"Quick!" Helen urged. "Let's get out of here before the ceilings fall down!"

As the girls ran to the hall, they met d.i.c.k, Emily, Mr. Daly, Mrs. Willoughby, and Maud. All cried out that they too had felt the vibrations, which now had ceased.

"This is very strange," d.i.c.k said. "This isn't earthquake territory."

On a hunch, Nancy suggested they telephone the Benton State Police and find out if the tremors were widespread. d.i.c.k made the call and with a puzzled expression reported that apparently the disturbance was confined to Lilac Inn.

Maud shrieked, "The building's falling apartt We'd better get out of here!"

"Go if you want to," Emily said sharply. "But the shaking has stopped and the building is still intact."

"I'll make an investigation," d.i.c.k offered. "There may be something wrong with the foundation." He suggested that Nancy and Mr. Daly accompany him to the cellar. "The rest of you wait here."

None of the three discovered anything out of the ordinary in the bas.e.m.e.nt.

Nancy smiled in relief. "Instead of falling down, this building seems to be unusually st.u.r.dy."

Mr. Daly, however, was extremely nervous. "This has never happened before," he said. "d.i.c.k, I strongly advise you to give up the inn. It's-it's just not safe here any more."

Emily's fiance shook his head, saying that he, like Nancy, was more determined than ever to solve the mysteries at Lilac Inn. The trio went back to the second floor and rea.s.sured the others.

"Are you all sure you never felt a similar vibration before?" d.i.c.k asked. "Shaking like that used to happen to our house when a very heavy truck went by."

"Maybe that's what happened this time," Helen said philosophically. "Let's go back to bed."

Nancy was not convinced by this explanation. Remembering the explosion and fire caused by the time bomb, she wondered if someone had planted an explosive underground to try to destroy or at least weaken Lilac Inn.

"Nancy," said Helen, when the girls were once more in bed, "why did you change your mind and come back to the inn tonight?"

When she heard about Nancy's masquerade in the lilac grove and its results, she praised her friend. "Next time, though, don't try such a risky thing alone," Helen scolded.

Nancy was almost asleep when a sudden thought struck her. Why had the "ghost" been t.i.tian-haired this time, instead of brunette?

When the girls went into the dining room the next morning, the other young people, Maud, and Mrs. Willoughby were already eating.

"Welcome back, Nancy," said John, grinning. "How's my beautiful sleuthing skin diver?"

"Ready to flip!" gibed Nancy.

Maud looked up from her grapefruit. "Will you go skin diving with a wig on?" she asked with an attempt to be facetious.

Nancy was slightly annoyed but gave no sign of this. She hoped Maud had not spread word around the inn of the wig episode.

Breakfast over, Nancy returned her car to the parking lot and then sought out Emily privately. She asked whether Maud had been gossiping about events of the previous evening. Emily said she was sure this was the case.

"Tell me, Emily," Nancy said, "how much do you know of Maud's background?"

"Very little. Only that Aunt Hazel met her about a year ago at a social gathering in River Heights. They became friendly. The next thing I heard was that Maud was going out West. Then, about a month ago, she showed up here. Aunt Hazel thought she would make a good social director, and d.i.c.k and I engaged her."

"Maud can be pleasant," Nancy commented, "and she does have musical talent. I really can't figure her out." To herself, Nancy conjectured on the possibility of Maud's using her position as a cover-up.

The young detective left the inn presently to do some sleuthing in the lilac grove. She met John part way there.

"I didn't want to mention it at breakfast, Nancy," he said, "but I'd like to see the exact site of your masquerade. Emily and d.i.c.k told me a little about it."

"I'll be glad to show you." Nancy led him to the lilac grove. She described vividly her encounter with her double. John listened intently.

"Nancy, you were in a dangerous spot. Maybe it's a lucky thing Carl Bard scared your impersonator away."

Nancy did not agree. "The sooner she is caught, the better. I must say, she does look much like me. I don't wonder you were fooled."

John laughed. "I think I wouldn't make the same mistake again. I much prefer the real Nancy."

Nancy blushed at his compliment. As the two looked about the lilac grove, Nancy saw a tiny object glinting in the sun. She picked it up. The object was of steel and shaped something like a can opener, except that there was a tiny wheel at the end.

"What's this?" she asked John.

He took the little device and stared at it. "I know where it belongs," he said. "I'll return it."

John put the object in his pocket, and Nancy had the feeling he had deliberately evaded her question.

"I wonder if there are any distinguishing footprints," he said, changing the subject.

Nancy frowned as she looked at the soft earth. There was a print-a peculiar one which she recognized-clearly outlined. It had been made by a skin diver's flipper. Nancy's mind flashed back to the night before, when she had left for the lilac grove. John's cottage had been dark. Maybe he had been sleeping. If not, where had he been?

"John," she said, looking the young man squarely in the eye, "were you skin diving last night?"

CHAPTER XV.

The Underwater Rescue JOHN McBRIDE looked startled at Nancy's sudden query. "No, I wasn't skin diving," he said, returning the girl's direct look. "Why?"

Nancy pointed to the marks on the ground. "They're flipper prints," she said. "There must be more! The jeep tread and these prints have to start and end some place."

John knelt down and studied the prints. "From the size, I'd judge these were made by a man."

Nancy suddenly recalled the similar prints she had seen and the distant sound of a motorboat she heard as she had approached the grove the previous night. She told John of this. "Perhaps," she said, "whoever wore the flippers was to meet my impersonator and leave by boat."

"Or perhaps the diver was aboard but got out of the boat away from sh.o.r.e and swam underwater to avoid being detected," John guessed.

Nancy nodded and thought, "If this is the case, it eliminates Maud Potter as a partic.i.p.ant in the rendezvous."

Nancy's mind flitted over possible suspects. One of the gardeners? He could easily have skin-diving equipment without anyone knowing it.

As Nancy and John walked toward the river, following the flipper treads, she asked, "John, do you know what 'blue pipes' are?"

"Sure. Lilacs," he said matter-of-factly.

Nancy hesitated, then revealed her theory about the term being a code or a signal. "It could even mean Lilac Inn."

He raised his eyebrows in amazement. "Wow! Some shrewd deducing!" he exclaimed.

John stopped suddenly and grasped Nancy's arm. As the girl stared at him in surprise, he said excitedly, "Nancy! You've given me a terrific idea! I can't explain until-well, someday you'll understand."

Nancy's curiosity was piqued. She naturally did not ask John what he meant. But she was certain that he was tackling some secret problem. What was it?

By now the couple had reached the river. John pointed out three flipper prints in soft dirt patches along the bank.

Nancy scanned the area. "I'd like to have a look underwater here," she said. "John, would you like to make a skin-diving date?"

John grinned. "You couldn't keep me ash.o.r.e."

The two returned to the inn. Nancy went first to the phone and called her father. Since she did not want to mention specific details in case someone was eavesdropping, she merely told him that "last night's meeting was most interesting."

"I understand," Mr. Drew said.

"Also, Dad, is it all right if I go skin diving? John will accompany me."

Mr. Drew gave his permission. "Perhaps you'll see some unusual fish," he added meaningfully.

"Could be, Dad. I'll let you know."

After Nancy had said good-by, she and John confided their plan and the reason for it to Helen, Emily, and d.i.c.k.

"All right," said Emily worriedly, "but watch out for spear throwers."

Soon Nancy and John, ready for skin diving, were back at the riverbank. They had decided to search underwater from the area of the flipper prints to the place Nancy had spotted the sharklike object.

They adjusted their face plates and tanks, and then they descended. Down-down they went, finally reaching the muddy bottom.

Their eyes darted here and there, observing schools of little fish; but nothing out of the ordinary came to sight. Nancy and John continued on, until they reached the place where Nancy had been before. They linked hands and walked cautiously along the river bottom.

Nancy pointed out the rocky overhang from which she had seen the shark shape projecting, John nodded.

To Nancy's disappointment, there was no sign of anything resembling the mysterious object. What had it been, she puzzled? A sunken boat that might have since drifted away?

Suddenly John stumbled and dropped Nancy's hand. Startled, she saw that his foot was wedged between two rocks obscured by weeds. He bent down and tugged, but to no avail.

At once Nancy went to his a.s.sistance. First, she pulled away the plant life surrounding the rocks. Then gently she tried to ease John's foot loose. It would not budge.

Nancy worked desperately to dislodge one of the rocks. Finally, with John's help, she succeeded in moving one of the stones. John's foot was free!

Exhausted, the couple rose to the surface and swam toward sh.o.r.e, gulping in fresh air. As soon as they sat down on the bank, John thanked Nancy for coming to his rescue. "You're a wonderful partner to have around, Nancy-sleuthing or skin diving," he said.

Nancy smiled. "Thanks, John. Let's have a look at your foot and see if it's injured."

John sighed. "Yes, Nurse."

They found that his foot was merely sc.r.a.ped. He and Nancy went underwater again to do some more sleuthing. But they uncovered nothing suspicious. Baffled, they swam back to their starting point and walked toward the inn.

On the way, Nancy saw Gil Gary tr.i.m.m.i.n.g a hedge nearby. She and John went over to him.

"Do you happen to know anyone else around here who skin-dives, Gil?" Nancy asked.

The gardener did not look up, and continued his tr.i.m.m.i.n.g. "Naw," he muttered. "River bottom's too muddy. It-"

He broke off. Nancy felt a surge of excitement. Why did Gil a.s.sume she was referring to the river? And did he know of its muddy condition from personal experience? Why had he not finished his answer?

"I suppose," she said, "some people prefer to travel by canoe." Nancy looked directly at the dock where one of the inn's canoes was tied up.

"S'pose so," Gil replied shortly.

John now stepped forward. "By the way, Gil, have you or Hank noticed any more tools missing lately?"

"Naw." The gardener shook his head.

John shrugged casually. "Just wondered, because Miss Drew and I came across a funny gadget this morning. Sort of like a can opener. Sound familiar?"

"No!" the gardener snapped. He flung his shears to the ground. "I'm goin' for lunch," he said, and retreated hastily.

Nancy and John exchanged triumphant glances. "He's nervous about something, all right," John said.

"He certainly doesn't act like an innocent person," Nancy reasoned.

There was just time before luncheon for Nancy and John to report in private to their three young friends.

When d.i.c.k heard of the gardener's reaction to Nancy's queries, he frowned. "Maybe I should have a talk with him myself," he said.

Nancy advised against this action for the present. "If Gil is connected with the diamond theft, we may learn through him who else is involved," she pointed out. "And, perhaps, the ident.i.ty of my double."

"You mean, all the culprits might be trapped at once?" Helen asked, and Nancy nodded.

As Nancy sat down at the table with the others, Jean came over. "Thanks again for the ride, Miss Drew," she said in her shy way. "It was real helpful."

At this moment Maud Potter entered the room. Jean quickly bent over and whispered to Nancy, "Don't forget! Watch out for that trouble-maker."

Nancy did not know what to think. She disliked Maud, but felt it was unfair to accept Jean's claim without proof. "Don't trust an informer too far," her father had once said.

The social director took her place. Looking at Emily, she announced, "Your aunt has a splitting headache and won't be down."

"Oh, poor dear." Emily jumped up. "I'll go see-"

"I wouldn't disturb her," Maud interrupted officiously. "She'll feel better after some rest."