Bacon was a great patriot, and dreamed of one day defending the entire English coastline with an insurmountable wall of brass. But as present engineering techniques did not allow for anything so monumental (for that matter, neither do today's), he decided to start small, and build a talking brass head that would tell him how to do it. (Strangely enough, these talking brass heads were considered nothing very extraordinary to construct; Robert Grosseteste, bishop of Lincoln, was said to have made one, Albert Magus another, and even the ancient Roman philosopher Boethius reportedly had one on hand.) Bacon built his according to all the specifications, but for some reason it wouldn't talk.
     Much frustrated, Bacon and his trusted assistant, Friar Bungay, went to the woods one night and raised a demon to ask it what the problem might be. After demurring at first, the demon finally told them what to do and said the head would talk in one month's time, though he couldn't say at exactly what day or hour. He warned the friars that if they didn't hear it before it had finished speaking, all would be lost.
     Consequently, the two friars went on a round-the-clock vigil. But after several weeks of constant attendance on the head, they decided to get some rest one night and delegated the responsibility to Miles, their servant.
And of course, that night turned out to be the fateful one. The head opened its brass jaws and said two words. 'Time is,' but Miles didn't think this was important enough to wake his masters for. Some time later, it said, 'Time was,' but again Miles figured this was pretty inconsequential stuff; in fact, since the head had so little to say, Miles started to jeer at it and sing bawdy songs.
     Half an hour more passed, and the head, unhinging its jaws one last time, said, 'Time is past,' and exploded into a thousand pieces. Bacon and Bungay, awakened by the deafening roar, raced into the room, but too late to learn anything from the smoldering ruins.

Raising Hell: a Consise History of the Black Arts -- and Those Who Dared to Practice Them, by Robert Masello, page 67