McGranahan escorted them up to the Industrial and Commercial department, which was situated on the third floor. The layout was mostly open-plan, stretching from the western outer wall to the eastern, with a row of enclosed rooms running the length of the building, partitioned off with aluminium and glass. McGranahan had his own private office in one of these, but he led them past it to a double-width apartment four doors along. It had a white screen on one wall and a data projector suspended from the ceiling towards the other end. Glass ran the length of the room, affording a view into the open-plan area, with venetian blinds providing the option of darkness and privacy. Opposite the glass on the windowless facing wall was a deep cabinet split horizontally, an arrangement that reminded Jasmine of the poster bins in a card shop. The top half displayed dozens of large frames, hinged for browsing, and the bottom half a grid of pigeonholes, each containing a rolled-up document. The office also accommodated a light-table, a photocopier and two very wide desks, upon one of which sat eight cardboard tubes.

‘This is the map room,’ McGranahan explained. ‘It’s a bit of a relic now that everything’s all computers and sat nav, but knowing us, it’ll be another ten years before we get around to finding another use for it.’

He nodded towards the room-length cabinet.

‘We’ll probably donate that lot to the People’s Palace or some other museum.’

‘What are they?’ Jasmine asked. She could only see the outside edges of the frames, but the one at the front looked like a map.

‘Street maps of Glasgow. The ones on top denote the sections of the grid, and in the corresponding boxes underneath are the bigger-scale equivalents, showing the layout of all the gas mains. There’s multiple versions of most of them, going back fifty-odd years, showing where the street layouts have changed.’

Ingrams wandered over to the desk bearing the cardboard tubes.

‘Are these the images?’ he asked.

‘That’s them, aye. I had our poor secretary Josephine down in the bowels of the building looking them out. I couldn’t believe how long it had been since we started doing them. If you’d asked, I’d have guessed ten years ago. It’s actually more than thirty, though Jim only wanted the first two.’

‘What exactly are they?’ Jasmine asked.

McGranahan walked over beside Ingrams and popped a red stopper from one of the tubes, pulling a rolled-up sheet of glossy paper from inside. He unrolled it and laid it out flat, which was when Jasmine noticed the sliding clamps at the edges of each table, there to hold such large documents in place.