Here, there was nowhere to go and only a monthly boat in winter – if we were lucky.

Mary went on ahead, her thin legs throwing long shadows across the bay, and I brushed away my thoughts and caught her up, tapping her shoulder and running off so that she could catch me in a game of chase.

Before lunch, I went to my room and found the pebble and put it in my pocket. As much as I wanted to confide in Bridget, I was aware that I did not want her to suspect that Mary was the one who had been in possession of such an object. It was clear she already believed Mary strange and I was keen not to add fuel to those doubts. Even so, I had a burning desire to learn more, to know their meaning; if William and Hettie made spells, what was the purpose they sought? Perhaps in understanding that, I might understand more of why Mary had kept one. It was to Ailsa’s I planned to go.

I was impatient for lessons to begin, and as soon as I heard the door close I made my way back outside. When I reached Ailsa’s, I knocked hard. When she greeted me, there was a smile in her eyes that told me she had expected me back.

This time, she offered me a seat.

‘And so?’ she began.

I drew out the pebble, placing it in front of her.

She pulled it to her curiously. ‘Where did you get this?’

‘I found it in Hettie’s room,’ I lied. ‘What does it mean?’

She opened the drawer, took out and filled her pipe. ‘I think you know the answer to that. It’s a spell.’

‘What spell?’ I asked.

‘It’s binding magic.’

I looked baffled.

‘Binding magic is made to attach one thing to another – but here,’ she touched the pebble, ‘this figure tells me that it was made to bind a person to something or someone else. When it is a person, for such a spell to work the hair must belong to that person too.’

‘But what does someone achieve by making it?’

Ailsa leaned back in her chair, blowing smoke into the air. ‘The idea is to tie the person to something for eternity. Commonly, it was made to bind one to a lover, but it could also be made to keep one tethered to a place.’

My chest relaxed. ‘This, then, is to bind someone to another? To make them love them and not leave them?’ And I ached with remorse that I had considered the pebble might have a more sinister meaning. Who more than Mary might long for that to be possible?

She turned the stone over in her palm.

‘Whose hair is this?’ she said, holding it to the light.

‘I don’t know. It’s of such an indeterminate shade, and probably dirty.’