The four children were in a great state of excitement. They could talk about nothing else but the secret passage and the prisoner in the tower, though when Dimmy was there they had to stop, and talk of other things.

“We simply must keep it all a secret,” said Mike. “I’m quite sure Dimmy would be scared. The only thing I’m wondering about is - how are we going to keep a watch on the tower of the Old House in the daytime, without Dimmy wondering what we are doing? It was easy enough at night - but in the daytime it won’t be so easy.”

“Well, we’ll have to be out of our rooms whilst Dimmy is cleaning them each day,” said Peggy. “But as soon as the cleaning is done we could take it in turns to go into the top bedroom and watch, without Dimmy knowing. We could have fairly long watches - say three hours. We needn’t keep our eyes on the tower all the time - we could read or something and keep looking up. I shall do my knitting.”

“And I shall do my jigsaw,” said Mike. “I can do that and keep looking up easily.”

“We’ll begin to-morrow morning,” said Jack. “I hope Dimmy doesn’t go up to our bedroom and find one of us there - she’ll think we’ve quarrelled or something!”

They took a look at the tower in the distance as they went to bed that night. But there was nothing to be seen. Nobody looked out. A dim light shone, that was all.

“There must be somebody there now,” said Jack. “Or they wouldn’t have a light. Goodness, I’m sure I shall never go to sleep to-night! My mind keeps thinking of secret caves!”

They did lie awake rather a long time, but at last they were all asleep and dreaming. They dreamt of caves and passages and towers and prisoners, and had just as exciting a time in their sleep as they had had in the daytime.

Mike looked at the distant tower as soon as he jumped out of bed next morning, but there was no one there. Jack took a glance as he was about to go downstairs - and he gave a cry.

“There’s someone at the window!”

Mike came rushing to see - but Jack pushed him back. “Don’t go too near our window. If we can see them they can see us - and it looks to me as if it’s only Mr. Diaz.”

The two boys kept back a little so that no one could see them. Yes - it was Mr. Diaz - and he was looking straight at their window.

“Keep quite still, Mike,” he said. “He’s just trying to find out how much we can see of his tower, I’m sure!”

Mr. Diaz drew back after a while. Dimmy rang the breakfast bell again downstairs, and Peggy came bounding up the winding staircase to find out what the boys were doing.

That day the children began their three-hourly watches - and it was just as Peggy was taking over from Jack about six o’clock that evening that they first saw the Prisoner!

Jack had been carving a wooden boat with his penknife, sitting patiently for three hours at one side of the window so that Mr. Diaz would not catch sight of him if he should happen to look out once more. Every minute or two Jack glanced over to the distant tower, but he had seen no one there.

Then Peggy came running up the stairs to take her turn at watching - and just as Jack was getting up from his chair, and Peggy was picking up her knitting, they both happened to glance at the far window.

And they both saw the same thing!

“It’s a little boy!” said Jack, in the greatest astonishment. “He doesn’t look more than seven or eight!”

“He doesn’t look English,” said Peggy. “Even from here he looks very dark-haired and dark-eyed.”

The little boy in the distant tower leaned on the window-sill. Jack took up the field-glasses that lay near at hand and looked through them. He could then see the little boy looking as near as if he were in the garden of Peep-Hole!

“He looks awfully pale and miserable.” said Jack. “Almost as if he were crying!”

“Let me see,” said Peggy. Jack gave her the glasses. She looked through them. “Yes,” she said. “He certainly does look sad. I’m not surprised, either, if he’s a prisoner!”

“Let’s wave to him!” said Jack suddenly. “He’ll be glad to see other children.” lack leaned right out of his window, and began waving violently.

At first the boy in the tower did not notice. Then Jack’s moving arm attracted his attention, and he stared. Jack almost fell out of the window, because he waved so hard. Peggy squeezed beside him and waved too. The boy smiled and waved back. First he put one hand out of the window and then both, and waved them like flags!

“Good! He’s seen us,” said Jack, pleased. “Now the next thing is - how are we going to find out who he is?”

Peggy had a good idea. “If we did some big letters in black ink, and held them up at the window one after the other, to spell out words, he would know we were friends!”

“Good idea!” said Jack. “It looks as if it’s going to be rainy to-night, so we could all come up here and do the letters then. Dimmy’s got a friend coming in to see her, I know, so she won’t mind us coming up here.”

“I wonder if she’s got some black ink,” said Peggy.

“We’ll ask her. I’ve got some sheets of drawing paper we can use.”

The little boy at the tower window suddenly disappeared and did not come back. “I expect somebody came into the tower room and he came away from the window in case they guessed that he was signalling to someone,” said Jack.

Mike and Nora came running in through the garden at that moment, for it was raining. They rushed up to the bedroom at the top of their tower to see why Jack hadn’t come down to the beach.

When they heard about the boy prisoner in the tower of the Old House, they wished that they had seen him too. They were thrilled when Jack told them that they were all going to make giant black letters so that they might spell out words to the prisoner.

Peggy ran to see if Dimmy had any black ink, but she hadn’t.

“I’ve only the ordinary blue ink,” said Dimmy, rummaging in her desk. “But look - here’s some black charcoal. Will that do instead?”

“Oh yes!” cried Peggy. “Thank you, Dimmy. You won’t mind if we all play in Mike’s bedroom this evening, will you? You are having a friend to keep you company, aren’t you?”

“Oh yes,” said Dimmy. “I’ll be glad to have you four monkeys out of my way! You do what you like up there, but have the windows open so that you get plenty of fresh air.”

“Oh, we’ll be very particular about the windows, Dimmy!” said Peggy, laughing, and she ran off with the box of black charcoal.

She took the big white drawing sheets from her box, and went up to Mike’s bedroom. She gave some to each of the children, and opened the box of black charcoal.

“We shall make our hands black!” she said. “Isn’t the charcoal nice and black, Mike? The letters we make will show up well, and the prisoner will easily be able to read them.”

“Make them about a foot and a half tall and as thick as you can,” said Jack, sketching out a big letter A. “I’ll do the first six letters, you do the next six, Mike, Peggy the next six, and Nora the next. Whoever has finished first can do the odd two letters left. Look at my big A! I guess the prisoner could easily see that from his window.”

It was indeed a fine big A, nearly as high as the stool on which Jack was sitting. It was thickly done too, and surely anybody would be able to read it from quite a distance.

It did not take the children very long to finish all the letters. Peggy had done hers first, so she did Y and Z too, though she was sure they would not want to use the Z.

They had kept their eye on the tower window, but the boy had not appeared again. Now, with the rainy sky, the dark was coming down. A faint light appeared in the distant tower window. For a moment the children saw the outline of a boy’s head and shoulders at the window, and then it was gone again.