“Doric Wells?” Prue suggested. She’d never been there, but Mum had told her it was a traditional city, resistant to technology. Grandpa Haywood had come from there.

“Correct. And to the south?” Master White moved her arms as though conducting the landscape.

Prue looked behind her where trees stretched into the distance. “The forest?” said Prue.

“Yes, but far beyond where the eye can see?”

“The city of Augustine,” said Agapantha.

“Yes, good. And to the north, we have not only Batterthwaite, but Scar Pike.”

Prue knew that place; her parents had taken her and Francis there when they were little. They called it the haunted mountain because it was so tall and bleak.

Master White took a rolled-up piece of paper from her pack then spread it out on the floor. It was a map detailing the cities and the landmarks she’d mentioned.

“For your first lesson with me, let’s get down to basics. Master Woolstenbury discovered that there are locations where the walls between our world and the spirit world are weaker –intersections which we as humans gravitate naturally towards but are usually unaware of.”

Prue frowned. It all sounded rather unscientific.

“Bear with me, Apprentice Haywood.” Master White smiled. She drew neat lines joining the sites and sat back. “What do you see?”

“The lines all cross right where we are,” said Edwin, jumping down from Prue’s shoulder for a closer look.

“Indeed, this factorium is situated in an intersecting weak spot between worlds. When Hannah Woolstenbury, then a promising young scientist, first started ghost hunting, as it was called several decades ago, she explored all the existing methods of spirit searching, practices which were generally ridiculed by the scientific community. It alienated her from many colleagues, and she was targeted by the Medlock press as a sham. She lost her position in society.”

Master White carefully took some papers from her bag; they were old newspaper cuttings with headlines such as: Ghost Sham Brings Shame to City; Give up the Ghost, Woolstenbury; and Once Eminent Scientist Laughing Stock of Medlock.

“A lesser person would have given in, but Hannah knew that although many were false leads and superstitions, there was something big waiting to be discovered. Through persistence, determination, a lot of trial and error, and a sharp scientific brain, she found a way.”

Master White took a book from her backpack and opened it before them. There were images of the different stages of the moon.

“One of the biggest breakthroughs came when Hannah discovered that not only were there physical intersections where the walls between worlds were weakest, but there were also time intersections.”

“When it’s a full moon?” said Prue.