‘There’s a series of protections woven around the building,’ said Nightingale. ‘They were last set up after the new phone lines were put in in 1941. If we introduce a new physical connection with the outside, it would create a weak spot.’

I stopped trying to be casual and concentrated on maintaining the werelight. It was a relief when Nightingale told me to stop.

‘Good,’ he said. ‘I think you’re almost ready to move on to the next form.’

I dropped the werelight and caught my breath. Nightingale wandered over to the adjoining bench, where I’d dismantled my old mobile phone and set up the microscope I’d found in a mahogany case in one of the storage cupboards.

He touched the brass and black-lacquer tube. ‘Do you know what this is?’ he asked.

‘An original Charles Perry number 5 microscope,’ I said. ‘I looked it up on the internet. Made in 1932.’ Nightingale nodded, and bent down to examine the insides of my phone.

‘You think magic did this?’ he asked.

‘I know it was the magic,’ I said. ‘I just don’t know how or why.’

Nightingale shifted uncomfortably. ‘Peter,’ he said. ‘You’re not the first apprentice with an inquiring mind, but I don’t want this getting in the way of your duties.’

‘Yes sir,’ I said. ‘I’ll keep it to my free time.’

‘You’re about to suggest the coach house,’ said Nightingale.

‘Sir?’

‘For this cable connection,’ said Nightingale. ‘The heavy defences tended to disturb the horses, so they skirt the coach house. I’m sure this cable connection of yours will be very useful.’

‘Yes sir.’

‘For all manner of entertainments,’ continued Nightingale.

‘Sir.’

‘Now,’ said Nightingale. ‘The next form – Impello.’

I couldn’t tell whether the coach house had originally been built with a first floor, to house footmen or whatever, which had then been knocked through in the 1920s, or whether the floor had been added by sticking a new ceiling on the garage when they bricked up the main gate. At some point someone had bolted a rather beautiful wrought-iron spiral staircase to the courtyard wall. When I’d first ventured up, I was surprised to find that a good third of the sloping roof on the south-facing side had been glazed. The glass was dirty on the outside and some of the panes were cracked, but it let in enough daylight to reveal a jumble of shapes shrouded by dust sheets. Unlike those in the rest of the Folly, these sheets were furry with dust – I didn’t think Molly had ever cleaned in there.