Bess looked doubtful. "I thought pumps made a thumping noise."

"Let's try to find the thing," Nancy suggested. "It sounds close."

The girls searched the wooded bluff nearby, and a short distance along the shore on either side, but found no explanation for the noise. Puzzled, they returned to the beach.

Nancy looked up at the big stone house. The light was still on. Could the machine be in there? Was the sound being carried by the wind? As the thought crossed her mind, she suddenly realized that a man stood silhouetted in a doorway.

"Someone's up there," Nancy told the others. "He must have seen our flashlights."

George giggled. "I suppose he thinks we're burglars!"

Cecily did not comment, but stared at the house as if fascinated. Nancy, seeing her frown, remembered her father's remark that Cecily had an interesting story. What was it? And did the big stone house have something to do with the girl's reason for coming to the lake?

"Let's go!" Bess urged.

"Good idea," Cecily agreed. "I wonder-" She stopped speaking and turned away.

The other girls did not ask for any explanation, but all of them felt that Cecily, though friendly enough, was rather secretive. Nancy chided herself, however, that there was no reason why Cecily should immediately start telling them her personal problems.

The man went inside the house again and in a few moments the humming noise stopped. The girls wondered if he had turned off whatever machine had been causing the sound. Within minutes the big stone house was in darkness and Nancy assumed that whoever lived there had gone to bed.

The girls picked their way carefully back to the cottage, unlocked it, and went inside. Bess, overcome by curiosity, asked Cecily if she had come to the lake for a late vacation.

"Why-yes," Cecily answered noncommittally. She yawned. "If you don't mind, I think I'll go to bed. I've had a tiring day."

The others agreed that this was true indeed. The day had not only been tiring for her, but apparently very worrisome. It was arranged that Cecily and Nancy would sleep in one of the two bedrooms, and George and Bess in the other. The cat, Satin, was made comfortable on a small pillow from one of Cecily's bags. The soft, furry animal, cuddled by the fireplace, began to purr, and was soon asleep.

The girls had a snack and then went to bed. Nancy dropped off to sleep almost immediately, but was rudely awakened by the shrill cry of a woman outside. She and Cecily sat upright at the same moment, then jumped out of bed and rushed into the living room. As the screaming continued, Bess and George also appeared and all four girls ran from the cottage.

"Who is it? Where is she?" Bess asked, glancing fearfully around in the darkness.

The screaming seemed to be coming from the woods just beyond the cottage. The girls could see nothing, so Nancy rushed back inside for her flashlight and beamed it at the trees.

Suddenly there was a rustle of leaves and a huge bird left its perch on the branches of an oak. It flew directly in front of the girls, then soared out over the lake.

Nancy began to laugh. "We got fooled that time, all right. That bird is a loon. I just remembered that their call can be easily mistaken for a woman's scream."

"Well, for Pete's sake!" said George in disgust. "All kinds of things have awakened me in my life but never a crazy old bird!"

As the girls re-entered the cottage, the black cat lazily rose, arched his back, then gave a huge yawn. Bess giggled. "Satin, you're the only creature here who knows the difference between a birdcall and a screaming woman!"

Again Satin settled down, the house was locked, and the girls went to bed. Nancy did not go to sleep at once and she noticed that Cecily seemed restless, too. The young sleuth thought it best not to speak to her new acquaintance, however, and finally dropped off to sleep.

Presently Nancy was awakened once more, this time by a loud miaowing. She wondered what was bothering the cat, and looked over to see if Cecily were awake. She noticed that the red-haired girl was not in her bed.

At first Nancy assumed that she had gone out to quiet Satin, but as the animal persisted in its miaowing, Nancy finally got up. To her amazement, Cecily was not in the cottage. The cat was wailing and scratching at the front door.

Nancy thought, "Perhaps Cecily couldn't sleep, and went for a walk." Thinking she might comfort the girl, Nancy quietly went outside but left Satin in the cottage.

The moon had now risen bright and clear, but its light did not reveal Cecily. A bit alarmed, Nancy called the girl's name softly. There was no answer.

"Oh, dear, I hope she didn't run away!" Nancy said to herself. "Maybe my being here with Bess and George disturbed her. But she did seem sincere in wanting us to stay."

On a hunch Nancy went back into the cottage. Cecily's suitcases were there, and also her pajamas, neatly folded on a chair. Apparently she had changed into outdoor clothes! Quickly Nancy put on slacks and a sweater. She took her flashlight and once more hurried outside. For a moment she was tempted to awaken Bess and George but did not want to waste precious time. She paused for a few seconds, trying to figure which direction Cecily might have followed.

"Toward the road to town?" Nancy asked herself.

She discarded this idea because the girl had not taken her luggage along. "And I don't see her along the lake front."

Suddenly the young sleuth recalled how fascinated Cecily had been by the large stone house near which they had heard the strange humming sound.

"Maybe Cecily went back there out of curiosity. Anyway, that's where I'm going to look."

There was the semblance of a path through the woods, a short distance above the water, leading toward the foot of the bluff. Nancy followed this, keeping her eyes open for the missing girl and calling her name. She did not see Cecily, nor was there any reply.

Nancy went on and on and in a short time the path curved away from the lake and went directly up the steep embankment. The thick woods shut out the brilliant moonlight and Nancy turned on her flashlight. Again she paused, undecided whether to go on. The path led up to the big house, and if this were Cecily's goal, perhaps she had taken it.

"I'll try it," Nancy determined.

She was about halfway up the slope when suddenly her heart gave a thump and she nearly stopped breathing. Off to one side of the path, the missing girl lay in a crumpled heap!

"Oh, Cecily!" Nancy exclaimed as she rushed over to the still figure.

She felt Cecily's pulse and found that she was alive. The unconscious girl had a large swelling on her temple. Had she fallen and hit her head? Or, worse, had she been struck by some unknown assailant?

Cautiously Nancy looked all around and listened. At the head of the steep path she noticed a short, thick log, blocking the exit to the top of the bluff. But there was no sign of life.

"I must revive Cecily," Nancy thought. She hastened down to the lake where she wet a clean handkerchief, then hurried back. As she was bathing the injured girl's face, she heard a twig crack on the bluff above her.

Nancy looked up quickly. To her horror, the heavy log was rolling at breakneck speed down the path directly toward them!

CHAPTER V.

The Iron Bird

THERE was not a second to lose. Nancy seized the unconscious girl and rolled off the path with her. The log sped past, grazing Nancy's shoulder. Moments later, she heard it thud to the beach below and then splash into the water.

As Nancy breathed a sigh of relief, her companion stirred and opened her eyes. "Cecily, are you all right?" Nancy asked anxiously.

For a moment the girl looked bewildered, then she said, "Yes," and sat up.

"I'm certainly thankful for that!" Nancy exclaimed.

The young detective rubbed her bruised shoulder and looked thoughtfully up the path. She wondered why the log had started rolling and remembered the twig she had heard crack. Had someone deliberately shoved the log down on them? No one was in sight and right now there was no chance of her finding out the answer to her question.

"Oh, Nancy! You've saved me twice!" Cecily said weakly. "Pl-please help me back to the cottage, and I'll tell you why I came here."