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?"