families did not deport themselves in such a manner. Her Florencita had never acted this way before—racing into the house like mad. Finally, shaking her head and mumbling to herself, she returned to the kitchen to finish her preparations for lunch.

The moment she disappeared through the kitchen door, Jo Ann hastened through the one opposite and called to Florence and Peggy, “Come on, maybe we can see something from the window in this back room.”

Much to their disappointment, the iron bars, set in the deep recess of the thick walls, prevented them from seeing anything except a part of the ruins of the old church directly across the narrow street.

“And so the mystery deepens,” laughed Jo Ann. “I’ve heard of bumping your head against a stone wall, but I’ve never understood what it meant till now.”

“Do you know what I think it is?” remarked Florence as they wandered back to the hall. “At one time there probably was an opening here”—she motioned toward the solid wall at the end of the hall—“then, sometime when they were fixing the house over, they closed it up. This house is very, very old, you know.”

“But why would they leave a hole on the outside?” Peggy asked.

“Oh, they probably didn’t think it mattered on that back street, and maybe the stones didn’t fit or something. These walls are so thick, you know, it wouldn’t make any difference. It’s too small to be a window, anyway.”

“Maybe so,” commented Peggy, “but it sounds funny to me.”

Jo Ann was silent. She was thinking—thinking hard. She thoroughly agreed with Florence that the house was old, but she was sure that the opening had not been left by a careless mistake.

“There’s a reason for it,” she told herself, “and I’m going to find out what it is.”

Just then Florence’s father, Dr. Blackwell, a tall, distinguished-looking, gray-haired man, came up the stairs. “Good morning, young ladies,” he greeted them pleasantly. “You look quite fresh and rested after keeping such late hours.”

“We’re feeling the best ever,” Jo Ann answered.

“Well, you certainly look it,” he declared, glancing from one to the other. “Florence, you have more color in your cheeks than I’ve seen for a long time. Miss Jo and Miss Peggy are having a good effect on you already.”

“Oh, Daddy, we’re having a wonderful time! But did you know there’s a mystery about our house? We’ve just made the queerest discovery!”

Dr. Blackwell laughed. “What is it, may I ask—some mysterious writing on the wall, or a pot of gold?”

“Neither. We’ve found a window that isn’t a window. It opens on the outside of the house but not on the inside.”

“Well, now, that’s strange, isn’t it?” he replied smilingly, as though humoring a small child.

“Really, Dr. Blackwell, there’s a mysterious window that should open at the end of this hall!” exclaimed Jo Ann, “but we can find no trace of an opening ever having been there.”

At that moment Felipe, combination chauffeur and house boy, announced lunch, and the subject was dropped as they all hastened into the dining room.

Peggy and Jo Ann were surprised to find, after their late breakfast, that they were quite hungry. As Felipe and Juana passed back and forth waiting on the table, Jo Ann thought how convenient it was to have servants who could not understand what you said. You didn’t have to be nearly as careful as you did at home with the Negro servants.

“I thought you girls might like to see something of our city,” Dr. Blackwell remarked as they were eating their dessert. “I’ve arranged for Felipe to take you for a drive this afternoon. I’d thought I’d be able to accompany you, but a doctor’s time’s never his own, so I’ll have to depend on Florence and Felipe to show you the city.”

“That’ll be fine!” exclaimed Peggy. “But we’re sorry you can’t go with us. Aren’t we, Jo?”

Jo Ann nodded an emphatic assent and then went on to remark to Dr. Blackwell, “We saw something very interesting this morning—the old church back of your house. I’ve been reading early American history a good deal lately, and this church seems very much the same type as the old missions in California.”

“Well, well!” smiled Dr. Blackwell in surprise. “I thought the modern young girl used her pretty head solely for thinking of frocks and furbelows.”

Peggy laughed. “Jo Ann hates dress-up clothes. She’d live in jodhpurs or knickers and shirts, if her mother didn’t make her get out of them occasionally. Jo’s enthusiastic over horses and dogs and swimming, but her chief hobby is nosing around old buildings.”

“There’s so much mystery and romance connected with historic buildings,” Jo Ann put in, shining-eyed.

“If you’re looking for mystery and romance,” Dr. Blackwell remarked, “there’s plenty of it to be found in this part of the country—that is, if you can only find the key to unlock it. I’ve been so busy studying the ancient system of sanitation—or lack of it—that I’ve had no time for anything of that sort.”

“If you get Jo started along that line she’ll never stop,” put in Peggy with a teasing glance at Jo Ann.

Dr. Blackwell smiled. “Then I’ll turn her over to a friend of mine—a prominent lawyer, who’ll be delighted to discuss the early history of this country with her. You know whom I mean, Florence—Señor Rodriguez.”

Florence nodded.

“He has the best equipped library in the city,” continued the doctor, “and you can dig into the past to your heart’s content, Miss Jo.”

“That’ll be wonderful!” cried Jo Ann excitedly. “I know I’ll enjoy meeting him and seeing his library. I adore books—especially about old historic buildings.”

As they rose from the table Peggy remarked, “Jo and I want to run across the Plaza to buy some postcards to send home. Do you want to go with us, Florence?”

Florence and her father exchanged smiling glances as she replied, “You can’t buy postcards now—the stores close for an hour or two in the middle of the day.”

“Oh, I forgot I’m in Mexico,” laughed Peggy.

“In tropical countries it’s the custom to take a siesta after lunch,” Dr. Blackwell explained. “People sleep in the hottest part of the day and do their work in the cool of the evening. It’s a very good custom, too, since the sun has a tendency to cause fever if one is in it too much.”

On hearing this the girls meekly followed Florence to their room, and when she removed her dress and shoes and dropped down on the bed, they followed her example.

“How still it is!” thought Jo Ann. Not a sound floated up from the street below; not a leaf stirred on the trees in the park across the way. Even nature seemed to be sleeping, so deep, so intense was the stillness.

Florence, from habit, was soon sound asleep. The other two girls whispered quietly for a while; then Peggy’s eyelids drooped, and she, too, succumbed to the restful quiet.

But Jo Ann could not sleep. There were too many things to think about. A visit to the Señor’s library—she’d love that. And that old church across the street—there must be some very interesting facts connected with it. She’d find out more about that later from the Señor’s books. But that window! It still puzzled her. There was something curious about it. What was that Dr. Blackwell had said about finding the key to unlock the mystery?