wellington boot, and in the time we had, that was the best we could do. After a bit more practice, we took our frame apart ready to transport to the competition the next day.