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“That’s what I’ll do—find the key and unlock the mystery of this strange window,” she told herself. “Won’t Dr. Blackwell be surprised when I tell him I’ve solved it?”

Acting on a sudden impulse she slipped out of bed quietly so as not to disturb Peggy and Florence. What she was going to do, she wanted to do alone. She put on her dress and some rubber-soled shoes, then, picking up a large sun hat from a chair, softly opened the door.

There at the head of the stairs sat Felipe, sound asleep. She hesitated only a moment, then crept softly past him and on down the stairs.

“One good thing about these houses is there’s no danger of a loose board or a creaky step giving you away,” she thought.

Not a soul was in sight outside—not even a dog. Quickly she ran down the street and around the corner, but drew back as the terrific heat struck her face. Heat waves radiated from the cobblestones, and the white stone walls, acting as double reflectors, turned the narrow street into a veritable furnace.

But nothing could stop her now. There was something she wanted to find out about the rear wall of the house. Pulling her hat down farther over her face, she squinted her eyes and gazed up at the glaring white walls above her. Quickly she scratched three marks on the wall, one directly below the kitchen window, one beneath the back room window, and the third beneath the mysterious opening; then she paced off the distance between the marks. She was positive now that she could mark the exact spot on the inner side of the wall where the opening should be.

Eager to escape from the intense heat, she hastened back to the house.

“Whew, it’s hot!” she exclaimed to herself. “These Mexicans show good judgment in sleeping at this time of the day. I don’t blame them in the least.”

Fanning herself with her hat, she dropped down on the lower step to cool off a moment. How refreshing was the coolness of the great hall! She wondered how it was possible to be so cool here and so hot outside.

She listened intently for a moment, but not a sound came from above. Apparently everyone was still asleep.

Softly she slipped up the stairs, step by step, till she caught a glimpse of the sleeping Felipe just as she had left him, his chair tipped back against the wall and his head dropped forward.

There was only one more step now. Holding her breath, she lifted her foot; then suddenly there was a loud bumping noise. She was so startled she almost lost her balance. Clutching at the wall, she stared before her. Felipe, roused by some strange instinct, had let his chair down with a bang.

“No—no—no! Muy mal [Very bad]!” he exclaimed, pointing to her face. He patted himself on the head and talked rapidly in Spanish in an effort to make her understand that the sun was “bad for the head,” as he expressed it.

Much disgusted with herself for getting caught, and eager to escape, she called back “Sí, sí [Yes, yes],” and hastened on to her room. Now that her plan to measure the hall had been interrupted, she would have to wait till a more opportune time for that. Searching for paper and pencil, she decided to do the next best thing—put the outside measurements down so there’d be no danger of forgetting them.

She was sitting by the window busily drawing a plan of the house when Florence called in a surprised voice, “Why, Jo Ann! Where have you been? Your face is as red as a beet.”

“It’s nothing,” she replied. “I just went down and measured the position of those windows on the back wall. And was it hot!”

“You mean you’ve been down there in the sun!” Florence could hardly believe her ears. “Jo, you shouldn’t have done that.”

At the sound of voices Peggy opened her eyes, then sat up in bed to stare at Jo Ann. “For goodness’ sake, Jo, what’ve you been up to now?” she asked.

“Oh, nothing,” Jo Ann answered crisply. Why couldn’t they leave her alone? She hadn’t committed a crime.

“But, Jo, your face! You’re so hot.”

“Well, if you must know, I’ve been searching for the key with which to unlock the mystery—as Dr. Blackwell suggested.”

CHAPTER III
A NEW DISCOVERY

Looking cool and dainty in their fluffy summer dresses, the girls came lightly down the stairs, ready for their drive around the city.

Florence smiled as she noticed the car waiting at the curb. It had been rubbed and polished till it shone, and Felipe, beaming like a child with a favorite toy, was leaning over, brushing a speck of dust off the hood.

Snatching off his hat on seeing the girls and grinning widely, he hastened to open the door of the car; then, standing very straight, he waited for them to be seated. His effort to uphold the dignity of his position as chauffeur was amusing. After tilting his hat—an old one of Dr. Blackwell’s—at a rakish angle, he proudly took his place at the wheel. Although the car was not as new and expensive as some they passed, he was sure none carried more beautiful passengers.

To the girls’ delight it was much cooler now; soft breezes were stirring, and the heat was vanishing with the sun, which was sinking behind the high range of mountains to the west of the city.

Slowly Felipe passed the cathedral and circled the attractive little plaza, while the girls gazed admiringly at the formal beds of brilliant blooming flowers and drank in the perfumed air, heavy with the fragrance of wild orange and oleander. Now and then through the foliage of the trees they could catch a glimpse of the bandstand in the center of the trees.

“They have band concerts two nights a week here,” Florence explained. “We’ll promenade awhile tomorrow night. I know you’ll enjoy it.”

“Promenade? What do you mean?” asked Peggy.

“Why, walk around the Plaza. On the nights when the band plays, the people of the better classes gather here and either promenade around the broad walk outside the square or sit on the benches to talk and listen to the music.”

“That sounds as if it’d be lots of fun,” approved Peggy.

At this moment the car came to a sudden stop. In attempting to turn off the broad drive around the Plaza into one of the old narrow streets of the business section, they were held up by the congested traffic. The enforced rest did not bother Felipe in the least. Calmly resting his elbows on the steering wheel, he waited for the way to become cleared.

“Why doesn’t he blow his horn?” asked Jo Ann. “Maybe that’d make them move.” She noticed, however, that of all the persons in the near-by cars held up in the traffic jam, not one seemed the least bit impatient.

“These people must have the patience of Job,” said Peggy. “If this were at home, you’d hear the horns blowing all down the line.”

“It takes more than a little thing like this to ruffle the slow, easy-going Mexican,” explained Florence. “He’s never in a hurry.”

In a short time they were on their way again, moving slowly through the narrow, busy streets. All classes of people and many nationalities were here, their different modes of dress interesting Peggy and Jo Ann, as did the stores with their queer signs and window displays. Although this was the busiest time of the day, they noticed that there was none of the bustling rush characteristic of American cities.

On through the less crowded streets Felipe steered the car into the residential section, passing