I was in Penrith to ring bells and found this clock mechanism inside the front door of the church. I blogged about escapement mechanisms earlier in the year and got a good chance to inspect one here. In the top picture, there is a curved piece of metal above the top brass cog. It is attached the pendulum. What happens is that you wind up the clock. The place where you attach the winding handle is apparent in the bottom photograph (it wasn't clear whether you are winding a spring or winding up a weight but you give the mechanism some form of potential energy). This then drives the top brass cog round. The cog locks with the escapement mechanism, the curved piece of metal. This momentarily stops the cog, but then the cog pushes the curved piece out of the way and in so doing pushes on the pendulum. The pendulum then swings back and catches the cog on the other side. The process repeats. A pendulum would naturally stop swinging after a while but the mechanism is a way of taking the potential energy and giving it to the pendulum as extra kinetic energy. So the clock drives the pendulum. I'd always thought that it was the other way round. So what's the point in the pendulum? Well, by catching on the cog wheel all the time and momentarily stopping it, it forces the cog wheel to keep to the speed of the pendulum. So the pendulum regulates the speed at which the cog wheel turns and hence keeps it in time if the hands are attached to the cog. The tick and the tock are when the escapement mechanism hits the cog wheel to stop it.
