A friend and I are on a massive sailing contraption, hurtling towards the Thames barrier. It slows down just at the last minute and then swings round heading back so fast that the scenery disappears into a blur. The speed is exhilerating. When we come to a stop I calculate that we covered a mile every 1.25 seconds. A passenger from a previous ride is being hoisted by a body harness having broken his leg from the pressure.