Swallow put a casual hand on my shoulder and said, ‘I think that there used to be a way across the gap to that other mountain. Look at that flat place over there. That mound of broken stone forming the talus had to come from somewhere. Maybe it was a bridge.’ He knelt to examine the edge and announced triumphantly that something had broken away.

I gathered my wits and turned to look at the Taillard Observatory.

It was set atop a great stepped platform of the same white, very smooth stone as the building. I went up onto it, admiring the precision of the stone cuts and the angles. The building was centred on the platform so as to leave a terrace on all sides. It was beautiful and strange to my eyes, its walls rounded so there were no corners, and the roof was smooth and curved as an egg, save for the split I had seen from below. There was no glass in it, but some queer sort of machine stood just inside, blocking the way, so that it would be impossible to enter through it.

There was a door round the other side.

‘It’s locked,’ Swallow said.

I looked at the door, which seemed to be made of plast. There was a lever handle and a slot for a key under it. It did not look very complicated but, remembering the device set up on the door to Ariel’s chamber on Herder Isle, which had been set to explode if anyone tampered with the lock, I did not touch it. Standing well back and making the other two do the same, I formed a probe and delicately worked the lock with my mind.

‘Is it open?’ Swallow asked, and when I nodded, he reached past me to pull the lever. The door swung wide and we stepped into what seemed to be a single enormous circular chamber centred on a huge Beforetime machine fixed to the floor by massive metal and hard plast fittings. It was one part of this that was blocking the opening in the wall.

‘What did they observe here?’ Swallow murmured.

‘The moon,’ Maruman sent absurdly, casting a brief malevolent look at the slice of sky visible through the split in the roof before padding over to sniff at the giant device fixed to the floor.

‘Jacob scribed that these machines and devices would help him to locate the city of his dreams,’ I said. ‘Maybe this was built to let people look at other places.’ I thought for a bit then I said, ‘He seemed to think his dream city was a settlement that had grown since the Great White. Apparently there were a number of settlements east of the mountains in the Beforetime and he wanted to be sure it was the one he thought it was.’

‘Pellmar Quadrants,’ Swallow said, nodding.