The Wizards' Balls

Two massive brass spheres which rotate on their own raised platform on the roof of Unseen University, designed to spin wildly and discharge any excess of magic if an experiment goes critical anywhere in the building, which happens frequently when students mishandle the raw firmament.

The Balls are probably still there after the culminating events of Thief of Time/Night Watch, in which the activation of the Glass Clock sent a resonance across the city which threw Carcer and Samuel Vimes across time and space, but we are told that in the immediate aftermath, there was a short and sharp rain of cogwheels in the University.

They are named in a humorous Ankh-Morpork style.