It went past us invisible and silent but for a blast of heat and the smell of diesel. Macky was swatted off the track to land in a crumple just the below the X in EXCELLENT. There was a gurgling sound and his leg twitched for a couple of seconds before he went quite still. Then he faded, and with him his graffiti.

‘Can I stop now?’ asked Lesley. The werelight remained dim – Macky was still drawing its power.

‘Just a little bit longer,’ I said.

I heard a faint rattle and looking back towards the mouth of the tunnel I saw a dim and transparent figure start spraying the outline of a balloon B.

Cyclical, I wrote in my notebook, repeating – insentient?

I told Lesley she could shut down her werelight and Macky vanished. Abigail, who had cautiously flattened herself against the wall of the tunnel, watched as me and Lesley did a quick search along the strip of ground beside the track. Halfway back towards the entrance I pulled the dusty and cracked remains of Macky’s spectacles from amongst the sand and scattered ballast. I held them in my hand and closed my eyes. When it comes to vestigia, metal and glass are both unpredictable but I caught, faintly, a couple of bars of a rock guitar solo.

I made a note of the glasses – physical confirmation of the ghost’s existence – and wondered whether to take them home. Would removing something that integral to the ghost from the location have an effect on it? And if removing it did damage or destroy the ghost, did it matter? Was a ghost a person?

I haven’t read even ten per cent of the books in the mundane library about ghosts. In fact I’ve mostly only read the textbooks that Nightingale has assigned me and stuff, like Wolfe and Polidori, that I’ve come across during an investigation. From what I have read it is clear that attitudes towards ghosts, amongst official wizards, have changed over time.

Sir Isaac Newton, founder of modern magic, seemed to regard them as an irritating distraction from the beauty of his nice clean universe. There was a mad rush during the seventeenth century to classify them in the manner of plants or animals and during the Enlightenment there was a lot of earnest discussion about free will. The Victorians divided neatly into those who regarded ghosts as souls to be saved and those who thought them a form of spiritual pollution – to be exorcised. In the 1930s, as relativity and quantum theory arrived to unsettle the leather upholstery of the Folly, the speculation got a bit excitable and the poor old spirits of the departed were seized upon as convenient test subjects for all manner of magical experiments. The consensus being that they were little more than gramophone recordings of past lives and therefore occupied the same ethical status as fruit flies in a genetics lab.