"What's the matter?" she said, rubbing her arm ruefully.

"Look up on the opposite ridge," he whispered. "Don't let yourself be seen, and tell me if you see what I see...

And put something over your hair. The sun shines on it so that you could be seen a mile away."

"But I haven't got anything," she said indignantly. "I'll be careful. Let me look," and she pulled up a root of heather and held it over her head. For a long minute she was silent, and then she whistled in astonishment,,

"David! It is Mrs. Thurston, but what on earth is she doing? She keeps moving about and pointing to something down here. What is she up to?"

David watched Mrs. Thurston's dark head move against the skyline; saw her light a cigarette with a characteristic action, and then sit down in the heather. Although the sun was very hot she was wearing what looked like a drab raincoat, and so was rather difficult to see until she stood up. Once the sun glinted on something she was holding, and David realized with a shock that she had a pair of field-glasses. She was still for a long time... Just watching. Then she got up and there was no doubt that she was watching the reservoir and the dam. She moved along, below the top of the ridge, but still in their sight.

"David," Peter hissed. "Do you see what she's doing? She's taking photographs now. Those are not the glasses that she's holding up to her face now - it's one of those little cameras that take such marvellous pictures."

He was certain too. There was no doubt about it, and he suddenly felt very uneasy. There was something odd and furtive about Mrs, Thurston this afternoon that he did not like.

Then they watched her make her way round so that she was looking down on the dam from the valley side. She was too far away now for them to see precisely what she was doing, but there was little doubt that she was taking photographs from a different angle.

Peter said, "What do you think she's doing, David?"

He rolled over on his back. "I don't know," he said shortly, "but I don't like it, and I think there's something fishy about her. Why should she want to take photographs of your father's reservoir? He told me how important all this water was to the Midlands, and she's doing nothing but spying about the place in secret. We must go and tell Mr. Sterling at once. And Mr. Ingles is a Home Guard... I'm going to tell him. Come on!"

Peter suddenly looked serious. "You don't think...?" she began. Then, "Look! She's coming back."

As they watched Mrs. Thurston returned the same way, climbed the ridge, and without once looking behind her, disappeared in the direction of the track that led to the top of the mountain.