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.
They knew it would only add to their concern.
Mr. Hardy was as puzzled as his sons. 'It's a funny thing,' he said. 'At this point it's impossible to tell which 'case' the person is referring to. If I knew, it might shed light on either one.'
The detective grinned and changed the subject. 'Right now, I want you all to come for a drive and have a look at the boys' surprise.'
'Swell!' Frank and Joe exclaimed in unison.
While Aunt Gertrude and Mrs. Hardy were getting ready, Frank and Joe went out to the car
with their father. Quickly the boys related their afternoon's experience, concluding with the
arrow incident.
The detective looked grim. 'Whoever is responsible for these warnings is certainly keeping close tabs on us.'
Mr. Hardy and his sons speculated for a few minutes on the fact that the pedal found in front of the house apparently had belonged to Ken's bike.
'I think Joe and I should go back tonight to the place where we had the picnic,' Frank told his father. 'In the darkness we'll have a better chance to sleuth without being seen. And there might be some clue we missed this afternoon.'
'I suppose you're right,' agreed his father. 'But be cautious.'
As Aunt Gertrude and Mrs. Hardy came out of the house, conversation about the mystery
ceased.
Everyone climbed into the sedan and Mr. Hardy drove off. Frank and Joe, seated alongside him,
were in a state of rising suspense. Was the surprise the one thing they wanted most of all?
CHAPTER VIII
The Strange Mill Wheel
A FEW minutes later Mr. Hardy was driving along the Bayport waterfront.
'Is the surprise here, Dad?' Joe asked excitedly.
'That's right.'
Mr. Hardy drove to a boathouse at the far end of the dock area and parked. He then invited the others to follow him. He walked to the door of a boathouse and unfastened the padlock.
Frank and Joe held their breaths as Mr. Hardy swung back the door. For a moment they stared
inside, speechless with delight. Finally Joe burst out, 'Exactly what we had hoped for, Dad!' and put an arm affectionately around his father.
'What a beauty!' Frank exclaimed and wrung Mr. Hardy's hand.
Rocking between the piles lay a sleek, completely equipped motorboat. It nudged gently
against clean white fenders as the waves from the bay worked their way under the boathouse
door.
The boys' mother exclaimed in delight, and even Aunt Gertrude was duly impressed by the
handsome craft.
'This is the same model we saw at the boat show,' Joe said admiringly. 'I never thought we'd own one.'
'She even has the name we picked out,' Frank observed excitedly. 'The Sleuth!'
Shiny brass letters were fitted on the bow of the boat, with the port of registry, Bayport,
underneath them.
