"Too bad, Chet," Joe said sympathetically. "Well, at least you can keep on relaxing."
Despite his disappointment, Chet grinned. "Right now I'm starved. "Let's go down to the river and have our picnic."
He thanked the guard, and the young people started to walk away. Suddenly Frank stopped and
looked back at the mill. Propped against the south wall was a bicycle. Quickly he ran over to
examine it. "This looks like a Belgian model," Frank thought. "Sure is," he told himself. "The same type Ken Blake has."
On impulse Frank pulled the pedal from his pocket and compared it to those on the bike. They
matched exactly. Frank noticed that one of the pedals looked much less worn than the other.
"As if it had been replaced recently," he reflected, wondering excitedly if someone had used this bicycle to deliver the warning note.
"And could this bike be Ken's?" the young detective asked himself.
He inspected the front-wheel spokes. None was twisted, but several had slight dents. "They could've been straightened out easily," Frank reasoned, "and the paint scratches on the fender touched up."
He felt his heart beat faster as he waved his companions to join him. When Frank pointed out
the clues to his brother, Joe agreed immediately.
"It could be the bicycle which was used to deliver the message-"
Joe was interrupted by a strange voice behind them. "Pardon me, but why are you so
interested in that bike?"
Frank quickly slipped the pedal into his pocket as the group swung around to face the speaker.
He was the man who had been clipping the hedges.
"Because just yesterday we met a boy, Ken Blake, who was riding a bike of the same model. We don't often see this Belgian make around."
For a moment the man looked surprised, then smiled. "Of course! Ken works here-does odd
jobs for us around the mill. You must be the boys he met yesterday when he was delivering
some copy to the printer."
"Yes," Frank replied. "When we asked Ken about his job he was very secretive."
"Well," the maintenance man said, "he has to be! This plant is doing top-secret work. All of us have been impressed with the necessity of not talking about Elekton at all."
"Is Ken around?" Joe asked nonchalantly. "We'd like to say hello."
"I'm afraid not," was the reply. "We sent him by bus this afternoon to do an errand. He won't be back until later." The man excused himself and resumed his clipping.
"We'd better eat." Iola giggled. "My poor brother is suffering."
"I sure am!" Chet rolled his eyes. Laughing, the picnickers started away.
Joe, who was in the rear, happened to glance up at the front of the mill. He was startled to
catch a glimpse of a face at one of the second-story windows. He stopped in his tracks.
"Ken Blake I" Joe said to himself.
As the young sleuth stared, mystified, the face disappeared from the window.
CHAPTER VI
A Mysterious Tunnel
PUZZLED, Joe continued looking up at the window of the old mill.
"What's the matter?" Iola asked him. "Did you see a ghost?"
In a low whisper Joe explained about the face which had disappeared. "I'm sure it was Ken Blake I saw at that window!"
The others followed his gaze. "No one's there now," Iola said. "Of course the glass in all the windows is old and wavy. The sunlight on them could cause an illusion."
Chet agreed. "How could Ken be here if he was sent on an errand?"
Joe stood for a minute, deep in thought. "I can't figure it out, but I'm sure that it was no illusion.
Come on, Frank. Let's go check."
While the others walked down the hill, the Hardys strode up to the maintenance man, who was
still trimming hedges.
"Are you sure Ken went into town?" Joe asked. "Just now I thought I saw him looking out a second-floor window."
"You couldn't have. You must have been dreaming." The man gave a jovial laugh.
Joe was still not convinced. Impulsively he asked, "Does Ken ever run any errands for you at night?"
"No," the man answered readily. "He leaves his bike here and walks home when we close at five-thirty."
"Does anyone else have access to the bike after that?" Frank queried.
"It's kept in an open storage area under the rear of the mill and could be taken from there easily."
Although obviously curious, the man did not ask the Hardys the reason for their questions. He
looked at his watch.
"Excuse me, boys, I'm late for lunch." He turned and hurried into the mill.
As the brothers hastened to catch up with Chet and the girls, Frank said, "Another thing which makes me wonder if that bicycle is connected with the warning is the description of the boy
who bought the pedal.
He could be Ken Blake."
"I agree," Joe said. "I'd sure like to question Ken."
"We'll come back another time," Frank proposed.
The group picked up the picnic hamper from the Queen and strolled down a narrow path,
through the woods leading to Willow River.
"Here's a good spot." Callie pointed to a shaded level area along the bank. "We haven't been in this section before."
Soon everyone was enjoying the delicious lunch the girls had prepared: chicken sandwiches,
potato salad, chocolate cake, and lemonade. While they were eating, the girls were the targets of good-natured kidding.
"Boy!" Joe exclaimed as he finished his piece of cake. "This is almost as good as my mother and Aunt Gertrude make."
"That's a compliment!" Chet said emphatically.
Callie's eyes twinkled. "I know it is. Joe's mother and aunt are the best cooks ever!"
Iola sniffed. "I don't know about this compliment stuff. There's something on your mind, Joe Hardy!"
Joe grinned. "How are you on apple pie and cream puffs and-?"
"Oh, stop it!" Iola commanded. "Otherwise, you won't get a second piece of cake!"
"I give up." Joe handed over his paper plate.
After lunch everyone but Chet was ready to relax in the sun. Normally he was the first one to
suggest a period of rest, even a nap, but now his new project was uppermost in his mind.
"Let's start to collect the specimens for my microscope," he urged his friends.
The Hardys groaned good-naturedly at Chet's enthusiasm, but readily agreed.
"We'll need some exercise to work off that meal." Frank grinned.
The girls packed the food wrappings in the hamper. Then, single file, the group walked
downstream, paying careful attention to the rocks and vegetation. Chet picked up several rocks and leaves, but discarded them as being too common.
"Are you looking for something from the Stone Age?" Joe quipped. "Maybe a prehistoric fossil?"
"Wouldn't you be surprised if I found one?" Chet retorted.
They followed a bend in the river and came to a small cove with a rocky, shelving beach. Here
the willow trees did not grow so thickly. The shoreline curved gently around to the right before it came to a halt in a sandy strip along the riverbank.
"What a nice spot," said Callie. "We'll have to come here again and wear our swim suits."
"Look!" cried Iola. "What's that?"
She pointed to a dark opening beneath a rocky ledge which bordered the beach.
"A cave!" exclaimed Joe and Frank together.
Intrigued, the five hurried along the beach for a closer look. Eagerly the Hardys and Chet
peered inside the entrance. The interior was damp, and the cave's walls were covered with
green growth.
"This'll be a perfect spot to look for specimens," Chet said. "Let's go in!"
The boys entered the cave. The girls, however, decided to stay outside.
"Too spooky-a
nd crowded!" Callie declared. "Iola and I will sun ourselves while you boys explore."
The Hardys and Chet could just about stand up in the low-ceilinged cave. Frank turned on his
pocket flashlight and pointed to an unusual yellow-green fungus on the right side of the cave.
"Here's a good sample of lichens, Chet."
Soon the boys were busy scraping various lichens off the rocks. Gradually they moved deeper
into the cave. Frank halted in front of a pile of rocks at the rear.
"There ought to be some interesting specimens behind these stones," he said. "They look loose enough to
move."
Together, the three boys rolled some of the rocks to one side. To their great surprise, the
stones had concealed another dark hole.
"Hey! This looks like a tunnel!"
Excitedly Joe poked his flashlight into the opening. In its beam they could see that the hole
appeared to extend into the side of the bank.
"Let's see where the tunnel goes!" Joe urged.
"Okay," Frank agreed eagerly. "We'll have to move more of these rocks before we can climb through. I wonder who put them here and why."
Rapidly the boys pushed rocks aside until the narrow tunnel entrance was completely exposed.
Joe crawled in first, then Frank.
Chet tried to squeeze his bulky form through the space but quickly backed out. "It's too tight for me," he groaned. "I'll stay here and collect more specimens. Anyhow, I'll bet some animal made the tunnel and it doesn't lead anywhere."
"I'm sure no animal did this," Joe called back, aiming his flashlight at the earthen walls of the tunnel. "Look how hard-packed the sides are-as if dug out by a shovel,"
Frank was of the same opinion. He pointed to rough-hewn wooden stakes placed at intervals
along the sides and across the ceiling. "I wonder who put those supports here-and when."
The Hardys crawled ahead carefully. There was just room in the passageway for a normal-sized
person to get through.
Presently Joe called back to his brother, "Look ahead! I can see a sharp bend to the right. Let's keep going."
Frank was about to reply when the brothers were startled by a girl's scream from outside.
"That's Callie!" Frank exclaimed. "Something's wrong!"
CHAPTER VII
Sleuthing by Microscope
FRANK and Joe scrambled through the tunnel and out of the cave. They found Chet and the girls
staring at an arrow embedded in the sandy beach.
"It-it almost hit us," Iola quavered. Callie, who was white-faced with fear, nodded.
Joe was furious. "Whoever shot it shouldn't be allowed to use such a dangerous weapon!" he burst out.
"That's a hunting arrow-it could have caused serious injury."
Chet gulped. "M-maybe the Indians haven't left here, after all," he said, trying to hide his nervousness.
Joe turned to dash off into the woods to search for the bowman.
"Wait!" Frank called. He had pulled the arrow from the sand. "This was done deliberately," he announced grimly, holding the arrow up for all of them to see. Attached to the shaft just below the feathers was a tiny piece of paper. It had been fastened on with adhesive tape.
Frank unrolled the paper and read the printed message aloud: " 'Danger. Hardys beware.' "
Chet and the girls shuddered and looked around fearfully, as though they expected to see the
bowman behind them. "You boys are involved in a new mystery!" Callie exclaimed. "Your own or your father's?"
Frank and Joe exchanged glances. It certainly seemed as though they were involved, but they
had no way of knowing which case. Did it involve the counterfeit money? Or was it the case
their father could not divulge?
"A warning did come to Dad," Frank admitted. "This one obviously was meant for Joe and me.
Whoever shot the arrow trailed us here."
Joe frowned. "I wonder if the same person sent both warnings."
"I still think Ken Blake could give us a clue," Frank said. "But we must remember that anybody could have taken the bike from the storage place under the mill."
Frank pocketed the latest warning, then the five searched quickly for any lead to the bowman.
They found none. When the group returned to the beach, Joe looked at the sky. "We're in for a storm-and not one of us has a raincoat." The bright summer sun had disappeared behind
towering banks of cumulus clouds. There were rumbles of heavy thunder, followed by vivid
flashes of lightning. The air had become humid and oppressive.
"Let's get out of here!" Chet urged. "This isn't a picnic any more!"
The young people hastened through the woods and up the road to Chet's jalopy. As they drove
off, rain began coming down in torrents. The sky grew blacker.
Callie shivered. "It seems so sinister-after that awful arrow."
Chet dropped his sister off at the Morton farm and at the same time picked up his new
microscope. He begged to try out the instrument on both warning notes and the Hardys
smilingly agreed, although they had an up-to-date model of their own.
By the time they had said good-by to Callie at her house, and Chet had driven the Queen into
the Hardys' driveway, the storm had ended. The sun shone brightly again.
Immediately the three boys went to the laboratory over the garage. Here Frank carefully dusted the arrow and the second warning note for prints. He blew the powder away, and Joe and Chet
looked over his shoulder as he peered through the magnifying glass.
"Nothing. Same as the warning to Dad. The person no doubt wore gloves."
"Now to compare this paper to the first note," Joe said.
"Right," his brother agreed, "You have the combination to the cabinet in Dad's study. Chet and I will rig up his microscope while you get the note from the file."
Frank and Chet focused and adjusted the microscope, making sure it was level on the table.
They plugged in the illuminator and checked to see that it did not provide too dazzling a
reflection. When Joe returned, Chet took the two pieces of paper and fitted them side by side
under the clips on the base.
"Okay. Want to take a look, fellows?"
Frank, then Joe, studied both papers. "The quality and texture are definitely the same," Frank observed.
Next, he lifted the second note from under the clips and slowly moved the paper back and forth under the lenses.
"A watermark!" he exclaimed, stepping back so the others could look at the small, faint imprint.
"Sure is!" said Joe. "A five-pointed star. This could be a valuable clue! We can try to track down exactly where this paper came from."
"And also the arrow," said Chet. "I'll make the rounds of sport stores in town."
"Swell, Chet. Thanks," Frank said.
After their friend had left, the Hardys consulted the classified directory for paper
manufacturers.
They made several calls without any luck. Finally they learned that the Quality Paper Company
in Bridgeport manufactured paper bearing the five-pointed star watermark. The brothers
wanted to go at once to get more information, but realized this errand would have to wait.
"Dad will be home soon," Frank reminded his brother. "We don't want to miss our surprise!"
"Right, And I'd like to tell him about the warning on the arrow."
When Chet returned from a round of the sports shops, he was glum. "I wasn't much help," he said. "The arrow isn't new, and all the stores I checked told me it was a standard model that could be purchased at any sports shop in the country."
"Never mind, Chet," said Frank. "At least giving your microscope a trial run helped us to spot the watermark on the second warning note. We've located a company that manufactures paper
with the star watermark."
Chet's face brightened. "Let me know if you find out anything else," he said, packing up his microscope.
"I guess I'll take off-and do some nature study for a change."
After he had driven off, Frank and Joe walked to the house. Their minds once more turned to
the surprise Mr. Hardy had for them.
"Wouldn't it be terrific if-" Joe said to Frank excitedly. "Do you think it is?"
"I'm just hoping." Frank grinned.
Just then a newsboy delivered the evening newspaper. The brothers entered the house and
went into the living room. Frank scanned the front page and pointed out an item about new
trouble in an Indiana electronics plant.
"That's where an explosion took place a couple of months ago," Joe remarked. "Sabotage, the investigators decided."
"And before that," Frank added, "the same thing happened at a rocket research lab in California. Another unsolved case."
"Seems almost like a chain reaction," Frank remarked.
Any mystery appealed to the boys, but they did not have much chance to discuss this one. The
telephone rang. Aunt Gertrude, after taking the call, burst into the living room. From the look on her face Frank and Joe could tell she was indignant, and at the same time, frightened.
"What's the matter, Aunty?" Joe asked.
"More threats-that's all!" she cried out. "This time by telephone. A man's voice-he sounded sinister-horrible!"
Mrs. Hardy came into the living room at that moment. "What did he say, Gertrude?" she asked.
Aunt Gertrude took a deep breath in an effort to calm down. " 'Hardy and his sons are playing with fire,"
the man said. 'They'll get burned if they don't lay off this case.'" Miss Hardy sniffed. "I don't know what case he meant. What kind of danger are you boys mixed up in now?"
Frank and Joe smiled wryly. "Aunt Gertrude," Frank replied, "we really don't know. But please try not to worry," he begged her and his mother. "You know that Dad and the two of us will be careful."
When Mr. Hardy came home a little later, his family told him about the threatening telephone
call. The boys, however, did not mention the arrow warning in the presence of their mother
and Aunt Gertrude.
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